league/commonmark

Impact

AttributesExtension ships with the library but must be explicitly registered on the Environment; it is not included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register AttributesExtension are not affected by this advisory.

Two paths in the extension re-process every attribute a node has already collected each time another attribute is applied to it. When the attributes carry distinct names, the collected set grows by one on every step and is walked again in full, so a run of n attributes costs O(n²).

1. Attribute nodes resolving to a common target (affected from 1.5.0).

AttributesListener::processDocument() merges each attribute node into the set accumulated for its target, then filters the result. Both operations traverse that entire set: AttributesHelper::mergeAttributes() rebuilds it with array_merge(), and AttributesHelper::filterAttributes() matches a regular expression against every name in it. A run of attribute nodes sharing one target therefore re-walks a set that grows by a key per node.

Two input shapes reach this path: adjacent inline attributes at the start of a block ({a0="v"}{a1="v"}…, where quoting the values is what keeps them separate — an unquoted value swallows the }{ that follows it), and a chain of attribute blocks held at their default target by reference definitions ({a0=v} / [a]: u / {a1=v} / [a]: u / …).

256 KB of adjacent inline attributes takes 20.0 seconds to convert, against 0.09 seconds once patched.

2. Consecutive attribute-block lines (affected from 2.0.0).

AttributesBlockContinueParser::tryContinue() merges each continuation line into the block's accumulated attributes, again rebuilding the whole set on every line. One distinct attribute per line ({a0=v} / {a1=v} / …) grows it by a key each time.

256 KB of such lines takes 1.9 seconds to convert while producing zero bytes of output, against 0.08 seconds once patched.

Relationship to GHSA-jjv6-8j6v-6j52. The fix released in 2.9.1 for that advisory made the class attribute cheap to accumulate, but left every other attribute name on the original path. Applications that upgraded to 2.9.1 or 2.9.2 remain exposed to this variant.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

Patches

The issue is patched in 2.10.0. Both paths now fold each node — or each line — into the accumulated attributes at a cost proportional to that node or line alone, rather than re-merging and re-filtering everything gathered so far. Rendered output is unchanged, down to the order in which attributes appear.

The listener path affects 1.5.0 through 2.9.2. The continuation-line path affects 2.0.0 through 2.9.2. The 1.x release line is no longer supported, so its users must upgrade to 2.10.0 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register AttributesExtension when converting untrusted Markdown. This fully removes both paths.
• If the extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, the cap must be small to meaningfully bound worst-case CPU time.

The attributes/allow allow-list added in 2.7.0 is not a mitigation. A non-empty allow-list happens to bound the first path, because unlisted names are discarded before they accumulate, but it does nothing for the second: continuation lines are merged while parsing, before any filtering takes place.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading.

Impact

AttributesExtension ships with the library but must be explicitly registered on the Environment; it is not included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register AttributesExtension are not affected by this advisory.

Two paths in the extension re-process every attribute a node has already collected each time another attribute is applied to it. When the attributes carry distinct names, the collected set grows by one on every step and is walked again in full, so a run of n attributes costs O(n²).

1. Attribute nodes resolving to a common target (affected from 1.5.0).

AttributesListener::processDocument() merges each attribute node into the set accumulated for its target, then filters the result. Both operations traverse that entire set: AttributesHelper::mergeAttributes() rebuilds it with array_merge(), and AttributesHelper::filterAttributes() matches a regular expression against every name in it. A run of attribute nodes sharing one target therefore re-walks a set that grows by a key per node.

Two input shapes reach this path: adjacent inline attributes at the start of a block ({a0="v"}{a1="v"}…, where quoting the values is what keeps them separate — an unquoted value swallows the }{ that follows it), and a chain of attribute blocks held at their default target by reference definitions ({a0=v} / [a]: u / {a1=v} / [a]: u / …).

256 KB of adjacent inline attributes takes 20.0 seconds to convert, against 0.09 seconds once patched.

2. Consecutive attribute-block lines (affected from 2.0.0).

AttributesBlockContinueParser::tryContinue() merges each continuation line into the block's accumulated attributes, again rebuilding the whole set on every line. One distinct attribute per line ({a0=v} / {a1=v} / …) grows it by a key each time.

256 KB of such lines takes 1.9 seconds to convert while producing zero bytes of output, against 0.08 seconds once patched.

Relationship to GHSA-jjv6-8j6v-6j52. The fix released in 2.9.1 for that advisory made the class attribute cheap to accumulate, but left every other attribute name on the original path. Applications that upgraded to 2.9.1 or 2.9.2 remain exposed to this variant.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

Patches

The issue is patched in 2.10.0. Both paths now fold each node — or each line — into the accumulated attributes at a cost proportional to that node or line alone, rather than re-merging and re-filtering everything gathered so far. Rendered output is unchanged, down to the order in which attributes appear.

The listener path affects 1.5.0 through 2.9.2. The continuation-line path affects 2.0.0 through 2.9.2. The 1.x release line is no longer supported, so its users must upgrade to 2.10.0 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register AttributesExtension when converting untrusted Markdown. This fully removes both paths.
• If the extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, the cap must be small to meaningfully bound worst-case CPU time.

The attributes/allow allow-list added in 2.7.0 is not a mitigation. A non-empty allow-list happens to bound the first path, because unlisted names are discarded before they accumulate, but it does nothing for the second: continuation lines are merged while parsing, before any filtering takes place.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading.

Impact

AttributesExtension ships with the library but must be explicitly registered on the Environment; it is not included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register AttributesExtension are not affected by this advisory.

Two paths in the extension re-process every attribute a node has already collected each time another attribute is applied to it. When the attributes carry distinct names, the collected set grows by one on every step and is walked again in full, so a run of n attributes costs O(n²).

1. Attribute nodes resolving to a common target (affected from 1.5.0).

AttributesListener::processDocument() merges each attribute node into the set accumulated for its target, then filters the result. Both operations traverse that entire set: AttributesHelper::mergeAttributes() rebuilds it with array_merge(), and AttributesHelper::filterAttributes() matches a regular expression against every name in it. A run of attribute nodes sharing one target therefore re-walks a set that grows by a key per node.

Two input shapes reach this path: adjacent inline attributes at the start of a block ({a0="v"}{a1="v"}…, where quoting the values is what keeps them separate — an unquoted value swallows the }{ that follows it), and a chain of attribute blocks held at their default target by reference definitions ({a0=v} / [a]: u / {a1=v} / [a]: u / …).

256 KB of adjacent inline attributes takes 20.0 seconds to convert, against 0.09 seconds once patched.

2. Consecutive attribute-block lines (affected from 2.0.0).

AttributesBlockContinueParser::tryContinue() merges each continuation line into the block's accumulated attributes, again rebuilding the whole set on every line. One distinct attribute per line ({a0=v} / {a1=v} / …) grows it by a key each time.

256 KB of such lines takes 1.9 seconds to convert while producing zero bytes of output, against 0.08 seconds once patched.

Relationship to GHSA-jjv6-8j6v-6j52. The fix released in 2.9.1 for that advisory made the class attribute cheap to accumulate, but left every other attribute name on the original path. Applications that upgraded to 2.9.1 or 2.9.2 remain exposed to this variant.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

Patches

The issue is patched in 2.10.0. Both paths now fold each node — or each line — into the accumulated attributes at a cost proportional to that node or line alone, rather than re-merging and re-filtering everything gathered so far. Rendered output is unchanged, down to the order in which attributes appear.

The listener path affects 1.5.0 through 2.9.2. The continuation-line path affects 2.0.0 through 2.9.2. The 1.x release line is no longer supported, so its users must upgrade to 2.10.0 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register AttributesExtension when converting untrusted Markdown. This fully removes both paths.
• If the extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, the cap must be small to meaningfully bound worst-case CPU time.

The attributes/allow allow-list added in 2.7.0 is not a mitigation. A non-empty allow-list happens to bound the first path, because unlisted names are discarded before they accumulate, but it does nothing for the second: continuation lines are merged while parsing, before any filtering takes place.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading.

Impact

Two first-party extensions contain quadratic parsing paths. Both ship with the library but must be explicitly registered on the Environment; neither is included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register SmartPunctExtension or AttributesExtension are not affected by this advisory.

1. SmartPunctExtension — quote replacement recopies the whole text node (affected from 2.0.0).

ReplaceUnpairedQuotesListener converts each unpaired Quote node back to a Text node and merges it into its neighbours via AdjacentTextMerger. The merge reads the left node's literal into a local variable, appends to that variable, and writes it back — and because the read aliases the node's string, every append copies the entire accumulated literal rather than only the bytes added. The listener runs this once per surviving unpaired quote against the same continuously growing text node, so the same buffer is fully re-copied a linear number of times.

A 1.2 MB document of alternating text segments and apostrophes takes 34.9 seconds to convert, against 0.069 seconds for the same input with the extension not registered.

Hardened configuration makes this worse rather than better: QuoteParser appends the Quote node to the AST before pushing it onto the delimiter stack, so max_delimiters_per_line removes the quote-pairing work while leaving every node the listener must process.

2. AttributesExtension — block-level attribute runs re-scan their siblings (affected from 1.5.0).

AttributesListener::findTargetAndDirection() walks the entire remaining sibling chain for every block-level Attributes node whose target is the following node. The backward half of that walk returns immediately for such nodes, and the forward half stops only at a sibling that is not itself an attributes node — which a contiguous run never provides — so a run of k nodes costs k(k-1)/2 steps.

An input placing each {#a} on its own line, with a single reference definition to keep the run contiguous, takes 28.4 seconds at 16,000 attribute blocks while producing zero bytes of output.

This is the block-level counterpart of GHSA-g2gp-3wwq-f4ph, patched in 2.9.0. That fix is incomplete: the early break it introduced is guarded on the node being an AttributesInline, so block-level Attributes nodes still re-scan. Applications that upgraded to 2.9.0 specifically to address GHSA-g2gp-3wwq-f4ph remain exposed to this variant.

3. AttributesExtension — class lists are rebuilt on every merge (affected from 1.5.0).

AttributesHelper::mergeAttributes() round-trips the accumulated class list through explode and implode on each merge. An #id attribute assigns a scalar and skips the branch entirely, but a .class attribute appends to an array which is then imploded to a string, written to the target node, and read back on the next iteration — so the ith merge pays a cost proportional to i three separate times.

{.c} repeated 32,000 times takes 33.5 seconds, against 0.26 seconds for byte-identical input using {#a} — a 130x gap that widens with input size. Both the inline and the block-level attribute paths are affected.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

No library-level configuration gates any of these paths. For the Attributes extension in particular, neither the attributes/allow allow-list nor the on* event-handler hardening added in 2.7.0 has any effect, because the expensive work happens while parsing and resolving the AST, before any attribute filtering or rendering takes place.

Patches

The issues are patched in 2.9.1 and later:

• Adjacent text merging now appends in place instead of reading, modifying, and writing back the whole literal, so a merge costs only the bytes added. This fixes the defect for every caller, not only the SmartPunct listener.
• AttributesListener now records the runs it has already walked, so each contiguous run of block-level attribute nodes is scanned once rather than once per node.
• Accumulated class lists no longer pass through mergeAttributes() repeatedly; the listener holds pending attributes and joins them in a single pass.

The SmartPunct path affects 2.0.0 through 2.9.0. The Attributes paths affect 1.5.0 through 2.9.0, including releases that already contain the 2.9.0 fix for GHSA-g2gp-3wwq-f4ph. The 1.x release line is no longer supported, so its users must upgrade to 2.9.1 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register SmartPunctExtension or AttributesExtension when converting untrusted Markdown. This fully removes the affected paths.
• If either extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU time.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading. Configuration options including attributes/allow, max_delimiters_per_line, max_nesting_level, html_input, and allow_unsafe_links do not mitigate these issues.

Summary

The AttributesExtension documents a security guarantee:

Note: Attributes starting with on (e.g. onclick or onerror) are capable of executing JavaScript code and are therefore never allowed by default. You must explicitly add them to the allow list if you want to use them.

— docs/2.x/extensions/attributes.md

Prefixing the attribute name with a single U+000C FORM FEED byte defeats that guarantee. {<FF>onclick="alert(1)"} passes through AttributesHelper::filterAttributes() untouched and is written verbatim into the output, where browsers parse it as a genuine onclick handler.

The same prefix defeats the allow_unsafe_links check, letting a javascript: URI through on href / src even when allow_unsafe_links is false.

This bypasses the fix shipped in the 2.7.0 security release ("Fix XSS in AttributesExtension", 43207253ea5f14867c77c697cd3838c446cadcea), which added filterAttributes() for the express purpose of blocking these attributes.

Throughout this report <FF> denotes a literal U+000C byte ("\x0C" in PHP). It is invisible in rendered text, so all payloads below are written with PHP escape sequences to stay unambiguous.

Details

Three behaviours combine.

1. \x0C survives the parser's trim().

AttributesHelper::SINGLE_ATTRIBUTE begins with \s*, and Cursor::match() returns $matches[0][0] — the entire match, including that leading whitespace. The result is cleaned with PHP's trim():

// src/Extension/Attributes/Util/AttributesHelper.php:62
while ($attribute = \trim((string) $attributeCursor->match('/^' . self::SINGLE_ATTRIBUTE . '/i'))) {

PCRE \s matches \x0C, but PHP's default trim() charlist is " \t\n\r\0\x0B" — it includes the vertical tab \x0B but not the form feed \x0C. The byte is therefore consumed by the regex, retained in the returned match, and not stripped. It ends up inside the attribute name:

// src/Extension/Attributes/Util/AttributesHelper.php:94
$attributes[\trim($name)] = \trim($value);   // $name === "\x0Conclick"

\x0C is the only byte with this property: every other character the HTML5 tokenizer treats as whitespace (\x09, \x0A, \x0D, \x20), plus \x0B, is in PHP's trim charlist. The PoC includes a \x0B case as a control, and it is correctly stripped.

2. The filter's string comparisons miss it.

filterAttributes() compares the raw name against literal strings:

// src/Extension/Attributes/Util/AttributesHelper.php:148-166
$attrNameLower = \strtolower($name);                            // "\x0conclick"
... ($attrNameLower === 'href' || $attrNameLower === 'src') ... // false
... \str_starts_with($attrNameLower, 'on') ...                  // false -> not removed

3. The renderer never escapes attribute names.

// src/Util/HtmlElement.php:123-129
$result .= ' ' . $key . '="' . Xml::escape($value) . '"';   // $key emitted raw

Because the HTML5 tokenizer treats \x0C as whitespace between attributes, the browser reads the name as plain onclick.

PoC

<?php
require 'vendor/autoload.php';

use League\CommonMark\Environment\Environment;
use League\CommonMark\Extension\Attributes\AttributesExtension;
use League\CommonMark\Extension\CommonMark\CommonMarkCoreExtension;
use League\CommonMark\MarkdownConverter;

// The most defensive configuration docs/2.x/security.md recommends.
$env = new Environment([
    'html_input'         => 'escape',
    'allow_unsafe_links' => false,
    'max_nesting_level'  => 100,
    // 'attributes' => ['allow' => [...]] deliberately left at its default []
]);
$env->addExtension(new CommonMarkCoreExtension());
$env->addExtension(new AttributesExtension());
$converter = new MarkdownConverter($env);

$FF = "\x0C";

echo $converter->convert('hello {onclick="alert(1)"}')->getContent();
// <p>hello</p>                                    <- filtered, as documented

echo $converter->convert('hello {' . $FF . 'onclick="alert(1)"}')->getContent();
// <p \x0Conclick="alert(1)">hello</p>             <- BYPASS

Full observed output (\x0C shown escaped; it is a literal single byte in the real output):

| # | Markdown input | Rendered output | Result | |---|---|---|---| | A | hello {onclick="alert(1)"} | <p>hello</p> | filtered (control) | | B | hello {\x0Conclick="alert(1)"} | <p \x0Conclick="alert(1)">hello</p> | bypass | | C | hello {\x0Bonclick="alert(1)"} | <p>hello</p> | filtered (control) | | D | [click](javascript:alert(1)) | <p><a>click</a></p> | filtered (control) | | E | [click](https://example.com){\x0Chref="javascript:alert(1)"} | <p><a \x0Chref="javascript:alert(1)" href="https://example.com">click</a></p> | bypass | | F | ![x](https://example.invalid/x.png){\x0Conerror="alert(1)"} | <p><img \x0Conerror="alert(1)" src="…" alt="x" /></p> | bypass | | G | # heading + newline + {\x0Conclick="alert(1)"} | <h1 \x0Conclick="alert(1)">heading</h1> | bypass (block syntax) |

In case E the injected href precedes the legitimate one. Per the HTML5 duplicate-attribute rule the first occurrence wins, so the javascript: URI is the one the browser actually uses.

Browser confirmation. Loading the library's unmodified output in Chrome for Testing 148:

<img> attribute names : ["onerror","src","alt"]      <- parsed as a real `onerror`
typeof img.onerror    : function                     <- bound as an event handler
handlers fired        : ["img-onerror"]              <- fired on load, no interaction
document.title        : XSS-FIRED
link href attribute   : "javascript:void(0)"
link href property    : "javascript:void(0)"         <- javascript: URI is the effective href
page errors           : []

The onerror case executes with no user interaction — rendering the attacker's Markdown is sufficient.

Verified against git HEAD (f966b17a) and against tag 2.9.0, on PHP 8.5.8.

Impact

Stored cross-site scripting in any application that renders untrusted Markdown with AttributesExtension enabled and attributes.allow left at its default [] — even when the application has followed every hardening step in docs/2.x/security.md (html_input => 'escape', allow_unsafe_links => false, max_nesting_level => 100).

Consequences are the usual for stored XSS: session and cookie theft, actions performed as the viewing user, and account takeover where the host application permits it. Because the payload can be attached to an image (onerror), it fires on page load without requiring the victim to interact with anything.

The affected configuration is the extension's default: attributes.allow defaults to [], and the documentation describes that default as safe with respect to on* attributes.

Workaround for users

Setting an explicit allow list takes the other branch of filterAttributes(), which drops the form-feed name because it is not in the list:

$config = ['attributes' => ['allow' => ['id', 'class', 'align']]];

Verified: hello {\x0Conclick="alert(1)"} then renders as <p>hello</p>.

Suggested fix

The narrow fix is to add \x0C to the trim charlist at AttributesHelper.php lines 62, 89, 90 and 94. That closes this instance but leaves the shape of the problem in place.

A more durable fix is to reject anything that is not a well-formed attribute name in filterAttributes(), reusing the constant the parser already defines (RegexHelper is already imported in that file):

foreach ($attributes as $name => $value) {
    // Names are compared against literal strings below and emitted without escaping,
    // so anything that isn't a plain attribute name must not get through.
    if (\preg_match('/^' . RegexHelper::PARTIAL_ATTRIBUTENAME . '$/i', $name) !== 1) {
        unset($attributes[$name]);
        continue;
    }

    $attrNameLower = \strtolower($name);
    // ... existing logic unchanged
}

As defence in depth, HtmlElement::__toString() could validate or escape $key. It currently trusts its callers to supply safe attribute names, and filterAttributes() is the only thing standing between that method and user-supplied input.

Impact

Affected versions of league/commonmark perform super-linear work on three independent parsing paths, all of which are reachable on a stock new CommonMarkConverter() with default configuration and no extensions registered. Each trigger fits on a single line of input, so no complex Markdown structure is required.

The three paths were introduced at different times. This advisory's version range is their union; the individual ranges are:

| Path | Affected from | Affected through | |---|---|---| | 1. Fenced code block detection | 0.6.0 | 2.9.0 | | 2. Reference link label lookup | 0.6.0 | 2.9.0 | | 3. Emphasis / strikethrough delimiters (*, _, ~) | 2.6.0 | 2.9.0 | | 3. Highlight delimiters (=) | 2.9.0 | 2.9.0 |

1. Fenced code block detection — quadratic, affected from 0.6.0.

FencedCodeStartParser matches the following pattern:

/^[ \t]*(?:`{3,}(?!.*`)|~{3,})/

The lookahead enforces the CommonMark rule that a backtick fence's info string may not itself contain a backtick, but neither the lookahead nor the backtick run it guards is atomic or possessive. On a line consisting of a long backtick run, filler text, and a single trailing backtick, the quantifier gives back one character at a time and re-runs the lookahead across the remainder of the line on every candidate fence length.

A 320 KB single line takes roughly 27 seconds to convert. The identical payload with one x character prefixed — which fails the parser's own leading-character guard — takes 0.011 seconds. preg_last_error() returns 0 at every input size tested, including runs of 160,000 characters, so PCRE never reaches pcre.backtrack_limit and this is sustained CPU consumption rather than an early bail-out.

2. Reference link label lookup — effectively quadratic, affected from 0.6.0.

When a shortcut or collapsed reference link is attempted, CloseBracketParser::tryParseReference() copies the entire span between the brackets and passes it to ReferenceMap::get(), which normalizes the label — up to four full passes over its length (trim, preg_replace, mb_check_encoding, and strtolower, or mb_convert_case on the non-ASCII path). Nested brackets produce one such lookup per closing bracket, each on a span two characters longer than the last.

In 2.x the normalization sits behind an early return for an empty reference map, so a single 8-byte reference definition anywhere in the document ([x]: y) is enough to unlock the path. At n = 64,000 nested brackets the same input takes 22.0 seconds with that line present versus 0.59 seconds without it. A single non-ASCII character inside the brackets forces the mb_convert_case branch, costing roughly 2.5x more again.

3. Emphasis, strikethrough, and highlight delimiter processing — super-linear, affected from 2.6.0.

DelimiterStack::processDelimiters() remains linear only because of the openersBottom memo, which bounds the backward opener scan — an argument that holds only if the memo's key space is O(1). EmphasisDelimiterProcessor::getCacheKey(), and the equivalents in StrikethroughDelimiterProcessor and MarkDelimiterProcessor, embed the closer's raw current run length in the key, leaving that space unbounded. An attacker spends O(n) bytes minting a growing number of distinct run lengths; each distinct length is a fresh key whose recorded bound starts at zero, forcing a full backward re-scan of the entire pile of openers.

The resulting work grows as roughly n^1.5. This is sub-quadratic, but the amplification over linear growth itself scales with input size, so it worsens as inputs grow: 800 KB of ordinary asterisks, letters, and spaces costs roughly 27 seconds on a stock converter.

This path is a regression introduced in 2.6.0. Before that release the cache key was the bare delimiter character — a bounded key space that amortized correctly. * and _ are affected on any default configuration from 2.6.0 onward. ~ (StrikethroughExtension, included in GithubFlavoredMarkdownConverter and GithubFlavoredMarkdownExtension) is affected from 2.6.0. = (HighlightExtension) is affected only from 2.9.0, when MarkDelimiterProcessor was declared cacheable.

Overall impact. An unauthenticated attacker who can submit Markdown for conversion can use a comparatively small request to consume disproportionate CPU time. Repeated or concurrent requests can occupy all available PHP workers and prevent legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed. Applications that process only trusted Markdown are not remotely exploitable.

Settings such as html_input, allow_unsafe_links, and max_nesting_level do not mitigate any of these, because the expensive work occurs during parsing, before rendering. max_delimiters_per_line bounds the third path only, and does so lossily — it silently discards emphasis once the cap is exhausted.

Patches

The issues are patched in 2.9.1 and later:

• The fenced code block quantifier is now possessive, which is behavior-identical here: any character given back moves a backtick into the lookahead's scan range, so every retry was guaranteed to fail regardless.
• Reference link lookups now apply the CommonMark 999-character link label limit before copying and normalizing the label, matching the limit already enforced when parsing reference definitions. Because a definition can never exceed that length, an over-length lookup label cannot match one directly. One edge case does change: a label longer than 999 characters that collapses to a shorter match once whitespace is normalized — for example [a followed by 998 spaces and b] against a [a b]: /url definition — previously rendered as a link and now renders literally. This follows cmark, which applies its own label-length cap before normalizing (cmark_reference_lookup()), and matches how this library has always handled the equivalent [text][label] form via LinkParserHelper::parseLinkLabel(). commonmark.js normalizes first and still resolves such labels.
• Delimiter processor cache keys now clamp the run length to the coarsest bucket that can change behavior — min(length, 2) for emphasis, min(length, 3) for strikethrough and highlight — restoring a bounded key space while preserving byte-identical output.

Versions from 0.6.0 through 2.9.0 are affected by at least one of these paths; see the table above for which paths apply to which releases. The 0.x and 1.x release lines are no longer supported, so their users must upgrade to 2.9.1 or later.

Workarounds

If you cannot upgrade immediately, enforce a maximum length for individual lines before passing input to the converter, in addition to a total request-size limit. A per-line limit matters because every trigger described above fits within a single line. Because the cost grows super-linearly, the cap must be genuinely small to bound worst-case CPU.

Setting max_delimiters_per_line reduces exposure to the delimiter path only, and does so by silently dropping emphasis from the rendered output. It has no effect on the fenced code or reference link paths.

Restricting conversion to trusted users, applying strict execution-time limits, rate-limiting requests, and limiting concurrent conversions all reduce exposure, but none is a complete substitute for upgrading.

Impact

AttributesExtension ships with the library but must be explicitly registered on the Environment; it is not included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register AttributesExtension are not affected by this advisory.

Two paths in the extension re-process every attribute a node has already collected each time another attribute is applied to it. When the attributes carry distinct names, the collected set grows by one on every step and is walked again in full, so a run of n attributes costs O(n²).

1. Attribute nodes resolving to a common target (affected from 1.5.0).

AttributesListener::processDocument() merges each attribute node into the set accumulated for its target, then filters the result. Both operations traverse that entire set: AttributesHelper::mergeAttributes() rebuilds it with array_merge(), and AttributesHelper::filterAttributes() matches a regular expression against every name in it. A run of attribute nodes sharing one target therefore re-walks a set that grows by a key per node.

Two input shapes reach this path: adjacent inline attributes at the start of a block ({a0="v"}{a1="v"}…, where quoting the values is what keeps them separate — an unquoted value swallows the }{ that follows it), and a chain of attribute blocks held at their default target by reference definitions ({a0=v} / [a]: u / {a1=v} / [a]: u / …).

256 KB of adjacent inline attributes takes 20.0 seconds to convert, against 0.09 seconds once patched.

2. Consecutive attribute-block lines (affected from 2.0.0).

AttributesBlockContinueParser::tryContinue() merges each continuation line into the block's accumulated attributes, again rebuilding the whole set on every line. One distinct attribute per line ({a0=v} / {a1=v} / …) grows it by a key each time.

256 KB of such lines takes 1.9 seconds to convert while producing zero bytes of output, against 0.08 seconds once patched.

Relationship to GHSA-jjv6-8j6v-6j52. The fix released in 2.9.1 for that advisory made the class attribute cheap to accumulate, but left every other attribute name on the original path. Applications that upgraded to 2.9.1 or 2.9.2 remain exposed to this variant.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

Patches

The issue is patched in 2.10.0. Both paths now fold each node — or each line — into the accumulated attributes at a cost proportional to that node or line alone, rather than re-merging and re-filtering everything gathered so far. Rendered output is unchanged, down to the order in which attributes appear.

The listener path affects 1.5.0 through 2.9.2. The continuation-line path affects 2.0.0 through 2.9.2. The 1.x release line is no longer supported, so its users must upgrade to 2.10.0 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register AttributesExtension when converting untrusted Markdown. This fully removes both paths.
• If the extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, the cap must be small to meaningfully bound worst-case CPU time.

The attributes/allow allow-list added in 2.7.0 is not a mitigation. A non-empty allow-list happens to bound the first path, because unlisted names are discarded before they accumulate, but it does nothing for the second: continuation lines are merged while parsing, before any filtering takes place.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading.

Impact

Two first-party extensions contain quadratic parsing paths. Both ship with the library but must be explicitly registered on the Environment; neither is included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register SmartPunctExtension or AttributesExtension are not affected by this advisory.

1. SmartPunctExtension — quote replacement recopies the whole text node (affected from 2.0.0).

ReplaceUnpairedQuotesListener converts each unpaired Quote node back to a Text node and merges it into its neighbours via AdjacentTextMerger. The merge reads the left node's literal into a local variable, appends to that variable, and writes it back — and because the read aliases the node's string, every append copies the entire accumulated literal rather than only the bytes added. The listener runs this once per surviving unpaired quote against the same continuously growing text node, so the same buffer is fully re-copied a linear number of times.

A 1.2 MB document of alternating text segments and apostrophes takes 34.9 seconds to convert, against 0.069 seconds for the same input with the extension not registered.

Hardened configuration makes this worse rather than better: QuoteParser appends the Quote node to the AST before pushing it onto the delimiter stack, so max_delimiters_per_line removes the quote-pairing work while leaving every node the listener must process.

2. AttributesExtension — block-level attribute runs re-scan their siblings (affected from 1.5.0).

AttributesListener::findTargetAndDirection() walks the entire remaining sibling chain for every block-level Attributes node whose target is the following node. The backward half of that walk returns immediately for such nodes, and the forward half stops only at a sibling that is not itself an attributes node — which a contiguous run never provides — so a run of k nodes costs k(k-1)/2 steps.

An input placing each {#a} on its own line, with a single reference definition to keep the run contiguous, takes 28.4 seconds at 16,000 attribute blocks while producing zero bytes of output.

This is the block-level counterpart of GHSA-g2gp-3wwq-f4ph, patched in 2.9.0. That fix is incomplete: the early break it introduced is guarded on the node being an AttributesInline, so block-level Attributes nodes still re-scan. Applications that upgraded to 2.9.0 specifically to address GHSA-g2gp-3wwq-f4ph remain exposed to this variant.

3. AttributesExtension — class lists are rebuilt on every merge (affected from 1.5.0).

AttributesHelper::mergeAttributes() round-trips the accumulated class list through explode and implode on each merge. An #id attribute assigns a scalar and skips the branch entirely, but a .class attribute appends to an array which is then imploded to a string, written to the target node, and read back on the next iteration — so the ith merge pays a cost proportional to i three separate times.

{.c} repeated 32,000 times takes 33.5 seconds, against 0.26 seconds for byte-identical input using {#a} — a 130x gap that widens with input size. Both the inline and the block-level attribute paths are affected.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

No library-level configuration gates any of these paths. For the Attributes extension in particular, neither the attributes/allow allow-list nor the on* event-handler hardening added in 2.7.0 has any effect, because the expensive work happens while parsing and resolving the AST, before any attribute filtering or rendering takes place.

Patches

The issues are patched in 2.9.1 and later:

• Adjacent text merging now appends in place instead of reading, modifying, and writing back the whole literal, so a merge costs only the bytes added. This fixes the defect for every caller, not only the SmartPunct listener.
• AttributesListener now records the runs it has already walked, so each contiguous run of block-level attribute nodes is scanned once rather than once per node.
• Accumulated class lists no longer pass through mergeAttributes() repeatedly; the listener holds pending attributes and joins them in a single pass.

The SmartPunct path affects 2.0.0 through 2.9.0. The Attributes paths affect 1.5.0 through 2.9.0, including releases that already contain the 2.9.0 fix for GHSA-g2gp-3wwq-f4ph. The 1.x release line is no longer supported, so its users must upgrade to 2.9.1 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register SmartPunctExtension or AttributesExtension when converting untrusted Markdown. This fully removes the affected paths.
• If either extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU time.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading. Configuration options including attributes/allow, max_delimiters_per_line, max_nesting_level, html_input, and allow_unsafe_links do not mitigate these issues.

Summary

The AttributesExtension documents a security guarantee:

Note: Attributes starting with on (e.g. onclick or onerror) are capable of executing JavaScript code and are therefore never allowed by default. You must explicitly add them to the allow list if you want to use them.

— docs/2.x/extensions/attributes.md

Prefixing the attribute name with a single U+000C FORM FEED byte defeats that guarantee. {<FF>onclick="alert(1)"} passes through AttributesHelper::filterAttributes() untouched and is written verbatim into the output, where browsers parse it as a genuine onclick handler.

The same prefix defeats the allow_unsafe_links check, letting a javascript: URI through on href / src even when allow_unsafe_links is false.

This bypasses the fix shipped in the 2.7.0 security release ("Fix XSS in AttributesExtension", 43207253ea5f14867c77c697cd3838c446cadcea), which added filterAttributes() for the express purpose of blocking these attributes.

Throughout this report <FF> denotes a literal U+000C byte ("\x0C" in PHP). It is invisible in rendered text, so all payloads below are written with PHP escape sequences to stay unambiguous.

Details

Three behaviours combine.

1. \x0C survives the parser's trim().

AttributesHelper::SINGLE_ATTRIBUTE begins with \s*, and Cursor::match() returns $matches[0][0] — the entire match, including that leading whitespace. The result is cleaned with PHP's trim():

// src/Extension/Attributes/Util/AttributesHelper.php:62
while ($attribute = \trim((string) $attributeCursor->match('/^' . self::SINGLE_ATTRIBUTE . '/i'))) {

PCRE \s matches \x0C, but PHP's default trim() charlist is " \t\n\r\0\x0B" — it includes the vertical tab \x0B but not the form feed \x0C. The byte is therefore consumed by the regex, retained in the returned match, and not stripped. It ends up inside the attribute name:

// src/Extension/Attributes/Util/AttributesHelper.php:94
$attributes[\trim($name)] = \trim($value);   // $name === "\x0Conclick"

\x0C is the only byte with this property: every other character the HTML5 tokenizer treats as whitespace (\x09, \x0A, \x0D, \x20), plus \x0B, is in PHP's trim charlist. The PoC includes a \x0B case as a control, and it is correctly stripped.

2. The filter's string comparisons miss it.

filterAttributes() compares the raw name against literal strings:

// src/Extension/Attributes/Util/AttributesHelper.php:148-166
$attrNameLower = \strtolower($name);                            // "\x0conclick"
... ($attrNameLower === 'href' || $attrNameLower === 'src') ... // false
... \str_starts_with($attrNameLower, 'on') ...                  // false -> not removed

3. The renderer never escapes attribute names.

// src/Util/HtmlElement.php:123-129
$result .= ' ' . $key . '="' . Xml::escape($value) . '"';   // $key emitted raw

Because the HTML5 tokenizer treats \x0C as whitespace between attributes, the browser reads the name as plain onclick.

PoC

<?php
require 'vendor/autoload.php';

use League\CommonMark\Environment\Environment;
use League\CommonMark\Extension\Attributes\AttributesExtension;
use League\CommonMark\Extension\CommonMark\CommonMarkCoreExtension;
use League\CommonMark\MarkdownConverter;

// The most defensive configuration docs/2.x/security.md recommends.
$env = new Environment([
    'html_input'         => 'escape',
    'allow_unsafe_links' => false,
    'max_nesting_level'  => 100,
    // 'attributes' => ['allow' => [...]] deliberately left at its default []
]);
$env->addExtension(new CommonMarkCoreExtension());
$env->addExtension(new AttributesExtension());
$converter = new MarkdownConverter($env);

$FF = "\x0C";

echo $converter->convert('hello {onclick="alert(1)"}')->getContent();
// <p>hello</p>                                    <- filtered, as documented

echo $converter->convert('hello {' . $FF . 'onclick="alert(1)"}')->getContent();
// <p \x0Conclick="alert(1)">hello</p>             <- BYPASS

Full observed output (\x0C shown escaped; it is a literal single byte in the real output):

| # | Markdown input | Rendered output | Result | |---|---|---|---| | A | hello {onclick="alert(1)"} | <p>hello</p> | filtered (control) | | B | hello {\x0Conclick="alert(1)"} | <p \x0Conclick="alert(1)">hello</p> | bypass | | C | hello {\x0Bonclick="alert(1)"} | <p>hello</p> | filtered (control) | | D | [click](javascript:alert(1)) | <p><a>click</a></p> | filtered (control) | | E | [click](https://example.com){\x0Chref="javascript:alert(1)"} | <p><a \x0Chref="javascript:alert(1)" href="https://example.com">click</a></p> | bypass | | F | ![x](https://example.invalid/x.png){\x0Conerror="alert(1)"} | <p><img \x0Conerror="alert(1)" src="…" alt="x" /></p> | bypass | | G | # heading + newline + {\x0Conclick="alert(1)"} | <h1 \x0Conclick="alert(1)">heading</h1> | bypass (block syntax) |

In case E the injected href precedes the legitimate one. Per the HTML5 duplicate-attribute rule the first occurrence wins, so the javascript: URI is the one the browser actually uses.

Browser confirmation. Loading the library's unmodified output in Chrome for Testing 148:

<img> attribute names : ["onerror","src","alt"]      <- parsed as a real `onerror`
typeof img.onerror    : function                     <- bound as an event handler
handlers fired        : ["img-onerror"]              <- fired on load, no interaction
document.title        : XSS-FIRED
link href attribute   : "javascript:void(0)"
link href property    : "javascript:void(0)"         <- javascript: URI is the effective href
page errors           : []

The onerror case executes with no user interaction — rendering the attacker's Markdown is sufficient.

Verified against git HEAD (f966b17a) and against tag 2.9.0, on PHP 8.5.8.

Impact

Stored cross-site scripting in any application that renders untrusted Markdown with AttributesExtension enabled and attributes.allow left at its default [] — even when the application has followed every hardening step in docs/2.x/security.md (html_input => 'escape', allow_unsafe_links => false, max_nesting_level => 100).

Consequences are the usual for stored XSS: session and cookie theft, actions performed as the viewing user, and account takeover where the host application permits it. Because the payload can be attached to an image (onerror), it fires on page load without requiring the victim to interact with anything.

The affected configuration is the extension's default: attributes.allow defaults to [], and the documentation describes that default as safe with respect to on* attributes.

Workaround for users

Setting an explicit allow list takes the other branch of filterAttributes(), which drops the form-feed name because it is not in the list:

$config = ['attributes' => ['allow' => ['id', 'class', 'align']]];

Verified: hello {\x0Conclick="alert(1)"} then renders as <p>hello</p>.

Suggested fix

The narrow fix is to add \x0C to the trim charlist at AttributesHelper.php lines 62, 89, 90 and 94. That closes this instance but leaves the shape of the problem in place.

A more durable fix is to reject anything that is not a well-formed attribute name in filterAttributes(), reusing the constant the parser already defines (RegexHelper is already imported in that file):

foreach ($attributes as $name => $value) {
    // Names are compared against literal strings below and emitted without escaping,
    // so anything that isn't a plain attribute name must not get through.
    if (\preg_match('/^' . RegexHelper::PARTIAL_ATTRIBUTENAME . '$/i', $name) !== 1) {
        unset($attributes[$name]);
        continue;
    }

    $attrNameLower = \strtolower($name);
    // ... existing logic unchanged
}

As defence in depth, HtmlElement::__toString() could validate or escape $key. It currently trusts its callers to supply safe attribute names, and filterAttributes() is the only thing standing between that method and user-supplied input.

Impact

Affected versions of league/commonmark perform super-linear work on three independent parsing paths, all of which are reachable on a stock new CommonMarkConverter() with default configuration and no extensions registered. Each trigger fits on a single line of input, so no complex Markdown structure is required.

The three paths were introduced at different times. This advisory's version range is their union; the individual ranges are:

| Path | Affected from | Affected through | |---|---|---| | 1. Fenced code block detection | 0.6.0 | 2.9.0 | | 2. Reference link label lookup | 0.6.0 | 2.9.0 | | 3. Emphasis / strikethrough delimiters (*, _, ~) | 2.6.0 | 2.9.0 | | 3. Highlight delimiters (=) | 2.9.0 | 2.9.0 |

1. Fenced code block detection — quadratic, affected from 0.6.0.

FencedCodeStartParser matches the following pattern:

/^[ \t]*(?:`{3,}(?!.*`)|~{3,})/

The lookahead enforces the CommonMark rule that a backtick fence's info string may not itself contain a backtick, but neither the lookahead nor the backtick run it guards is atomic or possessive. On a line consisting of a long backtick run, filler text, and a single trailing backtick, the quantifier gives back one character at a time and re-runs the lookahead across the remainder of the line on every candidate fence length.

A 320 KB single line takes roughly 27 seconds to convert. The identical payload with one x character prefixed — which fails the parser's own leading-character guard — takes 0.011 seconds. preg_last_error() returns 0 at every input size tested, including runs of 160,000 characters, so PCRE never reaches pcre.backtrack_limit and this is sustained CPU consumption rather than an early bail-out.

2. Reference link label lookup — effectively quadratic, affected from 0.6.0.

When a shortcut or collapsed reference link is attempted, CloseBracketParser::tryParseReference() copies the entire span between the brackets and passes it to ReferenceMap::get(), which normalizes the label — up to four full passes over its length (trim, preg_replace, mb_check_encoding, and strtolower, or mb_convert_case on the non-ASCII path). Nested brackets produce one such lookup per closing bracket, each on a span two characters longer than the last.

In 2.x the normalization sits behind an early return for an empty reference map, so a single 8-byte reference definition anywhere in the document ([x]: y) is enough to unlock the path. At n = 64,000 nested brackets the same input takes 22.0 seconds with that line present versus 0.59 seconds without it. A single non-ASCII character inside the brackets forces the mb_convert_case branch, costing roughly 2.5x more again.

3. Emphasis, strikethrough, and highlight delimiter processing — super-linear, affected from 2.6.0.

DelimiterStack::processDelimiters() remains linear only because of the openersBottom memo, which bounds the backward opener scan — an argument that holds only if the memo's key space is O(1). EmphasisDelimiterProcessor::getCacheKey(), and the equivalents in StrikethroughDelimiterProcessor and MarkDelimiterProcessor, embed the closer's raw current run length in the key, leaving that space unbounded. An attacker spends O(n) bytes minting a growing number of distinct run lengths; each distinct length is a fresh key whose recorded bound starts at zero, forcing a full backward re-scan of the entire pile of openers.

The resulting work grows as roughly n^1.5. This is sub-quadratic, but the amplification over linear growth itself scales with input size, so it worsens as inputs grow: 800 KB of ordinary asterisks, letters, and spaces costs roughly 27 seconds on a stock converter.

This path is a regression introduced in 2.6.0. Before that release the cache key was the bare delimiter character — a bounded key space that amortized correctly. * and _ are affected on any default configuration from 2.6.0 onward. ~ (StrikethroughExtension, included in GithubFlavoredMarkdownConverter and GithubFlavoredMarkdownExtension) is affected from 2.6.0. = (HighlightExtension) is affected only from 2.9.0, when MarkDelimiterProcessor was declared cacheable.

Overall impact. An unauthenticated attacker who can submit Markdown for conversion can use a comparatively small request to consume disproportionate CPU time. Repeated or concurrent requests can occupy all available PHP workers and prevent legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed. Applications that process only trusted Markdown are not remotely exploitable.

Settings such as html_input, allow_unsafe_links, and max_nesting_level do not mitigate any of these, because the expensive work occurs during parsing, before rendering. max_delimiters_per_line bounds the third path only, and does so lossily — it silently discards emphasis once the cap is exhausted.

Patches

The issues are patched in 2.9.1 and later:

• The fenced code block quantifier is now possessive, which is behavior-identical here: any character given back moves a backtick into the lookahead's scan range, so every retry was guaranteed to fail regardless.
• Reference link lookups now apply the CommonMark 999-character link label limit before copying and normalizing the label, matching the limit already enforced when parsing reference definitions. Because a definition can never exceed that length, an over-length lookup label cannot match one directly. One edge case does change: a label longer than 999 characters that collapses to a shorter match once whitespace is normalized — for example [a followed by 998 spaces and b] against a [a b]: /url definition — previously rendered as a link and now renders literally. This follows cmark, which applies its own label-length cap before normalizing (cmark_reference_lookup()), and matches how this library has always handled the equivalent [text][label] form via LinkParserHelper::parseLinkLabel(). commonmark.js normalizes first and still resolves such labels.
• Delimiter processor cache keys now clamp the run length to the coarsest bucket that can change behavior — min(length, 2) for emphasis, min(length, 3) for strikethrough and highlight — restoring a bounded key space while preserving byte-identical output.

Versions from 0.6.0 through 2.9.0 are affected by at least one of these paths; see the table above for which paths apply to which releases. The 0.x and 1.x release lines are no longer supported, so their users must upgrade to 2.9.1 or later.

Workarounds

If you cannot upgrade immediately, enforce a maximum length for individual lines before passing input to the converter, in addition to a total request-size limit. A per-line limit matters because every trigger described above fits within a single line. Because the cost grows super-linearly, the cap must be genuinely small to bound worst-case CPU.

Setting max_delimiters_per_line reduces exposure to the delimiter path only, and does so by silently dropping emphasis from the rendered output. It has no effect on the fenced code or reference link paths.

Restricting conversion to trusted users, applying strict execution-time limits, rate-limiting requests, and limiting concurrent conversions all reduce exposure, but none is a complete substitute for upgrading.

Impact

XmlRenderer pretty-prints XML by emitting depth-proportional indentation whitespace for every opening and closing tag. For a tree of depth n, the indentation alone sums to O(n²) bytes of output (and corresponding memory), reachable through MarkdownToXmlConverter — e.g. str_repeat('> ', $depth) . "x\n", a single line of nested blockquotes — or through a direct XmlRenderer::renderDocument() call on an attacker-influenced AST.

This affects applications that convert untrusted Markdown to XML, which is an opt-in output path. The parser's max_nesting_level bounds the depth of parser-created trees, but its default is high enough to reach damaging sizes, can be raised by the host application, and does not constrain custom or programmatically built ASTs handed straight to the renderer. The result is a memory / output-size amplification rather than a hard crash, which is why this issue is rated Medium rather than High. No confidentiality or integrity impact. XML rendering was introduced in 2.0.0 (first shipped in 2.0.0-beta1, June 2021) and has emitted depth-proportional indentation ever since, so all 2.x releases are affected (verified against 2.8.x, clean upstream 1902f60f). 1.x has no XML renderer and is not affected.

Workarounds

Applications converting untrusted Markdown to XML should:

• Lower max_nesting_level to a conservative value appropriate to expected content, so the parser refuses to build extremely deep trees. This is the most direct lever for parser-produced ASTs, but does not protect trees built programmatically and passed straight to XmlRenderer.
• Cap input size before conversion, since the amplification is driven by input-proportional depth.
• Constrain XML consumers with memory / output-size limits (and streaming or size caps on any downstream XML parser or storage) so one request cannot allocate unbounded output.
• Prefer HTML rendering for untrusted content where XML is not strictly required — the HTML renderer does not emit depth-proportional indentation and is not subject to this amplification.

Impact

UniqueSlugNormalizer::normalize() makes each slug document-unique by searching for an unused numeric suffix, but restarts that search from 1 on every collision. The k-th heading that collapses to the same base slug performs k−1 array lookups, so K colliding slugs cost Σ(k−1) = O(K²). An attacker can force every heading onto a single base slug trivially — many empty ATX headings, identical heading text, or punctuation-only headings that normalize to the empty string.

The path is reached whenever the shared slug normalizer runs over attacker-controlled text. That happens when HeadingPermalinkExtension is registered (its HeadingPermalinkProcessor normalizes every heading), independently through FootnoteExtension (its AnonymousFootnoteRefParser normalizes every ^[label] reference), and on any TableOfContentsExtension site (which requires HeadingPermalinkExtension to be co-registered). The default slug_normalizer/unique setting (UniqueSlugNormalizerInterface::PER_DOCUMENT) accumulates collisions across the whole document. No authentication is required — a small document body turns into seconds of CPU and denies service. Availability impact only. UniqueSlugNormalizer was introduced in 2.0.0 (first shipped in 2.0.0-beta1, May 2021); the 1.x heading-permalink slug generator performed no de-duplication and is not affected. All 2.x releases (including 2.8.x) are affected.

Workarounds

Integrators who cannot upgrade immediately can:

• Set slug_normalizer/unique to false / UniqueSlugNormalizerInterface::DISABLED, which stops the de-duplication scan entirely — at the cost of losing id uniqueness (colliding headings then share an anchor).
• Disable HeadingPermalinkExtension (and TableOfContentsExtension, which depends on it), and FootnoteExtension where anonymous footnotes reach the same normalizer, for untrusted Markdown.
• Cap the accepted document size / heading count upstream so K cannot reach the quadratic danger zone.

Each of these trades off functionality or correctness; upgrading to the patched release (which removes the quadratic behavior while keeping unique ids and identical output) is the recommended remediation.

Impact

The Footnote extension records one backref per footnote reference and then appends the entire backref list for every footnote definition block in the document, without ever de-duplicating or removing repeated definitions of the same label (GatherFootnotesListener, populated by NumberFootnotesListener). A document that references a single label N times and also supplies N duplicate [^a]: definitions of that label therefore produces N × N FootnoteBackref nodes, so output size, parse time, and peak memory are all O(N²).

Reaching the vulnerable path requires FootnoteExtension to be registered on the Environment. This is opt-in, but is a commonly enabled GFM-style feature; no other non-default configuration is required. An unauthenticated attacker can expand a ~10 KB request into a ~62 MB HTML response, ~3 s of CPU, and ~440 MB of peak memory — enough to OOM-kill a default 128 MB PHP worker and deny service. Availability impact only; no confidentiality or integrity effect. The Footnote extension was introduced in 1.5.0 (May 2020) with this backref logic present from the first commit, so all releases from 1.5.0 onward (including every 2.x through 2.8.x) are affected.

Workarounds

There is no library-level option to cap the number of footnotes, references, or definitions, so no configuration switch prevents the amplification. Integrators who cannot upgrade should:

• Disable the Footnote extension for untrusted input, or
• Enforce a strict input-size limit before conversion — but note this is a weak control here, since the ~10 KB payload that already triggers the 62 MB / ~440 MB blowup is well within typical request-body limits, so any cap must be aggressively small to help.

Upgrading to the patched release is the recommended remediation.

Impact

With the Attributes extension enabled, AttributesListener::findTargetAndDirection() resolves each attribute node's target by walking outward through its siblings. For a run of N adjacent inline attribute blocks placed at the start of a block (with nothing to their left), each node scans the entire sibling list to the far-right end before giving up and falling back to the parent. Each resolution is therefore Θ(N) and the whole run is Θ(N²).

Reaching the path requires AttributesExtension (opt-in, but first-party: League\CommonMark\Extension\Attributes\AttributesExtension). No other configuration matters — the quadratic walk runs unconditionally during parsing and is not gated by the attributes/allow allow-list, the on* hardening added in 2.7.0, or allow_unsafe_links. An unauthenticated attacker can submit a ~32 KB input ({#a} repeated 8,000 times) that takes over 5 seconds to convert, with time growing quadratically in input length — a cheap denial of service. Availability impact only. The Attributes extension was introduced in 1.5.0 (May 2020) with this outward-walk resolver present from the first commit, so all releases from 1.5.0 onward (including every 2.x) are affected.

Workarounds

There is no library-level configuration that gates the quadratic walk. Integrators who cannot upgrade can only reduce exposure indirectly:

• Disable the Attributes extension for untrusted input, or
• Impose a strict maximum input length before conversion — noting that because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU.

Upgrading to a release containing the fix is recommended.

Impact

Affected versions of league/commonmark can have quadratic time complexity when parsing specially crafted Markdown lines. In practical terms, doubling the length of an affected line can make the parser perform roughly four times as much work. The parser identifies locations using character positions, but regular-expression matches report byte positions. These positions differ when a UTF-8 character uses more than one byte. Several parsing paths repeatedly rescan growing portions of the line to translate between the two positions. The Autolink extension can also copy and validate the remaining line at every URL-like prefix.

In current 2.x releases, a single non-ASCII character anywhere on a line can place that whole line on the slower multibyte path. An attacker can combine it with a long run of leading whitespace or repeated Markdown punctuation, causing increasingly large rescans. When the Autolink extension is enabled, repeated URL-like prefixes provide another trigger, even on ASCII-only lines. Each trigger fits within one long line, so complex Markdown structure is unnecessary.

An attacker who can submit Markdown for conversion can use a comparatively small request to consume disproportionate CPU time and allocation activity. Repeated or concurrent requests can occupy all available PHP workers and prevent legitimate requests from completing. The core paths affect CommonMarkConverter, GithubFlavoredMarkdownConverter, and custom environments. The autolink-specific path affects applications using AutolinkExtension or GithubFlavoredMarkdownExtension. Applications that process only trusted Markdown are not remotely exploitable. The impact is limited to availability: it does not disclose data, change rendered output, or bypass rendering restrictions. Settings such as html_input and allow_unsafe_links do not mitigate the issue because the expensive work occurs before rendering.

Patches

The issue is patched in 2.9.0 and later. Starting in that release, the parser records UTF-8 character-to-byte positions incrementally, converts ordered regular-expression match positions without restarting from the beginning of the line, and matches autolinks against the original line instead of copying every remaining suffix. The affected work then grows in direct proportion to the input size while preserving existing Markdown output and configuration behavior. Versions from 0.6.0 through 2.8.3 are affected. The 0.x and 1.x release lines are no longer supported, so their users must upgrade to 2.9.0 or later.

Workarounds

If you cannot upgrade immediately, reject or truncate inputs with excessively long individual lines before passing them to the converter. A total request-size limit is also useful, but a per-line limit is important because every demonstrated trigger fits on one line. Choose limits appropriate for the application and enforce them before Markdown parsing begins. Restricting conversion to trusted users, applying strict execution-time limits, rate-limiting requests, and limiting concurrent conversions can further reduce exposure, but these measures are not complete substitutes for upgrading.

Disabling AutolinkExtension and avoiding GithubFlavoredMarkdownExtension removes the autolink-specific trigger, but the core multibyte parsing paths remain reachable in the standard parser. Existing nesting, delimiter, raw-HTML, and unsafe-link configuration options do not eliminate all affected paths. Applications that must continue processing untrusted Markdown should therefore enforce input limits even when autolinking is disabled.

Summary

The AttributesExtension's href/src unsafe-link filter (AttributesHelper::filterAttributes()) can be bypassed by embedding control bytes in a javascript: URL that browsers discard before parsing the scheme. Two variants:

• Tab/newline inside the scheme — a literal ASCII TAB (0x09), CR (0x0D), or LF (0x0A), e.g. java<TAB>script:alert(1). Per the WHATWG URL Standard's "basic URL parser" step 3, browsers "remove all ASCII tab or newline from input".
• Leading C0 controls — e.g. <0x01>javascript:alert(1). Per step 1 of the same algorithm, browsers remove any leading or trailing C0 control or space. (A leading space alone does not bypass, because parseAttributes() already trim()s the value; other C0 bytes are not trimmed.)

The filter is a literal anchored-prefix regex (RegexHelper::isLinkPotentiallyUnsafe() / REGEX_UNSAFE_PROTOCOL) that matches neither obfuscated form, so in both cases the browser still executes javascript:alert(1).

This is confirmed reproducible even with allow_unsafe_links => false set — i.e. even applications that have followed the library's own documented hardening guidance for untrusted input remain exploitable.

This is a sibling gap in the same defense that CVE-2025-46734 (GHSA-3527-qv2q-pfvx) fixed in v2.7.0 — that fix made href/src respect allow_unsafe_links, but did not normalize control bytes before checking, so these obfuscation techniques were never covered.

Vulnerability

Files:

• src/Util/RegexHelper.php:69 (REGEX_UNSAFE_PROTOCOL), :239-242 (isLinkPotentiallyUnsafe())
• src/Extension/Attributes/Util/AttributesHelper.php:149-179 (filterAttributes())

CWE: CWE-79 (Improper Neutralization of Input During Web Page Generation / XSS) — primary

• CWE-692 (Incomplete Denylist to Cross-Site Scripting) — the anchored-prefix denylist in REGEX_UNSAFE_PROTOCOL is incomplete. This is a composite of CWE-184 and CWE-79, so it captures the full "incomplete denylist → XSS" chain on its own.
• CWE-86 (Improper Neutralization of Invalid Characters in Identifiers in Web Pages) — the specific evasion technique: control bytes embedded within the URI scheme identifier, which the browser strips before resolving it.

Root Cause

// src/Util/RegexHelper.php
public const REGEX_UNSAFE_PROTOCOL = '/^(?:javascript|vbscript|file|data):/i';

public static function isLinkPotentiallyUnsafe(string $url): bool
{
    return \preg_match(self::REGEX_UNSAFE_PROTOCOL, $url) !== 0 && \preg_match(self::REGEX_SAFE_DATA_PROTOCOL, $url) === 0;
}

// src/Extension/Attributes/Util/AttributesHelper.php
foreach ($attributes as $name => $value) {
    $attrNameLower = \strtolower($name);
    if (! $allowUnsafeLinks && ($attrNameLower === 'href' || $attrNameLower === 'src') && \is_string($value) && RegexHelper::isLinkPotentiallyUnsafe($value)) {
        unset($attributes[$name]);
        continue;
    }
    ...

The Attributes extension's own quote-value grammar (PARTIAL_DOUBLEQUOTEDVALUE = '"[^"]*"') accepts any byte except " inside quotes, including raw tab/CR/LF and other C0 controls, and parseAttributes() only trim()s (leading/trailing, and only the default charlist " \t\n\r\0\x0B" — so a leading \x01 survives). Critically, the core Markdown link-destination path (LinkParserHelper → UrlEncoder::unescapeAndEncode()) percent-encodes every control byte before this same safety check ever runs — but the Attributes extension's href/src handling has no equivalent normalization step, so the raw control byte reaches both the check and the final HTML output (Xml::escape() only escapes & < > " ', not tab/CR/LF, since they're legal bytes inside an HTML attribute).

Attack Scenario

1. An application enables the (commonly-used) AttributesExtension and sets allow_unsafe_links => false — the project's own documented hardening step for untrusted input.
2. An attacker submits Markdown: [Click me](javascript:alert(0)){href="java<TAB>script:alert(document.cookie)"} (TAB is one literal 0x09 byte).
3. The library emits <a href="java<TAB>script:alert(document.cookie)">Click me</a> — isLinkPotentiallyUnsafe() doesn't match the tab-split scheme, so the filter takes no action.
4. A victim viewing/clicking the link has the browser strip the embedded TAB and execute javascript:alert(document.cookie) in the victim's session — stored XSS, cookie theft, account takeover potential.

Why the payload needs an unsafe core destination. Step 2 above deliberately uses [Click me](javascript:alert(0)) rather than a normal link. LinkRenderer overwrites attrs['href'] with the node's own URL unless that URL is itself judged unsafe — so [x](https://example.com){href="java<TAB>script:..."} renders the harmless href="https://example.com", and an empty destination [x](){href="..."} renders href="". The attacker therefore supplies a core destination that the filter does catch, which suppresses the overwrite and lets the attribute-supplied href reach the final tag. This is no obstacle in practice — the attacker writes the entire Markdown document.

Two related forms that are not exploitable, noted so the fix isn't over-scoped:

• Attaching the attribute to a non-link block — hi {href="java<TAB>script:alert(1)"} — does bypass the filter and emits <p href="java<TAB>script:alert(1)">, but href on a <p> is inert: there is nothing to navigate. (An earlier draft of this report described this as a "simpler, unconditional variant" of the attack; it is a filter bypass, not an XSS.)
• <img src> is unaffected, since ImageRenderer unconditionally overwrites src from the core URL regardless of the safety verdict.

Recommended Fix

Normalize inside RegexHelper::isLinkPotentiallyUnsafe() before testing, mirroring the WHATWG URL parser's own normalization. This covers both variants, fixes every call site at once (LinkRenderer, ImageRenderer, and any third-party callers), and needs no changes in the Attributes extension.

Affected Versions

>= 1.5.0, <= 2.8.3 - every release that ships the AttributesExtension. Verified by installing each version and rendering the payloads with allow_unsafe_links => false. The attribute-value grammar (PARTIAL_DOUBLEQUOTEDVALUE = '"[^"]*"') has accepted raw control bytes since the extension was introduced, and none of the intervening parser rewrites narrowed it.

Prior Related Advisories

GHSA-3527-qv2q-pfvx / CVE-2025-46734 fixed a different Attributes-extension XSS (unallowlisted on* handlers, href/src not respecting allow_unsafe_links at all) in v2.7.0. This issue bypasses the specific href/src protection that fix introduced (the control-byte normalization gap was not part of that fix) - but the obfuscated inputs also work on older versions.

Impact

AttributesExtension ships with the library but must be explicitly registered on the Environment; it is not included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register AttributesExtension are not affected by this advisory.

Two paths in the extension re-process every attribute a node has already collected each time another attribute is applied to it. When the attributes carry distinct names, the collected set grows by one on every step and is walked again in full, so a run of n attributes costs O(n²).

1. Attribute nodes resolving to a common target (affected from 1.5.0).

AttributesListener::processDocument() merges each attribute node into the set accumulated for its target, then filters the result. Both operations traverse that entire set: AttributesHelper::mergeAttributes() rebuilds it with array_merge(), and AttributesHelper::filterAttributes() matches a regular expression against every name in it. A run of attribute nodes sharing one target therefore re-walks a set that grows by a key per node.

Two input shapes reach this path: adjacent inline attributes at the start of a block ({a0="v"}{a1="v"}…, where quoting the values is what keeps them separate — an unquoted value swallows the }{ that follows it), and a chain of attribute blocks held at their default target by reference definitions ({a0=v} / [a]: u / {a1=v} / [a]: u / …).

256 KB of adjacent inline attributes takes 20.0 seconds to convert, against 0.09 seconds once patched.

2. Consecutive attribute-block lines (affected from 2.0.0).

AttributesBlockContinueParser::tryContinue() merges each continuation line into the block's accumulated attributes, again rebuilding the whole set on every line. One distinct attribute per line ({a0=v} / {a1=v} / …) grows it by a key each time.

256 KB of such lines takes 1.9 seconds to convert while producing zero bytes of output, against 0.08 seconds once patched.

Relationship to GHSA-jjv6-8j6v-6j52. The fix released in 2.9.1 for that advisory made the class attribute cheap to accumulate, but left every other attribute name on the original path. Applications that upgraded to 2.9.1 or 2.9.2 remain exposed to this variant.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

Patches

The issue is patched in 2.10.0. Both paths now fold each node — or each line — into the accumulated attributes at a cost proportional to that node or line alone, rather than re-merging and re-filtering everything gathered so far. Rendered output is unchanged, down to the order in which attributes appear.

The listener path affects 1.5.0 through 2.9.2. The continuation-line path affects 2.0.0 through 2.9.2. The 1.x release line is no longer supported, so its users must upgrade to 2.10.0 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register AttributesExtension when converting untrusted Markdown. This fully removes both paths.
• If the extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, the cap must be small to meaningfully bound worst-case CPU time.

The attributes/allow allow-list added in 2.7.0 is not a mitigation. A non-empty allow-list happens to bound the first path, because unlisted names are discarded before they accumulate, but it does nothing for the second: continuation lines are merged while parsing, before any filtering takes place.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading.

Impact

Two first-party extensions contain quadratic parsing paths. Both ship with the library but must be explicitly registered on the Environment; neither is included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register SmartPunctExtension or AttributesExtension are not affected by this advisory.

1. SmartPunctExtension — quote replacement recopies the whole text node (affected from 2.0.0).

ReplaceUnpairedQuotesListener converts each unpaired Quote node back to a Text node and merges it into its neighbours via AdjacentTextMerger. The merge reads the left node's literal into a local variable, appends to that variable, and writes it back — and because the read aliases the node's string, every append copies the entire accumulated literal rather than only the bytes added. The listener runs this once per surviving unpaired quote against the same continuously growing text node, so the same buffer is fully re-copied a linear number of times.

A 1.2 MB document of alternating text segments and apostrophes takes 34.9 seconds to convert, against 0.069 seconds for the same input with the extension not registered.

Hardened configuration makes this worse rather than better: QuoteParser appends the Quote node to the AST before pushing it onto the delimiter stack, so max_delimiters_per_line removes the quote-pairing work while leaving every node the listener must process.

2. AttributesExtension — block-level attribute runs re-scan their siblings (affected from 1.5.0).

AttributesListener::findTargetAndDirection() walks the entire remaining sibling chain for every block-level Attributes node whose target is the following node. The backward half of that walk returns immediately for such nodes, and the forward half stops only at a sibling that is not itself an attributes node — which a contiguous run never provides — so a run of k nodes costs k(k-1)/2 steps.

An input placing each {#a} on its own line, with a single reference definition to keep the run contiguous, takes 28.4 seconds at 16,000 attribute blocks while producing zero bytes of output.

This is the block-level counterpart of GHSA-g2gp-3wwq-f4ph, patched in 2.9.0. That fix is incomplete: the early break it introduced is guarded on the node being an AttributesInline, so block-level Attributes nodes still re-scan. Applications that upgraded to 2.9.0 specifically to address GHSA-g2gp-3wwq-f4ph remain exposed to this variant.

3. AttributesExtension — class lists are rebuilt on every merge (affected from 1.5.0).

AttributesHelper::mergeAttributes() round-trips the accumulated class list through explode and implode on each merge. An #id attribute assigns a scalar and skips the branch entirely, but a .class attribute appends to an array which is then imploded to a string, written to the target node, and read back on the next iteration — so the ith merge pays a cost proportional to i three separate times.

{.c} repeated 32,000 times takes 33.5 seconds, against 0.26 seconds for byte-identical input using {#a} — a 130x gap that widens with input size. Both the inline and the block-level attribute paths are affected.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

No library-level configuration gates any of these paths. For the Attributes extension in particular, neither the attributes/allow allow-list nor the on* event-handler hardening added in 2.7.0 has any effect, because the expensive work happens while parsing and resolving the AST, before any attribute filtering or rendering takes place.

Patches

The issues are patched in 2.9.1 and later:

• Adjacent text merging now appends in place instead of reading, modifying, and writing back the whole literal, so a merge costs only the bytes added. This fixes the defect for every caller, not only the SmartPunct listener.
• AttributesListener now records the runs it has already walked, so each contiguous run of block-level attribute nodes is scanned once rather than once per node.
• Accumulated class lists no longer pass through mergeAttributes() repeatedly; the listener holds pending attributes and joins them in a single pass.

The SmartPunct path affects 2.0.0 through 2.9.0. The Attributes paths affect 1.5.0 through 2.9.0, including releases that already contain the 2.9.0 fix for GHSA-g2gp-3wwq-f4ph. The 1.x release line is no longer supported, so its users must upgrade to 2.9.1 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register SmartPunctExtension or AttributesExtension when converting untrusted Markdown. This fully removes the affected paths.
• If either extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU time.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading. Configuration options including attributes/allow, max_delimiters_per_line, max_nesting_level, html_input, and allow_unsafe_links do not mitigate these issues.

Summary

The AttributesExtension documents a security guarantee:

Note: Attributes starting with on (e.g. onclick or onerror) are capable of executing JavaScript code and are therefore never allowed by default. You must explicitly add them to the allow list if you want to use them.

— docs/2.x/extensions/attributes.md

Prefixing the attribute name with a single U+000C FORM FEED byte defeats that guarantee. {<FF>onclick="alert(1)"} passes through AttributesHelper::filterAttributes() untouched and is written verbatim into the output, where browsers parse it as a genuine onclick handler.

The same prefix defeats the allow_unsafe_links check, letting a javascript: URI through on href / src even when allow_unsafe_links is false.

This bypasses the fix shipped in the 2.7.0 security release ("Fix XSS in AttributesExtension", 43207253ea5f14867c77c697cd3838c446cadcea), which added filterAttributes() for the express purpose of blocking these attributes.

Throughout this report <FF> denotes a literal U+000C byte ("\x0C" in PHP). It is invisible in rendered text, so all payloads below are written with PHP escape sequences to stay unambiguous.

Details

Three behaviours combine.

1. \x0C survives the parser's trim().

AttributesHelper::SINGLE_ATTRIBUTE begins with \s*, and Cursor::match() returns $matches[0][0] — the entire match, including that leading whitespace. The result is cleaned with PHP's trim():

// src/Extension/Attributes/Util/AttributesHelper.php:62
while ($attribute = \trim((string) $attributeCursor->match('/^' . self::SINGLE_ATTRIBUTE . '/i'))) {

PCRE \s matches \x0C, but PHP's default trim() charlist is " \t\n\r\0\x0B" — it includes the vertical tab \x0B but not the form feed \x0C. The byte is therefore consumed by the regex, retained in the returned match, and not stripped. It ends up inside the attribute name:

// src/Extension/Attributes/Util/AttributesHelper.php:94
$attributes[\trim($name)] = \trim($value);   // $name === "\x0Conclick"

\x0C is the only byte with this property: every other character the HTML5 tokenizer treats as whitespace (\x09, \x0A, \x0D, \x20), plus \x0B, is in PHP's trim charlist. The PoC includes a \x0B case as a control, and it is correctly stripped.

2. The filter's string comparisons miss it.

filterAttributes() compares the raw name against literal strings:

// src/Extension/Attributes/Util/AttributesHelper.php:148-166
$attrNameLower = \strtolower($name);                            // "\x0conclick"
... ($attrNameLower === 'href' || $attrNameLower === 'src') ... // false
... \str_starts_with($attrNameLower, 'on') ...                  // false -> not removed

3. The renderer never escapes attribute names.

// src/Util/HtmlElement.php:123-129
$result .= ' ' . $key . '="' . Xml::escape($value) . '"';   // $key emitted raw

Because the HTML5 tokenizer treats \x0C as whitespace between attributes, the browser reads the name as plain onclick.

PoC

<?php
require 'vendor/autoload.php';

use League\CommonMark\Environment\Environment;
use League\CommonMark\Extension\Attributes\AttributesExtension;
use League\CommonMark\Extension\CommonMark\CommonMarkCoreExtension;
use League\CommonMark\MarkdownConverter;

// The most defensive configuration docs/2.x/security.md recommends.
$env = new Environment([
    'html_input'         => 'escape',
    'allow_unsafe_links' => false,
    'max_nesting_level'  => 100,
    // 'attributes' => ['allow' => [...]] deliberately left at its default []
]);
$env->addExtension(new CommonMarkCoreExtension());
$env->addExtension(new AttributesExtension());
$converter = new MarkdownConverter($env);

$FF = "\x0C";

echo $converter->convert('hello {onclick="alert(1)"}')->getContent();
// <p>hello</p>                                    <- filtered, as documented

echo $converter->convert('hello {' . $FF . 'onclick="alert(1)"}')->getContent();
// <p \x0Conclick="alert(1)">hello</p>             <- BYPASS

Full observed output (\x0C shown escaped; it is a literal single byte in the real output):

| # | Markdown input | Rendered output | Result | |---|---|---|---| | A | hello {onclick="alert(1)"} | <p>hello</p> | filtered (control) | | B | hello {\x0Conclick="alert(1)"} | <p \x0Conclick="alert(1)">hello</p> | bypass | | C | hello {\x0Bonclick="alert(1)"} | <p>hello</p> | filtered (control) | | D | [click](javascript:alert(1)) | <p><a>click</a></p> | filtered (control) | | E | [click](https://example.com){\x0Chref="javascript:alert(1)"} | <p><a \x0Chref="javascript:alert(1)" href="https://example.com">click</a></p> | bypass | | F | ![x](https://example.invalid/x.png){\x0Conerror="alert(1)"} | <p><img \x0Conerror="alert(1)" src="…" alt="x" /></p> | bypass | | G | # heading + newline + {\x0Conclick="alert(1)"} | <h1 \x0Conclick="alert(1)">heading</h1> | bypass (block syntax) |

In case E the injected href precedes the legitimate one. Per the HTML5 duplicate-attribute rule the first occurrence wins, so the javascript: URI is the one the browser actually uses.

Browser confirmation. Loading the library's unmodified output in Chrome for Testing 148:

<img> attribute names : ["onerror","src","alt"]      <- parsed as a real `onerror`
typeof img.onerror    : function                     <- bound as an event handler
handlers fired        : ["img-onerror"]              <- fired on load, no interaction
document.title        : XSS-FIRED
link href attribute   : "javascript:void(0)"
link href property    : "javascript:void(0)"         <- javascript: URI is the effective href
page errors           : []

The onerror case executes with no user interaction — rendering the attacker's Markdown is sufficient.

Verified against git HEAD (f966b17a) and against tag 2.9.0, on PHP 8.5.8.

Impact

Stored cross-site scripting in any application that renders untrusted Markdown with AttributesExtension enabled and attributes.allow left at its default [] — even when the application has followed every hardening step in docs/2.x/security.md (html_input => 'escape', allow_unsafe_links => false, max_nesting_level => 100).

Consequences are the usual for stored XSS: session and cookie theft, actions performed as the viewing user, and account takeover where the host application permits it. Because the payload can be attached to an image (onerror), it fires on page load without requiring the victim to interact with anything.

The affected configuration is the extension's default: attributes.allow defaults to [], and the documentation describes that default as safe with respect to on* attributes.

Workaround for users

Setting an explicit allow list takes the other branch of filterAttributes(), which drops the form-feed name because it is not in the list:

$config = ['attributes' => ['allow' => ['id', 'class', 'align']]];

Verified: hello {\x0Conclick="alert(1)"} then renders as <p>hello</p>.

Suggested fix

The narrow fix is to add \x0C to the trim charlist at AttributesHelper.php lines 62, 89, 90 and 94. That closes this instance but leaves the shape of the problem in place.

A more durable fix is to reject anything that is not a well-formed attribute name in filterAttributes(), reusing the constant the parser already defines (RegexHelper is already imported in that file):

foreach ($attributes as $name => $value) {
    // Names are compared against literal strings below and emitted without escaping,
    // so anything that isn't a plain attribute name must not get through.
    if (\preg_match('/^' . RegexHelper::PARTIAL_ATTRIBUTENAME . '$/i', $name) !== 1) {
        unset($attributes[$name]);
        continue;
    }

    $attrNameLower = \strtolower($name);
    // ... existing logic unchanged
}

As defence in depth, HtmlElement::__toString() could validate or escape $key. It currently trusts its callers to supply safe attribute names, and filterAttributes() is the only thing standing between that method and user-supplied input.

Impact

Affected versions of league/commonmark perform super-linear work on three independent parsing paths, all of which are reachable on a stock new CommonMarkConverter() with default configuration and no extensions registered. Each trigger fits on a single line of input, so no complex Markdown structure is required.

The three paths were introduced at different times. This advisory's version range is their union; the individual ranges are:

| Path | Affected from | Affected through | |---|---|---| | 1. Fenced code block detection | 0.6.0 | 2.9.0 | | 2. Reference link label lookup | 0.6.0 | 2.9.0 | | 3. Emphasis / strikethrough delimiters (*, _, ~) | 2.6.0 | 2.9.0 | | 3. Highlight delimiters (=) | 2.9.0 | 2.9.0 |

1. Fenced code block detection — quadratic, affected from 0.6.0.

FencedCodeStartParser matches the following pattern:

/^[ \t]*(?:`{3,}(?!.*`)|~{3,})/

The lookahead enforces the CommonMark rule that a backtick fence's info string may not itself contain a backtick, but neither the lookahead nor the backtick run it guards is atomic or possessive. On a line consisting of a long backtick run, filler text, and a single trailing backtick, the quantifier gives back one character at a time and re-runs the lookahead across the remainder of the line on every candidate fence length.

A 320 KB single line takes roughly 27 seconds to convert. The identical payload with one x character prefixed — which fails the parser's own leading-character guard — takes 0.011 seconds. preg_last_error() returns 0 at every input size tested, including runs of 160,000 characters, so PCRE never reaches pcre.backtrack_limit and this is sustained CPU consumption rather than an early bail-out.

2. Reference link label lookup — effectively quadratic, affected from 0.6.0.

When a shortcut or collapsed reference link is attempted, CloseBracketParser::tryParseReference() copies the entire span between the brackets and passes it to ReferenceMap::get(), which normalizes the label — up to four full passes over its length (trim, preg_replace, mb_check_encoding, and strtolower, or mb_convert_case on the non-ASCII path). Nested brackets produce one such lookup per closing bracket, each on a span two characters longer than the last.

In 2.x the normalization sits behind an early return for an empty reference map, so a single 8-byte reference definition anywhere in the document ([x]: y) is enough to unlock the path. At n = 64,000 nested brackets the same input takes 22.0 seconds with that line present versus 0.59 seconds without it. A single non-ASCII character inside the brackets forces the mb_convert_case branch, costing roughly 2.5x more again.

3. Emphasis, strikethrough, and highlight delimiter processing — super-linear, affected from 2.6.0.

DelimiterStack::processDelimiters() remains linear only because of the openersBottom memo, which bounds the backward opener scan — an argument that holds only if the memo's key space is O(1). EmphasisDelimiterProcessor::getCacheKey(), and the equivalents in StrikethroughDelimiterProcessor and MarkDelimiterProcessor, embed the closer's raw current run length in the key, leaving that space unbounded. An attacker spends O(n) bytes minting a growing number of distinct run lengths; each distinct length is a fresh key whose recorded bound starts at zero, forcing a full backward re-scan of the entire pile of openers.

The resulting work grows as roughly n^1.5. This is sub-quadratic, but the amplification over linear growth itself scales with input size, so it worsens as inputs grow: 800 KB of ordinary asterisks, letters, and spaces costs roughly 27 seconds on a stock converter.

This path is a regression introduced in 2.6.0. Before that release the cache key was the bare delimiter character — a bounded key space that amortized correctly. * and _ are affected on any default configuration from 2.6.0 onward. ~ (StrikethroughExtension, included in GithubFlavoredMarkdownConverter and GithubFlavoredMarkdownExtension) is affected from 2.6.0. = (HighlightExtension) is affected only from 2.9.0, when MarkDelimiterProcessor was declared cacheable.

Overall impact. An unauthenticated attacker who can submit Markdown for conversion can use a comparatively small request to consume disproportionate CPU time. Repeated or concurrent requests can occupy all available PHP workers and prevent legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed. Applications that process only trusted Markdown are not remotely exploitable.

Settings such as html_input, allow_unsafe_links, and max_nesting_level do not mitigate any of these, because the expensive work occurs during parsing, before rendering. max_delimiters_per_line bounds the third path only, and does so lossily — it silently discards emphasis once the cap is exhausted.

Patches

The issues are patched in 2.9.1 and later:

• The fenced code block quantifier is now possessive, which is behavior-identical here: any character given back moves a backtick into the lookahead's scan range, so every retry was guaranteed to fail regardless.
• Reference link lookups now apply the CommonMark 999-character link label limit before copying and normalizing the label, matching the limit already enforced when parsing reference definitions. Because a definition can never exceed that length, an over-length lookup label cannot match one directly. One edge case does change: a label longer than 999 characters that collapses to a shorter match once whitespace is normalized — for example [a followed by 998 spaces and b] against a [a b]: /url definition — previously rendered as a link and now renders literally. This follows cmark, which applies its own label-length cap before normalizing (cmark_reference_lookup()), and matches how this library has always handled the equivalent [text][label] form via LinkParserHelper::parseLinkLabel(). commonmark.js normalizes first and still resolves such labels.
• Delimiter processor cache keys now clamp the run length to the coarsest bucket that can change behavior — min(length, 2) for emphasis, min(length, 3) for strikethrough and highlight — restoring a bounded key space while preserving byte-identical output.

Versions from 0.6.0 through 2.9.0 are affected by at least one of these paths; see the table above for which paths apply to which releases. The 0.x and 1.x release lines are no longer supported, so their users must upgrade to 2.9.1 or later.

Workarounds

If you cannot upgrade immediately, enforce a maximum length for individual lines before passing input to the converter, in addition to a total request-size limit. A per-line limit matters because every trigger described above fits within a single line. Because the cost grows super-linearly, the cap must be genuinely small to bound worst-case CPU.

Setting max_delimiters_per_line reduces exposure to the delimiter path only, and does so by silently dropping emphasis from the rendered output. It has no effect on the fenced code or reference link paths.

Restricting conversion to trusted users, applying strict execution-time limits, rate-limiting requests, and limiting concurrent conversions all reduce exposure, but none is a complete substitute for upgrading.

Impact

XmlRenderer pretty-prints XML by emitting depth-proportional indentation whitespace for every opening and closing tag. For a tree of depth n, the indentation alone sums to O(n²) bytes of output (and corresponding memory), reachable through MarkdownToXmlConverter — e.g. str_repeat('> ', $depth) . "x\n", a single line of nested blockquotes — or through a direct XmlRenderer::renderDocument() call on an attacker-influenced AST.

This affects applications that convert untrusted Markdown to XML, which is an opt-in output path. The parser's max_nesting_level bounds the depth of parser-created trees, but its default is high enough to reach damaging sizes, can be raised by the host application, and does not constrain custom or programmatically built ASTs handed straight to the renderer. The result is a memory / output-size amplification rather than a hard crash, which is why this issue is rated Medium rather than High. No confidentiality or integrity impact. XML rendering was introduced in 2.0.0 (first shipped in 2.0.0-beta1, June 2021) and has emitted depth-proportional indentation ever since, so all 2.x releases are affected (verified against 2.8.x, clean upstream 1902f60f). 1.x has no XML renderer and is not affected.

Workarounds

Applications converting untrusted Markdown to XML should:

• Lower max_nesting_level to a conservative value appropriate to expected content, so the parser refuses to build extremely deep trees. This is the most direct lever for parser-produced ASTs, but does not protect trees built programmatically and passed straight to XmlRenderer.
• Cap input size before conversion, since the amplification is driven by input-proportional depth.
• Constrain XML consumers with memory / output-size limits (and streaming or size caps on any downstream XML parser or storage) so one request cannot allocate unbounded output.
• Prefer HTML rendering for untrusted content where XML is not strictly required — the HTML renderer does not emit depth-proportional indentation and is not subject to this amplification.

Impact

UniqueSlugNormalizer::normalize() makes each slug document-unique by searching for an unused numeric suffix, but restarts that search from 1 on every collision. The k-th heading that collapses to the same base slug performs k−1 array lookups, so K colliding slugs cost Σ(k−1) = O(K²). An attacker can force every heading onto a single base slug trivially — many empty ATX headings, identical heading text, or punctuation-only headings that normalize to the empty string.

The path is reached whenever the shared slug normalizer runs over attacker-controlled text. That happens when HeadingPermalinkExtension is registered (its HeadingPermalinkProcessor normalizes every heading), independently through FootnoteExtension (its AnonymousFootnoteRefParser normalizes every ^[label] reference), and on any TableOfContentsExtension site (which requires HeadingPermalinkExtension to be co-registered). The default slug_normalizer/unique setting (UniqueSlugNormalizerInterface::PER_DOCUMENT) accumulates collisions across the whole document. No authentication is required — a small document body turns into seconds of CPU and denies service. Availability impact only. UniqueSlugNormalizer was introduced in 2.0.0 (first shipped in 2.0.0-beta1, May 2021); the 1.x heading-permalink slug generator performed no de-duplication and is not affected. All 2.x releases (including 2.8.x) are affected.

Workarounds

Integrators who cannot upgrade immediately can:

• Set slug_normalizer/unique to false / UniqueSlugNormalizerInterface::DISABLED, which stops the de-duplication scan entirely — at the cost of losing id uniqueness (colliding headings then share an anchor).
• Disable HeadingPermalinkExtension (and TableOfContentsExtension, which depends on it), and FootnoteExtension where anonymous footnotes reach the same normalizer, for untrusted Markdown.
• Cap the accepted document size / heading count upstream so K cannot reach the quadratic danger zone.

Each of these trades off functionality or correctness; upgrading to the patched release (which removes the quadratic behavior while keeping unique ids and identical output) is the recommended remediation.

Impact

The Footnote extension records one backref per footnote reference and then appends the entire backref list for every footnote definition block in the document, without ever de-duplicating or removing repeated definitions of the same label (GatherFootnotesListener, populated by NumberFootnotesListener). A document that references a single label N times and also supplies N duplicate [^a]: definitions of that label therefore produces N × N FootnoteBackref nodes, so output size, parse time, and peak memory are all O(N²).

Reaching the vulnerable path requires FootnoteExtension to be registered on the Environment. This is opt-in, but is a commonly enabled GFM-style feature; no other non-default configuration is required. An unauthenticated attacker can expand a ~10 KB request into a ~62 MB HTML response, ~3 s of CPU, and ~440 MB of peak memory — enough to OOM-kill a default 128 MB PHP worker and deny service. Availability impact only; no confidentiality or integrity effect. The Footnote extension was introduced in 1.5.0 (May 2020) with this backref logic present from the first commit, so all releases from 1.5.0 onward (including every 2.x through 2.8.x) are affected.

Workarounds

There is no library-level option to cap the number of footnotes, references, or definitions, so no configuration switch prevents the amplification. Integrators who cannot upgrade should:

• Disable the Footnote extension for untrusted input, or
• Enforce a strict input-size limit before conversion — but note this is a weak control here, since the ~10 KB payload that already triggers the 62 MB / ~440 MB blowup is well within typical request-body limits, so any cap must be aggressively small to help.

Upgrading to the patched release is the recommended remediation.

Impact

With the Attributes extension enabled, AttributesListener::findTargetAndDirection() resolves each attribute node's target by walking outward through its siblings. For a run of N adjacent inline attribute blocks placed at the start of a block (with nothing to their left), each node scans the entire sibling list to the far-right end before giving up and falling back to the parent. Each resolution is therefore Θ(N) and the whole run is Θ(N²).

Reaching the path requires AttributesExtension (opt-in, but first-party: League\CommonMark\Extension\Attributes\AttributesExtension). No other configuration matters — the quadratic walk runs unconditionally during parsing and is not gated by the attributes/allow allow-list, the on* hardening added in 2.7.0, or allow_unsafe_links. An unauthenticated attacker can submit a ~32 KB input ({#a} repeated 8,000 times) that takes over 5 seconds to convert, with time growing quadratically in input length — a cheap denial of service. Availability impact only. The Attributes extension was introduced in 1.5.0 (May 2020) with this outward-walk resolver present from the first commit, so all releases from 1.5.0 onward (including every 2.x) are affected.

Workarounds

There is no library-level configuration that gates the quadratic walk. Integrators who cannot upgrade can only reduce exposure indirectly:

• Disable the Attributes extension for untrusted input, or
• Impose a strict maximum input length before conversion — noting that because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU.

Upgrading to a release containing the fix is recommended.

Impact

Affected versions of league/commonmark can have quadratic time complexity when parsing specially crafted Markdown lines. In practical terms, doubling the length of an affected line can make the parser perform roughly four times as much work. The parser identifies locations using character positions, but regular-expression matches report byte positions. These positions differ when a UTF-8 character uses more than one byte. Several parsing paths repeatedly rescan growing portions of the line to translate between the two positions. The Autolink extension can also copy and validate the remaining line at every URL-like prefix.

In current 2.x releases, a single non-ASCII character anywhere on a line can place that whole line on the slower multibyte path. An attacker can combine it with a long run of leading whitespace or repeated Markdown punctuation, causing increasingly large rescans. When the Autolink extension is enabled, repeated URL-like prefixes provide another trigger, even on ASCII-only lines. Each trigger fits within one long line, so complex Markdown structure is unnecessary.

An attacker who can submit Markdown for conversion can use a comparatively small request to consume disproportionate CPU time and allocation activity. Repeated or concurrent requests can occupy all available PHP workers and prevent legitimate requests from completing. The core paths affect CommonMarkConverter, GithubFlavoredMarkdownConverter, and custom environments. The autolink-specific path affects applications using AutolinkExtension or GithubFlavoredMarkdownExtension. Applications that process only trusted Markdown are not remotely exploitable. The impact is limited to availability: it does not disclose data, change rendered output, or bypass rendering restrictions. Settings such as html_input and allow_unsafe_links do not mitigate the issue because the expensive work occurs before rendering.

Patches

The issue is patched in 2.9.0 and later. Starting in that release, the parser records UTF-8 character-to-byte positions incrementally, converts ordered regular-expression match positions without restarting from the beginning of the line, and matches autolinks against the original line instead of copying every remaining suffix. The affected work then grows in direct proportion to the input size while preserving existing Markdown output and configuration behavior. Versions from 0.6.0 through 2.8.3 are affected. The 0.x and 1.x release lines are no longer supported, so their users must upgrade to 2.9.0 or later.

Workarounds

If you cannot upgrade immediately, reject or truncate inputs with excessively long individual lines before passing them to the converter. A total request-size limit is also useful, but a per-line limit is important because every demonstrated trigger fits on one line. Choose limits appropriate for the application and enforce them before Markdown parsing begins. Restricting conversion to trusted users, applying strict execution-time limits, rate-limiting requests, and limiting concurrent conversions can further reduce exposure, but these measures are not complete substitutes for upgrading.

Disabling AutolinkExtension and avoiding GithubFlavoredMarkdownExtension removes the autolink-specific trigger, but the core multibyte parsing paths remain reachable in the standard parser. Existing nesting, delimiter, raw-HTML, and unsafe-link configuration options do not eliminate all affected paths. Applications that must continue processing untrusted Markdown should therefore enforce input limits even when autolinking is disabled.

Summary

The AttributesExtension's href/src unsafe-link filter (AttributesHelper::filterAttributes()) can be bypassed by embedding control bytes in a javascript: URL that browsers discard before parsing the scheme. Two variants:

• Tab/newline inside the scheme — a literal ASCII TAB (0x09), CR (0x0D), or LF (0x0A), e.g. java<TAB>script:alert(1). Per the WHATWG URL Standard's "basic URL parser" step 3, browsers "remove all ASCII tab or newline from input".
• Leading C0 controls — e.g. <0x01>javascript:alert(1). Per step 1 of the same algorithm, browsers remove any leading or trailing C0 control or space. (A leading space alone does not bypass, because parseAttributes() already trim()s the value; other C0 bytes are not trimmed.)

The filter is a literal anchored-prefix regex (RegexHelper::isLinkPotentiallyUnsafe() / REGEX_UNSAFE_PROTOCOL) that matches neither obfuscated form, so in both cases the browser still executes javascript:alert(1).

This is confirmed reproducible even with allow_unsafe_links => false set — i.e. even applications that have followed the library's own documented hardening guidance for untrusted input remain exploitable.

This is a sibling gap in the same defense that CVE-2025-46734 (GHSA-3527-qv2q-pfvx) fixed in v2.7.0 — that fix made href/src respect allow_unsafe_links, but did not normalize control bytes before checking, so these obfuscation techniques were never covered.

Vulnerability

Files:

• src/Util/RegexHelper.php:69 (REGEX_UNSAFE_PROTOCOL), :239-242 (isLinkPotentiallyUnsafe())
• src/Extension/Attributes/Util/AttributesHelper.php:149-179 (filterAttributes())

CWE: CWE-79 (Improper Neutralization of Input During Web Page Generation / XSS) — primary

• CWE-692 (Incomplete Denylist to Cross-Site Scripting) — the anchored-prefix denylist in REGEX_UNSAFE_PROTOCOL is incomplete. This is a composite of CWE-184 and CWE-79, so it captures the full "incomplete denylist → XSS" chain on its own.
• CWE-86 (Improper Neutralization of Invalid Characters in Identifiers in Web Pages) — the specific evasion technique: control bytes embedded within the URI scheme identifier, which the browser strips before resolving it.

Root Cause

// src/Util/RegexHelper.php
public const REGEX_UNSAFE_PROTOCOL = '/^(?:javascript|vbscript|file|data):/i';

public static function isLinkPotentiallyUnsafe(string $url): bool
{
    return \preg_match(self::REGEX_UNSAFE_PROTOCOL, $url) !== 0 && \preg_match(self::REGEX_SAFE_DATA_PROTOCOL, $url) === 0;
}

// src/Extension/Attributes/Util/AttributesHelper.php
foreach ($attributes as $name => $value) {
    $attrNameLower = \strtolower($name);
    if (! $allowUnsafeLinks && ($attrNameLower === 'href' || $attrNameLower === 'src') && \is_string($value) && RegexHelper::isLinkPotentiallyUnsafe($value)) {
        unset($attributes[$name]);
        continue;
    }
    ...

The Attributes extension's own quote-value grammar (PARTIAL_DOUBLEQUOTEDVALUE = '"[^"]*"') accepts any byte except " inside quotes, including raw tab/CR/LF and other C0 controls, and parseAttributes() only trim()s (leading/trailing, and only the default charlist " \t\n\r\0\x0B" — so a leading \x01 survives). Critically, the core Markdown link-destination path (LinkParserHelper → UrlEncoder::unescapeAndEncode()) percent-encodes every control byte before this same safety check ever runs — but the Attributes extension's href/src handling has no equivalent normalization step, so the raw control byte reaches both the check and the final HTML output (Xml::escape() only escapes & < > " ', not tab/CR/LF, since they're legal bytes inside an HTML attribute).

Attack Scenario

1. An application enables the (commonly-used) AttributesExtension and sets allow_unsafe_links => false — the project's own documented hardening step for untrusted input.
2. An attacker submits Markdown: [Click me](javascript:alert(0)){href="java<TAB>script:alert(document.cookie)"} (TAB is one literal 0x09 byte).
3. The library emits <a href="java<TAB>script:alert(document.cookie)">Click me</a> — isLinkPotentiallyUnsafe() doesn't match the tab-split scheme, so the filter takes no action.
4. A victim viewing/clicking the link has the browser strip the embedded TAB and execute javascript:alert(document.cookie) in the victim's session — stored XSS, cookie theft, account takeover potential.

Why the payload needs an unsafe core destination. Step 2 above deliberately uses [Click me](javascript:alert(0)) rather than a normal link. LinkRenderer overwrites attrs['href'] with the node's own URL unless that URL is itself judged unsafe — so [x](https://example.com){href="java<TAB>script:..."} renders the harmless href="https://example.com", and an empty destination [x](){href="..."} renders href="". The attacker therefore supplies a core destination that the filter does catch, which suppresses the overwrite and lets the attribute-supplied href reach the final tag. This is no obstacle in practice — the attacker writes the entire Markdown document.

Two related forms that are not exploitable, noted so the fix isn't over-scoped:

• Attaching the attribute to a non-link block — hi {href="java<TAB>script:alert(1)"} — does bypass the filter and emits <p href="java<TAB>script:alert(1)">, but href on a <p> is inert: there is nothing to navigate. (An earlier draft of this report described this as a "simpler, unconditional variant" of the attack; it is a filter bypass, not an XSS.)
• <img src> is unaffected, since ImageRenderer unconditionally overwrites src from the core URL regardless of the safety verdict.

Recommended Fix

Normalize inside RegexHelper::isLinkPotentiallyUnsafe() before testing, mirroring the WHATWG URL parser's own normalization. This covers both variants, fixes every call site at once (LinkRenderer, ImageRenderer, and any third-party callers), and needs no changes in the Attributes extension.

Affected Versions

>= 1.5.0, <= 2.8.3 - every release that ships the AttributesExtension. Verified by installing each version and rendering the payloads with allow_unsafe_links => false. The attribute-value grammar (PARTIAL_DOUBLEQUOTEDVALUE = '"[^"]*"') has accepted raw control bytes since the extension was introduced, and none of the intervening parser rewrites narrowed it.

Prior Related Advisories

GHSA-3527-qv2q-pfvx / CVE-2025-46734 fixed a different Attributes-extension XSS (unallowlisted on* handlers, href/src not respecting allow_unsafe_links at all) in v2.7.0. This issue bypasses the specific href/src protection that fix introduced (the control-byte normalization gap was not part of that fix) - but the obfuscated inputs also work on older versions.

Impact

AttributesExtension ships with the library but must be explicitly registered on the Environment; it is not included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register AttributesExtension are not affected by this advisory.

Two paths in the extension re-process every attribute a node has already collected each time another attribute is applied to it. When the attributes carry distinct names, the collected set grows by one on every step and is walked again in full, so a run of n attributes costs O(n²).

1. Attribute nodes resolving to a common target (affected from 1.5.0).

AttributesListener::processDocument() merges each attribute node into the set accumulated for its target, then filters the result. Both operations traverse that entire set: AttributesHelper::mergeAttributes() rebuilds it with array_merge(), and AttributesHelper::filterAttributes() matches a regular expression against every name in it. A run of attribute nodes sharing one target therefore re-walks a set that grows by a key per node.

Two input shapes reach this path: adjacent inline attributes at the start of a block ({a0="v"}{a1="v"}…, where quoting the values is what keeps them separate — an unquoted value swallows the }{ that follows it), and a chain of attribute blocks held at their default target by reference definitions ({a0=v} / [a]: u / {a1=v} / [a]: u / …).

256 KB of adjacent inline attributes takes 20.0 seconds to convert, against 0.09 seconds once patched.

2. Consecutive attribute-block lines (affected from 2.0.0).

AttributesBlockContinueParser::tryContinue() merges each continuation line into the block's accumulated attributes, again rebuilding the whole set on every line. One distinct attribute per line ({a0=v} / {a1=v} / …) grows it by a key each time.

256 KB of such lines takes 1.9 seconds to convert while producing zero bytes of output, against 0.08 seconds once patched.

Relationship to GHSA-jjv6-8j6v-6j52. The fix released in 2.9.1 for that advisory made the class attribute cheap to accumulate, but left every other attribute name on the original path. Applications that upgraded to 2.9.1 or 2.9.2 remain exposed to this variant.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

Patches

The issue is patched in 2.10.0. Both paths now fold each node — or each line — into the accumulated attributes at a cost proportional to that node or line alone, rather than re-merging and re-filtering everything gathered so far. Rendered output is unchanged, down to the order in which attributes appear.

The listener path affects 1.5.0 through 2.9.2. The continuation-line path affects 2.0.0 through 2.9.2. The 1.x release line is no longer supported, so its users must upgrade to 2.10.0 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register AttributesExtension when converting untrusted Markdown. This fully removes both paths.
• If the extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, the cap must be small to meaningfully bound worst-case CPU time.

The attributes/allow allow-list added in 2.7.0 is not a mitigation. A non-empty allow-list happens to bound the first path, because unlisted names are discarded before they accumulate, but it does nothing for the second: continuation lines are merged while parsing, before any filtering takes place.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading.

Impact

Two first-party extensions contain quadratic parsing paths. Both ship with the library but must be explicitly registered on the Environment; neither is included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register SmartPunctExtension or AttributesExtension are not affected by this advisory.

1. SmartPunctExtension — quote replacement recopies the whole text node (affected from 2.0.0).

ReplaceUnpairedQuotesListener converts each unpaired Quote node back to a Text node and merges it into its neighbours via AdjacentTextMerger. The merge reads the left node's literal into a local variable, appends to that variable, and writes it back — and because the read aliases the node's string, every append copies the entire accumulated literal rather than only the bytes added. The listener runs this once per surviving unpaired quote against the same continuously growing text node, so the same buffer is fully re-copied a linear number of times.

A 1.2 MB document of alternating text segments and apostrophes takes 34.9 seconds to convert, against 0.069 seconds for the same input with the extension not registered.

Hardened configuration makes this worse rather than better: QuoteParser appends the Quote node to the AST before pushing it onto the delimiter stack, so max_delimiters_per_line removes the quote-pairing work while leaving every node the listener must process.

2. AttributesExtension — block-level attribute runs re-scan their siblings (affected from 1.5.0).

AttributesListener::findTargetAndDirection() walks the entire remaining sibling chain for every block-level Attributes node whose target is the following node. The backward half of that walk returns immediately for such nodes, and the forward half stops only at a sibling that is not itself an attributes node — which a contiguous run never provides — so a run of k nodes costs k(k-1)/2 steps.

An input placing each {#a} on its own line, with a single reference definition to keep the run contiguous, takes 28.4 seconds at 16,000 attribute blocks while producing zero bytes of output.

This is the block-level counterpart of GHSA-g2gp-3wwq-f4ph, patched in 2.9.0. That fix is incomplete: the early break it introduced is guarded on the node being an AttributesInline, so block-level Attributes nodes still re-scan. Applications that upgraded to 2.9.0 specifically to address GHSA-g2gp-3wwq-f4ph remain exposed to this variant.

3. AttributesExtension — class lists are rebuilt on every merge (affected from 1.5.0).

AttributesHelper::mergeAttributes() round-trips the accumulated class list through explode and implode on each merge. An #id attribute assigns a scalar and skips the branch entirely, but a .class attribute appends to an array which is then imploded to a string, written to the target node, and read back on the next iteration — so the ith merge pays a cost proportional to i three separate times.

{.c} repeated 32,000 times takes 33.5 seconds, against 0.26 seconds for byte-identical input using {#a} — a 130x gap that widens with input size. Both the inline and the block-level attribute paths are affected.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

No library-level configuration gates any of these paths. For the Attributes extension in particular, neither the attributes/allow allow-list nor the on* event-handler hardening added in 2.7.0 has any effect, because the expensive work happens while parsing and resolving the AST, before any attribute filtering or rendering takes place.

Patches

The issues are patched in 2.9.1 and later:

• Adjacent text merging now appends in place instead of reading, modifying, and writing back the whole literal, so a merge costs only the bytes added. This fixes the defect for every caller, not only the SmartPunct listener.
• AttributesListener now records the runs it has already walked, so each contiguous run of block-level attribute nodes is scanned once rather than once per node.
• Accumulated class lists no longer pass through mergeAttributes() repeatedly; the listener holds pending attributes and joins them in a single pass.

The SmartPunct path affects 2.0.0 through 2.9.0. The Attributes paths affect 1.5.0 through 2.9.0, including releases that already contain the 2.9.0 fix for GHSA-g2gp-3wwq-f4ph. The 1.x release line is no longer supported, so its users must upgrade to 2.9.1 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register SmartPunctExtension or AttributesExtension when converting untrusted Markdown. This fully removes the affected paths.
• If either extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU time.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading. Configuration options including attributes/allow, max_delimiters_per_line, max_nesting_level, html_input, and allow_unsafe_links do not mitigate these issues.

Summary

The AttributesExtension documents a security guarantee:

Note: Attributes starting with on (e.g. onclick or onerror) are capable of executing JavaScript code and are therefore never allowed by default. You must explicitly add them to the allow list if you want to use them.

— docs/2.x/extensions/attributes.md

Prefixing the attribute name with a single U+000C FORM FEED byte defeats that guarantee. {<FF>onclick="alert(1)"} passes through AttributesHelper::filterAttributes() untouched and is written verbatim into the output, where browsers parse it as a genuine onclick handler.

The same prefix defeats the allow_unsafe_links check, letting a javascript: URI through on href / src even when allow_unsafe_links is false.

This bypasses the fix shipped in the 2.7.0 security release ("Fix XSS in AttributesExtension", 43207253ea5f14867c77c697cd3838c446cadcea), which added filterAttributes() for the express purpose of blocking these attributes.

Throughout this report <FF> denotes a literal U+000C byte ("\x0C" in PHP). It is invisible in rendered text, so all payloads below are written with PHP escape sequences to stay unambiguous.

Details

Three behaviours combine.

1. \x0C survives the parser's trim().

AttributesHelper::SINGLE_ATTRIBUTE begins with \s*, and Cursor::match() returns $matches[0][0] — the entire match, including that leading whitespace. The result is cleaned with PHP's trim():

// src/Extension/Attributes/Util/AttributesHelper.php:62
while ($attribute = \trim((string) $attributeCursor->match('/^' . self::SINGLE_ATTRIBUTE . '/i'))) {

PCRE \s matches \x0C, but PHP's default trim() charlist is " \t\n\r\0\x0B" — it includes the vertical tab \x0B but not the form feed \x0C. The byte is therefore consumed by the regex, retained in the returned match, and not stripped. It ends up inside the attribute name:

// src/Extension/Attributes/Util/AttributesHelper.php:94
$attributes[\trim($name)] = \trim($value);   // $name === "\x0Conclick"

\x0C is the only byte with this property: every other character the HTML5 tokenizer treats as whitespace (\x09, \x0A, \x0D, \x20), plus \x0B, is in PHP's trim charlist. The PoC includes a \x0B case as a control, and it is correctly stripped.

2. The filter's string comparisons miss it.

filterAttributes() compares the raw name against literal strings:

// src/Extension/Attributes/Util/AttributesHelper.php:148-166
$attrNameLower = \strtolower($name);                            // "\x0conclick"
... ($attrNameLower === 'href' || $attrNameLower === 'src') ... // false
... \str_starts_with($attrNameLower, 'on') ...                  // false -> not removed

3. The renderer never escapes attribute names.

// src/Util/HtmlElement.php:123-129
$result .= ' ' . $key . '="' . Xml::escape($value) . '"';   // $key emitted raw

Because the HTML5 tokenizer treats \x0C as whitespace between attributes, the browser reads the name as plain onclick.

PoC

<?php
require 'vendor/autoload.php';

use League\CommonMark\Environment\Environment;
use League\CommonMark\Extension\Attributes\AttributesExtension;
use League\CommonMark\Extension\CommonMark\CommonMarkCoreExtension;
use League\CommonMark\MarkdownConverter;

// The most defensive configuration docs/2.x/security.md recommends.
$env = new Environment([
    'html_input'         => 'escape',
    'allow_unsafe_links' => false,
    'max_nesting_level'  => 100,
    // 'attributes' => ['allow' => [...]] deliberately left at its default []
]);
$env->addExtension(new CommonMarkCoreExtension());
$env->addExtension(new AttributesExtension());
$converter = new MarkdownConverter($env);

$FF = "\x0C";

echo $converter->convert('hello {onclick="alert(1)"}')->getContent();
// <p>hello</p>                                    <- filtered, as documented

echo $converter->convert('hello {' . $FF . 'onclick="alert(1)"}')->getContent();
// <p \x0Conclick="alert(1)">hello</p>             <- BYPASS

Full observed output (\x0C shown escaped; it is a literal single byte in the real output):

| # | Markdown input | Rendered output | Result | |---|---|---|---| | A | hello {onclick="alert(1)"} | <p>hello</p> | filtered (control) | | B | hello {\x0Conclick="alert(1)"} | <p \x0Conclick="alert(1)">hello</p> | bypass | | C | hello {\x0Bonclick="alert(1)"} | <p>hello</p> | filtered (control) | | D | [click](javascript:alert(1)) | <p><a>click</a></p> | filtered (control) | | E | [click](https://example.com){\x0Chref="javascript:alert(1)"} | <p><a \x0Chref="javascript:alert(1)" href="https://example.com">click</a></p> | bypass | | F | ![x](https://example.invalid/x.png){\x0Conerror="alert(1)"} | <p><img \x0Conerror="alert(1)" src="…" alt="x" /></p> | bypass | | G | # heading + newline + {\x0Conclick="alert(1)"} | <h1 \x0Conclick="alert(1)">heading</h1> | bypass (block syntax) |

In case E the injected href precedes the legitimate one. Per the HTML5 duplicate-attribute rule the first occurrence wins, so the javascript: URI is the one the browser actually uses.

Browser confirmation. Loading the library's unmodified output in Chrome for Testing 148:

<img> attribute names : ["onerror","src","alt"]      <- parsed as a real `onerror`
typeof img.onerror    : function                     <- bound as an event handler
handlers fired        : ["img-onerror"]              <- fired on load, no interaction
document.title        : XSS-FIRED
link href attribute   : "javascript:void(0)"
link href property    : "javascript:void(0)"         <- javascript: URI is the effective href
page errors           : []

The onerror case executes with no user interaction — rendering the attacker's Markdown is sufficient.

Verified against git HEAD (f966b17a) and against tag 2.9.0, on PHP 8.5.8.

Impact

Stored cross-site scripting in any application that renders untrusted Markdown with AttributesExtension enabled and attributes.allow left at its default [] — even when the application has followed every hardening step in docs/2.x/security.md (html_input => 'escape', allow_unsafe_links => false, max_nesting_level => 100).

Consequences are the usual for stored XSS: session and cookie theft, actions performed as the viewing user, and account takeover where the host application permits it. Because the payload can be attached to an image (onerror), it fires on page load without requiring the victim to interact with anything.

The affected configuration is the extension's default: attributes.allow defaults to [], and the documentation describes that default as safe with respect to on* attributes.

Workaround for users

Setting an explicit allow list takes the other branch of filterAttributes(), which drops the form-feed name because it is not in the list:

$config = ['attributes' => ['allow' => ['id', 'class', 'align']]];

Verified: hello {\x0Conclick="alert(1)"} then renders as <p>hello</p>.

Suggested fix

The narrow fix is to add \x0C to the trim charlist at AttributesHelper.php lines 62, 89, 90 and 94. That closes this instance but leaves the shape of the problem in place.

A more durable fix is to reject anything that is not a well-formed attribute name in filterAttributes(), reusing the constant the parser already defines (RegexHelper is already imported in that file):

foreach ($attributes as $name => $value) {
    // Names are compared against literal strings below and emitted without escaping,
    // so anything that isn't a plain attribute name must not get through.
    if (\preg_match('/^' . RegexHelper::PARTIAL_ATTRIBUTENAME . '$/i', $name) !== 1) {
        unset($attributes[$name]);
        continue;
    }

    $attrNameLower = \strtolower($name);
    // ... existing logic unchanged
}

As defence in depth, HtmlElement::__toString() could validate or escape $key. It currently trusts its callers to supply safe attribute names, and filterAttributes() is the only thing standing between that method and user-supplied input.

Impact

Affected versions of league/commonmark perform super-linear work on three independent parsing paths, all of which are reachable on a stock new CommonMarkConverter() with default configuration and no extensions registered. Each trigger fits on a single line of input, so no complex Markdown structure is required.

The three paths were introduced at different times. This advisory's version range is their union; the individual ranges are:

| Path | Affected from | Affected through | |---|---|---| | 1. Fenced code block detection | 0.6.0 | 2.9.0 | | 2. Reference link label lookup | 0.6.0 | 2.9.0 | | 3. Emphasis / strikethrough delimiters (*, _, ~) | 2.6.0 | 2.9.0 | | 3. Highlight delimiters (=) | 2.9.0 | 2.9.0 |

1. Fenced code block detection — quadratic, affected from 0.6.0.

FencedCodeStartParser matches the following pattern:

/^[ \t]*(?:`{3,}(?!.*`)|~{3,})/

The lookahead enforces the CommonMark rule that a backtick fence's info string may not itself contain a backtick, but neither the lookahead nor the backtick run it guards is atomic or possessive. On a line consisting of a long backtick run, filler text, and a single trailing backtick, the quantifier gives back one character at a time and re-runs the lookahead across the remainder of the line on every candidate fence length.

A 320 KB single line takes roughly 27 seconds to convert. The identical payload with one x character prefixed — which fails the parser's own leading-character guard — takes 0.011 seconds. preg_last_error() returns 0 at every input size tested, including runs of 160,000 characters, so PCRE never reaches pcre.backtrack_limit and this is sustained CPU consumption rather than an early bail-out.

2. Reference link label lookup — effectively quadratic, affected from 0.6.0.

When a shortcut or collapsed reference link is attempted, CloseBracketParser::tryParseReference() copies the entire span between the brackets and passes it to ReferenceMap::get(), which normalizes the label — up to four full passes over its length (trim, preg_replace, mb_check_encoding, and strtolower, or mb_convert_case on the non-ASCII path). Nested brackets produce one such lookup per closing bracket, each on a span two characters longer than the last.

In 2.x the normalization sits behind an early return for an empty reference map, so a single 8-byte reference definition anywhere in the document ([x]: y) is enough to unlock the path. At n = 64,000 nested brackets the same input takes 22.0 seconds with that line present versus 0.59 seconds without it. A single non-ASCII character inside the brackets forces the mb_convert_case branch, costing roughly 2.5x more again.

3. Emphasis, strikethrough, and highlight delimiter processing — super-linear, affected from 2.6.0.

DelimiterStack::processDelimiters() remains linear only because of the openersBottom memo, which bounds the backward opener scan — an argument that holds only if the memo's key space is O(1). EmphasisDelimiterProcessor::getCacheKey(), and the equivalents in StrikethroughDelimiterProcessor and MarkDelimiterProcessor, embed the closer's raw current run length in the key, leaving that space unbounded. An attacker spends O(n) bytes minting a growing number of distinct run lengths; each distinct length is a fresh key whose recorded bound starts at zero, forcing a full backward re-scan of the entire pile of openers.

The resulting work grows as roughly n^1.5. This is sub-quadratic, but the amplification over linear growth itself scales with input size, so it worsens as inputs grow: 800 KB of ordinary asterisks, letters, and spaces costs roughly 27 seconds on a stock converter.

This path is a regression introduced in 2.6.0. Before that release the cache key was the bare delimiter character — a bounded key space that amortized correctly. * and _ are affected on any default configuration from 2.6.0 onward. ~ (StrikethroughExtension, included in GithubFlavoredMarkdownConverter and GithubFlavoredMarkdownExtension) is affected from 2.6.0. = (HighlightExtension) is affected only from 2.9.0, when MarkDelimiterProcessor was declared cacheable.

Overall impact. An unauthenticated attacker who can submit Markdown for conversion can use a comparatively small request to consume disproportionate CPU time. Repeated or concurrent requests can occupy all available PHP workers and prevent legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed. Applications that process only trusted Markdown are not remotely exploitable.

Settings such as html_input, allow_unsafe_links, and max_nesting_level do not mitigate any of these, because the expensive work occurs during parsing, before rendering. max_delimiters_per_line bounds the third path only, and does so lossily — it silently discards emphasis once the cap is exhausted.

Patches

The issues are patched in 2.9.1 and later:

• The fenced code block quantifier is now possessive, which is behavior-identical here: any character given back moves a backtick into the lookahead's scan range, so every retry was guaranteed to fail regardless.
• Reference link lookups now apply the CommonMark 999-character link label limit before copying and normalizing the label, matching the limit already enforced when parsing reference definitions. Because a definition can never exceed that length, an over-length lookup label cannot match one directly. One edge case does change: a label longer than 999 characters that collapses to a shorter match once whitespace is normalized — for example [a followed by 998 spaces and b] against a [a b]: /url definition — previously rendered as a link and now renders literally. This follows cmark, which applies its own label-length cap before normalizing (cmark_reference_lookup()), and matches how this library has always handled the equivalent [text][label] form via LinkParserHelper::parseLinkLabel(). commonmark.js normalizes first and still resolves such labels.
• Delimiter processor cache keys now clamp the run length to the coarsest bucket that can change behavior — min(length, 2) for emphasis, min(length, 3) for strikethrough and highlight — restoring a bounded key space while preserving byte-identical output.

Versions from 0.6.0 through 2.9.0 are affected by at least one of these paths; see the table above for which paths apply to which releases. The 0.x and 1.x release lines are no longer supported, so their users must upgrade to 2.9.1 or later.

Workarounds

If you cannot upgrade immediately, enforce a maximum length for individual lines before passing input to the converter, in addition to a total request-size limit. A per-line limit matters because every trigger described above fits within a single line. Because the cost grows super-linearly, the cap must be genuinely small to bound worst-case CPU.

Setting max_delimiters_per_line reduces exposure to the delimiter path only, and does so by silently dropping emphasis from the rendered output. It has no effect on the fenced code or reference link paths.

Restricting conversion to trusted users, applying strict execution-time limits, rate-limiting requests, and limiting concurrent conversions all reduce exposure, but none is a complete substitute for upgrading.

Impact

XmlRenderer pretty-prints XML by emitting depth-proportional indentation whitespace for every opening and closing tag. For a tree of depth n, the indentation alone sums to O(n²) bytes of output (and corresponding memory), reachable through MarkdownToXmlConverter — e.g. str_repeat('> ', $depth) . "x\n", a single line of nested blockquotes — or through a direct XmlRenderer::renderDocument() call on an attacker-influenced AST.

This affects applications that convert untrusted Markdown to XML, which is an opt-in output path. The parser's max_nesting_level bounds the depth of parser-created trees, but its default is high enough to reach damaging sizes, can be raised by the host application, and does not constrain custom or programmatically built ASTs handed straight to the renderer. The result is a memory / output-size amplification rather than a hard crash, which is why this issue is rated Medium rather than High. No confidentiality or integrity impact. XML rendering was introduced in 2.0.0 (first shipped in 2.0.0-beta1, June 2021) and has emitted depth-proportional indentation ever since, so all 2.x releases are affected (verified against 2.8.x, clean upstream 1902f60f). 1.x has no XML renderer and is not affected.

Workarounds

Applications converting untrusted Markdown to XML should:

• Lower max_nesting_level to a conservative value appropriate to expected content, so the parser refuses to build extremely deep trees. This is the most direct lever for parser-produced ASTs, but does not protect trees built programmatically and passed straight to XmlRenderer.
• Cap input size before conversion, since the amplification is driven by input-proportional depth.
• Constrain XML consumers with memory / output-size limits (and streaming or size caps on any downstream XML parser or storage) so one request cannot allocate unbounded output.
• Prefer HTML rendering for untrusted content where XML is not strictly required — the HTML renderer does not emit depth-proportional indentation and is not subject to this amplification.

Impact

UniqueSlugNormalizer::normalize() makes each slug document-unique by searching for an unused numeric suffix, but restarts that search from 1 on every collision. The k-th heading that collapses to the same base slug performs k−1 array lookups, so K colliding slugs cost Σ(k−1) = O(K²). An attacker can force every heading onto a single base slug trivially — many empty ATX headings, identical heading text, or punctuation-only headings that normalize to the empty string.

The path is reached whenever the shared slug normalizer runs over attacker-controlled text. That happens when HeadingPermalinkExtension is registered (its HeadingPermalinkProcessor normalizes every heading), independently through FootnoteExtension (its AnonymousFootnoteRefParser normalizes every ^[label] reference), and on any TableOfContentsExtension site (which requires HeadingPermalinkExtension to be co-registered). The default slug_normalizer/unique setting (UniqueSlugNormalizerInterface::PER_DOCUMENT) accumulates collisions across the whole document. No authentication is required — a small document body turns into seconds of CPU and denies service. Availability impact only. UniqueSlugNormalizer was introduced in 2.0.0 (first shipped in 2.0.0-beta1, May 2021); the 1.x heading-permalink slug generator performed no de-duplication and is not affected. All 2.x releases (including 2.8.x) are affected.

Workarounds

Integrators who cannot upgrade immediately can:

• Set slug_normalizer/unique to false / UniqueSlugNormalizerInterface::DISABLED, which stops the de-duplication scan entirely — at the cost of losing id uniqueness (colliding headings then share an anchor).
• Disable HeadingPermalinkExtension (and TableOfContentsExtension, which depends on it), and FootnoteExtension where anonymous footnotes reach the same normalizer, for untrusted Markdown.
• Cap the accepted document size / heading count upstream so K cannot reach the quadratic danger zone.

Each of these trades off functionality or correctness; upgrading to the patched release (which removes the quadratic behavior while keeping unique ids and identical output) is the recommended remediation.

Impact

The Footnote extension records one backref per footnote reference and then appends the entire backref list for every footnote definition block in the document, without ever de-duplicating or removing repeated definitions of the same label (GatherFootnotesListener, populated by NumberFootnotesListener). A document that references a single label N times and also supplies N duplicate [^a]: definitions of that label therefore produces N × N FootnoteBackref nodes, so output size, parse time, and peak memory are all O(N²).

Reaching the vulnerable path requires FootnoteExtension to be registered on the Environment. This is opt-in, but is a commonly enabled GFM-style feature; no other non-default configuration is required. An unauthenticated attacker can expand a ~10 KB request into a ~62 MB HTML response, ~3 s of CPU, and ~440 MB of peak memory — enough to OOM-kill a default 128 MB PHP worker and deny service. Availability impact only; no confidentiality or integrity effect. The Footnote extension was introduced in 1.5.0 (May 2020) with this backref logic present from the first commit, so all releases from 1.5.0 onward (including every 2.x through 2.8.x) are affected.

Workarounds

There is no library-level option to cap the number of footnotes, references, or definitions, so no configuration switch prevents the amplification. Integrators who cannot upgrade should:

• Disable the Footnote extension for untrusted input, or
• Enforce a strict input-size limit before conversion — but note this is a weak control here, since the ~10 KB payload that already triggers the 62 MB / ~440 MB blowup is well within typical request-body limits, so any cap must be aggressively small to help.

Upgrading to the patched release is the recommended remediation.

Impact

With the Attributes extension enabled, AttributesListener::findTargetAndDirection() resolves each attribute node's target by walking outward through its siblings. For a run of N adjacent inline attribute blocks placed at the start of a block (with nothing to their left), each node scans the entire sibling list to the far-right end before giving up and falling back to the parent. Each resolution is therefore Θ(N) and the whole run is Θ(N²).

Reaching the path requires AttributesExtension (opt-in, but first-party: League\CommonMark\Extension\Attributes\AttributesExtension). No other configuration matters — the quadratic walk runs unconditionally during parsing and is not gated by the attributes/allow allow-list, the on* hardening added in 2.7.0, or allow_unsafe_links. An unauthenticated attacker can submit a ~32 KB input ({#a} repeated 8,000 times) that takes over 5 seconds to convert, with time growing quadratically in input length — a cheap denial of service. Availability impact only. The Attributes extension was introduced in 1.5.0 (May 2020) with this outward-walk resolver present from the first commit, so all releases from 1.5.0 onward (including every 2.x) are affected.

Workarounds

There is no library-level configuration that gates the quadratic walk. Integrators who cannot upgrade can only reduce exposure indirectly:

• Disable the Attributes extension for untrusted input, or
• Impose a strict maximum input length before conversion — noting that because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU.

Upgrading to a release containing the fix is recommended.

Impact

Affected versions of league/commonmark can have quadratic time complexity when parsing specially crafted Markdown lines. In practical terms, doubling the length of an affected line can make the parser perform roughly four times as much work. The parser identifies locations using character positions, but regular-expression matches report byte positions. These positions differ when a UTF-8 character uses more than one byte. Several parsing paths repeatedly rescan growing portions of the line to translate between the two positions. The Autolink extension can also copy and validate the remaining line at every URL-like prefix.

In current 2.x releases, a single non-ASCII character anywhere on a line can place that whole line on the slower multibyte path. An attacker can combine it with a long run of leading whitespace or repeated Markdown punctuation, causing increasingly large rescans. When the Autolink extension is enabled, repeated URL-like prefixes provide another trigger, even on ASCII-only lines. Each trigger fits within one long line, so complex Markdown structure is unnecessary.

An attacker who can submit Markdown for conversion can use a comparatively small request to consume disproportionate CPU time and allocation activity. Repeated or concurrent requests can occupy all available PHP workers and prevent legitimate requests from completing. The core paths affect CommonMarkConverter, GithubFlavoredMarkdownConverter, and custom environments. The autolink-specific path affects applications using AutolinkExtension or GithubFlavoredMarkdownExtension. Applications that process only trusted Markdown are not remotely exploitable. The impact is limited to availability: it does not disclose data, change rendered output, or bypass rendering restrictions. Settings such as html_input and allow_unsafe_links do not mitigate the issue because the expensive work occurs before rendering.

Patches

The issue is patched in 2.9.0 and later. Starting in that release, the parser records UTF-8 character-to-byte positions incrementally, converts ordered regular-expression match positions without restarting from the beginning of the line, and matches autolinks against the original line instead of copying every remaining suffix. The affected work then grows in direct proportion to the input size while preserving existing Markdown output and configuration behavior. Versions from 0.6.0 through 2.8.3 are affected. The 0.x and 1.x release lines are no longer supported, so their users must upgrade to 2.9.0 or later.

Workarounds

If you cannot upgrade immediately, reject or truncate inputs with excessively long individual lines before passing them to the converter. A total request-size limit is also useful, but a per-line limit is important because every demonstrated trigger fits on one line. Choose limits appropriate for the application and enforce them before Markdown parsing begins. Restricting conversion to trusted users, applying strict execution-time limits, rate-limiting requests, and limiting concurrent conversions can further reduce exposure, but these measures are not complete substitutes for upgrading.

Disabling AutolinkExtension and avoiding GithubFlavoredMarkdownExtension removes the autolink-specific trigger, but the core multibyte parsing paths remain reachable in the standard parser. Existing nesting, delimiter, raw-HTML, and unsafe-link configuration options do not eliminate all affected paths. Applications that must continue processing untrusted Markdown should therefore enforce input limits even when autolinking is disabled.

Summary

The AttributesExtension's href/src unsafe-link filter (AttributesHelper::filterAttributes()) can be bypassed by embedding control bytes in a javascript: URL that browsers discard before parsing the scheme. Two variants:

• Tab/newline inside the scheme — a literal ASCII TAB (0x09), CR (0x0D), or LF (0x0A), e.g. java<TAB>script:alert(1). Per the WHATWG URL Standard's "basic URL parser" step 3, browsers "remove all ASCII tab or newline from input".
• Leading C0 controls — e.g. <0x01>javascript:alert(1). Per step 1 of the same algorithm, browsers remove any leading or trailing C0 control or space. (A leading space alone does not bypass, because parseAttributes() already trim()s the value; other C0 bytes are not trimmed.)

The filter is a literal anchored-prefix regex (RegexHelper::isLinkPotentiallyUnsafe() / REGEX_UNSAFE_PROTOCOL) that matches neither obfuscated form, so in both cases the browser still executes javascript:alert(1).

This is confirmed reproducible even with allow_unsafe_links => false set — i.e. even applications that have followed the library's own documented hardening guidance for untrusted input remain exploitable.

This is a sibling gap in the same defense that CVE-2025-46734 (GHSA-3527-qv2q-pfvx) fixed in v2.7.0 — that fix made href/src respect allow_unsafe_links, but did not normalize control bytes before checking, so these obfuscation techniques were never covered.

Vulnerability

Files:

• src/Util/RegexHelper.php:69 (REGEX_UNSAFE_PROTOCOL), :239-242 (isLinkPotentiallyUnsafe())
• src/Extension/Attributes/Util/AttributesHelper.php:149-179 (filterAttributes())

CWE: CWE-79 (Improper Neutralization of Input During Web Page Generation / XSS) — primary

• CWE-692 (Incomplete Denylist to Cross-Site Scripting) — the anchored-prefix denylist in REGEX_UNSAFE_PROTOCOL is incomplete. This is a composite of CWE-184 and CWE-79, so it captures the full "incomplete denylist → XSS" chain on its own.
• CWE-86 (Improper Neutralization of Invalid Characters in Identifiers in Web Pages) — the specific evasion technique: control bytes embedded within the URI scheme identifier, which the browser strips before resolving it.

Root Cause

// src/Util/RegexHelper.php
public const REGEX_UNSAFE_PROTOCOL = '/^(?:javascript|vbscript|file|data):/i';

public static function isLinkPotentiallyUnsafe(string $url): bool
{
    return \preg_match(self::REGEX_UNSAFE_PROTOCOL, $url) !== 0 && \preg_match(self::REGEX_SAFE_DATA_PROTOCOL, $url) === 0;
}

// src/Extension/Attributes/Util/AttributesHelper.php
foreach ($attributes as $name => $value) {
    $attrNameLower = \strtolower($name);
    if (! $allowUnsafeLinks && ($attrNameLower === 'href' || $attrNameLower === 'src') && \is_string($value) && RegexHelper::isLinkPotentiallyUnsafe($value)) {
        unset($attributes[$name]);
        continue;
    }
    ...

The Attributes extension's own quote-value grammar (PARTIAL_DOUBLEQUOTEDVALUE = '"[^"]*"') accepts any byte except " inside quotes, including raw tab/CR/LF and other C0 controls, and parseAttributes() only trim()s (leading/trailing, and only the default charlist " \t\n\r\0\x0B" — so a leading \x01 survives). Critically, the core Markdown link-destination path (LinkParserHelper → UrlEncoder::unescapeAndEncode()) percent-encodes every control byte before this same safety check ever runs — but the Attributes extension's href/src handling has no equivalent normalization step, so the raw control byte reaches both the check and the final HTML output (Xml::escape() only escapes & < > " ', not tab/CR/LF, since they're legal bytes inside an HTML attribute).

Attack Scenario

1. An application enables the (commonly-used) AttributesExtension and sets allow_unsafe_links => false — the project's own documented hardening step for untrusted input.
2. An attacker submits Markdown: [Click me](javascript:alert(0)){href="java<TAB>script:alert(document.cookie)"} (TAB is one literal 0x09 byte).
3. The library emits <a href="java<TAB>script:alert(document.cookie)">Click me</a> — isLinkPotentiallyUnsafe() doesn't match the tab-split scheme, so the filter takes no action.
4. A victim viewing/clicking the link has the browser strip the embedded TAB and execute javascript:alert(document.cookie) in the victim's session — stored XSS, cookie theft, account takeover potential.

Why the payload needs an unsafe core destination. Step 2 above deliberately uses [Click me](javascript:alert(0)) rather than a normal link. LinkRenderer overwrites attrs['href'] with the node's own URL unless that URL is itself judged unsafe — so [x](https://example.com){href="java<TAB>script:..."} renders the harmless href="https://example.com", and an empty destination [x](){href="..."} renders href="". The attacker therefore supplies a core destination that the filter does catch, which suppresses the overwrite and lets the attribute-supplied href reach the final tag. This is no obstacle in practice — the attacker writes the entire Markdown document.

Two related forms that are not exploitable, noted so the fix isn't over-scoped:

• Attaching the attribute to a non-link block — hi {href="java<TAB>script:alert(1)"} — does bypass the filter and emits <p href="java<TAB>script:alert(1)">, but href on a <p> is inert: there is nothing to navigate. (An earlier draft of this report described this as a "simpler, unconditional variant" of the attack; it is a filter bypass, not an XSS.)
• <img src> is unaffected, since ImageRenderer unconditionally overwrites src from the core URL regardless of the safety verdict.

Recommended Fix

Normalize inside RegexHelper::isLinkPotentiallyUnsafe() before testing, mirroring the WHATWG URL parser's own normalization. This covers both variants, fixes every call site at once (LinkRenderer, ImageRenderer, and any third-party callers), and needs no changes in the Attributes extension.

Affected Versions

>= 1.5.0, <= 2.8.3 - every release that ships the AttributesExtension. Verified by installing each version and rendering the payloads with allow_unsafe_links => false. The attribute-value grammar (PARTIAL_DOUBLEQUOTEDVALUE = '"[^"]*"') has accepted raw control bytes since the extension was introduced, and none of the intervening parser rewrites narrowed it.

Prior Related Advisories

GHSA-3527-qv2q-pfvx / CVE-2025-46734 fixed a different Attributes-extension XSS (unallowlisted on* handlers, href/src not respecting allow_unsafe_links at all) in v2.7.0. This issue bypasses the specific href/src protection that fix introduced (the control-byte normalization gap was not part of that fix) - but the obfuscated inputs also work on older versions.

Impact

The DomainFilteringAdapter in the Embed extension is vulnerable to an allowlist bypass due to a missing hostname boundary assertion in the domain-matching regex. An attacker-controlled domain like youtube.com.evil passes the allowlist check when youtube.com is an allowed domain.

This enables two attack vectors:

• SSRF: The OscaroteroEmbedAdapter makes server-side HTTP requests to the embed URL via the embed/embed library. A bypassed domain filter causes the server to make outbound requests to an attacker-controlled host, potentially probing internal services or exfiltrating request metadata.
• XSS: EmbedRenderer outputs the oEmbed response HTML directly into the page with no sanitization. An attacker controlling the bypassed domain can return arbitrary HTML/JavaScript in their oEmbed response, which is rendered verbatim.

Any application using the Embed extension and relying on allowed_domains to restrict domains when processing untrusted Markdown input is affected.

Patches

This has been patched in version 2.8.2. The fix replaces the regex-based domain check with explicit hostname parsing using parse_url(), ensuring exact domain and subdomain matching only.

Workarounds

• Disable the Embed extension, or restrict its use to trusted users
• Provide your own domain-filtering implementation of EmbedAdapterInterface
• Enable a Content Security Policy (CSP) and outbound firewall restrictions

Impact

AttributesExtension ships with the library but must be explicitly registered on the Environment; it is not included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register AttributesExtension are not affected by this advisory.

Two paths in the extension re-process every attribute a node has already collected each time another attribute is applied to it. When the attributes carry distinct names, the collected set grows by one on every step and is walked again in full, so a run of n attributes costs O(n²).

1. Attribute nodes resolving to a common target (affected from 1.5.0).

AttributesListener::processDocument() merges each attribute node into the set accumulated for its target, then filters the result. Both operations traverse that entire set: AttributesHelper::mergeAttributes() rebuilds it with array_merge(), and AttributesHelper::filterAttributes() matches a regular expression against every name in it. A run of attribute nodes sharing one target therefore re-walks a set that grows by a key per node.

Two input shapes reach this path: adjacent inline attributes at the start of a block ({a0="v"}{a1="v"}…, where quoting the values is what keeps them separate — an unquoted value swallows the }{ that follows it), and a chain of attribute blocks held at their default target by reference definitions ({a0=v} / [a]: u / {a1=v} / [a]: u / …).

256 KB of adjacent inline attributes takes 20.0 seconds to convert, against 0.09 seconds once patched.

2. Consecutive attribute-block lines (affected from 2.0.0).

AttributesBlockContinueParser::tryContinue() merges each continuation line into the block's accumulated attributes, again rebuilding the whole set on every line. One distinct attribute per line ({a0=v} / {a1=v} / …) grows it by a key each time.

256 KB of such lines takes 1.9 seconds to convert while producing zero bytes of output, against 0.08 seconds once patched.

Relationship to GHSA-jjv6-8j6v-6j52. The fix released in 2.9.1 for that advisory made the class attribute cheap to accumulate, but left every other attribute name on the original path. Applications that upgraded to 2.9.1 or 2.9.2 remain exposed to this variant.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

Patches

The issue is patched in 2.10.0. Both paths now fold each node — or each line — into the accumulated attributes at a cost proportional to that node or line alone, rather than re-merging and re-filtering everything gathered so far. Rendered output is unchanged, down to the order in which attributes appear.

The listener path affects 1.5.0 through 2.9.2. The continuation-line path affects 2.0.0 through 2.9.2. The 1.x release line is no longer supported, so its users must upgrade to 2.10.0 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register AttributesExtension when converting untrusted Markdown. This fully removes both paths.
• If the extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, the cap must be small to meaningfully bound worst-case CPU time.

The attributes/allow allow-list added in 2.7.0 is not a mitigation. A non-empty allow-list happens to bound the first path, because unlisted names are discarded before they accumulate, but it does nothing for the second: continuation lines are merged while parsing, before any filtering takes place.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading.

Impact

Two first-party extensions contain quadratic parsing paths. Both ship with the library but must be explicitly registered on the Environment; neither is included in CommonMarkConverter, GithubFlavoredMarkdownConverter, or GithubFlavoredMarkdownExtension. Applications that do not register SmartPunctExtension or AttributesExtension are not affected by this advisory.

1. SmartPunctExtension — quote replacement recopies the whole text node (affected from 2.0.0).

ReplaceUnpairedQuotesListener converts each unpaired Quote node back to a Text node and merges it into its neighbours via AdjacentTextMerger. The merge reads the left node's literal into a local variable, appends to that variable, and writes it back — and because the read aliases the node's string, every append copies the entire accumulated literal rather than only the bytes added. The listener runs this once per surviving unpaired quote against the same continuously growing text node, so the same buffer is fully re-copied a linear number of times.

A 1.2 MB document of alternating text segments and apostrophes takes 34.9 seconds to convert, against 0.069 seconds for the same input with the extension not registered.

Hardened configuration makes this worse rather than better: QuoteParser appends the Quote node to the AST before pushing it onto the delimiter stack, so max_delimiters_per_line removes the quote-pairing work while leaving every node the listener must process.

2. AttributesExtension — block-level attribute runs re-scan their siblings (affected from 1.5.0).

AttributesListener::findTargetAndDirection() walks the entire remaining sibling chain for every block-level Attributes node whose target is the following node. The backward half of that walk returns immediately for such nodes, and the forward half stops only at a sibling that is not itself an attributes node — which a contiguous run never provides — so a run of k nodes costs k(k-1)/2 steps.

An input placing each {#a} on its own line, with a single reference definition to keep the run contiguous, takes 28.4 seconds at 16,000 attribute blocks while producing zero bytes of output.

This is the block-level counterpart of GHSA-g2gp-3wwq-f4ph, patched in 2.9.0. That fix is incomplete: the early break it introduced is guarded on the node being an AttributesInline, so block-level Attributes nodes still re-scan. Applications that upgraded to 2.9.0 specifically to address GHSA-g2gp-3wwq-f4ph remain exposed to this variant.

3. AttributesExtension — class lists are rebuilt on every merge (affected from 1.5.0).

AttributesHelper::mergeAttributes() round-trips the accumulated class list through explode and implode on each merge. An #id attribute assigns a scalar and skips the branch entirely, but a .class attribute appends to an array which is then imploded to a string, written to the target node, and read back on the next iteration — so the ith merge pays a cost proportional to i three separate times.

{.c} repeated 32,000 times takes 33.5 seconds, against 0.26 seconds for byte-identical input using {#a} — a 130x gap that widens with input size. Both the inline and the block-level attribute paths are affected.

Overall impact. An unauthenticated attacker who can submit Markdown to an affected application can consume disproportionate CPU time with a comparatively small request, occupying PHP workers and preventing legitimate requests from completing. The impact is limited to availability: no data is disclosed, rendered output is unchanged, and no rendering restriction is bypassed.

No library-level configuration gates any of these paths. For the Attributes extension in particular, neither the attributes/allow allow-list nor the on* event-handler hardening added in 2.7.0 has any effect, because the expensive work happens while parsing and resolving the AST, before any attribute filtering or rendering takes place.

Patches

The issues are patched in 2.9.1 and later:

• Adjacent text merging now appends in place instead of reading, modifying, and writing back the whole literal, so a merge costs only the bytes added. This fixes the defect for every caller, not only the SmartPunct listener.
• AttributesListener now records the runs it has already walked, so each contiguous run of block-level attribute nodes is scanned once rather than once per node.
• Accumulated class lists no longer pass through mergeAttributes() repeatedly; the listener holds pending attributes and joins them in a single pass.

The SmartPunct path affects 2.0.0 through 2.9.0. The Attributes paths affect 1.5.0 through 2.9.0, including releases that already contain the 2.9.0 fix for GHSA-g2gp-3wwq-f4ph. The 1.x release line is no longer supported, so its users must upgrade to 2.9.1 or later.

Workarounds

If you cannot upgrade immediately:

• Do not register SmartPunctExtension or AttributesExtension when converting untrusted Markdown. This fully removes the affected paths.
• If either extension is required, impose a strict maximum input length before conversion. Because the cost is quadratic, even a modest cap must be small to meaningfully bound worst-case CPU time.

Restricting conversion to trusted users, applying strict execution-time limits, and rate-limiting requests reduce exposure but are not substitutes for upgrading. Configuration options including attributes/allow, max_delimiters_per_line, max_nesting_level, html_input, and allow_unsafe_links do not mitigate these issues.

Summary

The AttributesExtension documents a security guarantee:

Note: Attributes starting with on (e.g. onclick or onerror) are capable of executing JavaScript code and are therefore never allowed by default. You must explicitly add them to the allow list if you want to use them.

— docs/2.x/extensions/attributes.md

Prefixing the attribute name with a single U+000C FORM FEED byte defeats that guarantee. {<FF>onclick="alert(1)"} passes through AttributesHelper::filterAttributes() untouched and is written verbatim into the output, where browsers parse it as a genuine onclick handler.

The same prefix defeats the allow_unsafe_links check, letting a javascript: URI through on href / src even when allow_unsafe_links is false.

This bypasses the fix shipped in the 2.7.0 security release ("Fix XSS in AttributesExtension", 43207253ea5f14867c77c697cd3838c446cadcea), which added filterAttributes() for the express purpose of blocking these attributes.

Throughout this report <FF> denotes a literal U+000C byte ("\x0C" in PHP). It is invisible in rendered text, so all payloads below are written with PHP escape sequences to stay unambiguous.

Details

Three behaviours combine.

1. \x0C survives the parser's trim().

AttributesHelper::SINGLE_ATTRIBUTE begins with \s*, and Cursor::match() returns $matches[0][0] — the entire match, including that leading whitespace. The result is cleaned with PHP's trim():

// src/Extension/Attributes/Util/AttributesHelper.php:62
while ($attribute = \trim((string) $attributeCursor->match('/^' . self::SINGLE_ATTRIBUTE . '/i'))) {

PCRE \s matches \x0C, but PHP's default trim() charlist is " \t\n\r\0\x0B" — it includes the vertical tab \x0B but not the form feed \x0C. The byte is therefore consumed by the regex, retained in the returned match, and not stripped. It ends up inside the attribute name:

// src/Extension/Attributes/Util/AttributesHelper.php:94
$attributes[\trim($name)] = \trim($value);   // $name === "\x0Conclick"

\x0C is the only byte with this property: every other character the HTML5 tokenizer treats as whitespace (\x09, \x0A, \x0D, \x20), plus \x0B, is in PHP's trim charlist. The PoC includes a \x0B case as a control, and it is correctly stripped.

2. The filter's string comparisons miss it.

filterAttributes() compares the raw name against literal strings:

// src/Extension/Attributes/Util/AttributesHelper.php:148-166
$attrNameLower = \strtolower($name);                            // "\x0conclick"
... ($attrNameLower === 'href' || $attrNameLower === 'src') ... // false
... \str_starts_with($attrNameLower, 'on') ...                  // false -> not removed

3. The renderer never escapes attribute names.

// src/Util/HtmlElement.php:123-129
$result .= ' ' . $key . '="' . Xml::escape($value) . '"';   // $key emitted raw

Because the HTML5 tokenizer treats \x0C as whitespace between attributes, the browser reads the name as plain onclick.

PoC

<?php
require 'vendor/autoload.php';

use League\CommonMark\Environment\Environment;
use League\CommonMark\Extension\Attributes\AttributesExtension;
use League\CommonMark\Extension\CommonMark\CommonMarkCoreExtension;
use League\CommonMark\MarkdownConverter;

// The most defensive configuration docs/2.x/security.md recommends.
$env = new Environment([
    'html_input'         => 'escape',
    'allow_unsafe_links' => false,
    'max_nesting_level'  => 100,
    // 'attributes' => ['allow' => [...]] deliberately left at its default []
]);
$env->addExtension(new CommonMarkCoreExtension());
$env->addExtension(new AttributesExtension());
$converter = new MarkdownConverter($env);

$FF = "\x0C";

echo $converter->convert('hello {onclick="alert(1)"}')->getContent();
// <p>hello</p>                                    <- filtered, as documented

echo $converter->convert('hello {' . $FF . 'onclick="alert(1)"}')->getContent();
// <p \x0Conclick="alert(1)">hello</p>             <- BYPASS

Full observed output (\x0C shown escaped; it is a literal single byte in the real output):

| # | Markdown input | Rendered output | Result | |---|---|---|---| | A | hello {onclick="alert(1)"} | <p>hello</p> | filtered (control) | | B | hello {\x0Conclick="alert(1)"} | <p \x0Conclick="alert(1)">hello</p> | bypass | | C | hello {\x0Bonclick="alert(1)"} | <p>hello</p> | filtered (control) | | D | [click](javascript:alert(1)) | <p><a>click</a></p> | filtered (control) | | E | [click](https://example.com){\x0Chref="javascript:alert(1)"} | <p><a \x0Chref="javascript:alert(1)" href="https://example.com">click</a></p> | bypass | | F | ![x](https://example.invalid/x.png){\x0Conerror="alert(1)"} | <p><img \x0Conerror="alert(1)" src="…" alt="x" /></p> | bypass | | G | # heading + newline + {\x0Conclick="alert(1)"} | <h1 \x0Conclick="alert(1)">heading</h1> | bypass (block syntax) |

In case E the injected href precedes the legitimate one. Per the HTML5 duplicate-attribute rule the first occurrence wins, so the javascript: URI is the one the browser actually uses.

Browser confirmation. Loading the library's unmodified output in Chrome for Testing 148:

<img> attribute names : ["onerror","src","alt"]      <- parsed as a real `onerror`
typeof img.onerror    : function                     <- bound as an event handler
handlers fired        : ["img-onerror"]              <- fired on load, no interaction
document.title        : XSS-FIRED
link href attribute   : "javascript:void(0)"
link href property    : "javascript:void(0)"         <- javascript: URI is the effective href
page errors           : []

The onerror case executes with no user interaction — rendering the attacker's Markdown is sufficient.

Verified against git HEAD (f966b17a) and against tag 2.9.0, on PHP 8.5.8.

Impact

Stored cross-site scripting in any application that renders untrusted Markdown with AttributesExtension enabled and attributes.allow left at its default [] — even when the application has followed every hardening step in docs/2.x/security.md (html_input => 'escape', allow_unsafe_links => false, max_nesting_level => 100).

Consequences are the usual for stored XSS: session and cookie theft, actions performed as the viewing user, and account takeover where the host application permits it. Because the payload can be attached to an image (onerror), it fires on page load without requiring the victim to interact with anything.

The affected configuration is the extension's default: attributes.allow defaults to [], and the documentation describes that default as safe with respect to on* attributes.

Workaround for users

Setting an explicit allow list takes the other branch of filterAttributes(), which drops the form-feed name because it is not in the list:

$config = ['attributes' => ['allow' => ['id', 'class', 'align']]];

Verified: hello {\x0Conclick="alert(1)"} then renders as <p>hello</p>.

Suggested fix

The narrow fix is to add \x0C to the trim charlist at AttributesHelper.php lines 62, 89, 90 and 94. That closes this instance but leaves the shape of the problem in place.

A more durable fix is to reject anything that is not a well-formed attribute name in filterAttributes(), reusing the constant the parser already defines (RegexHelper is already imported in that file):

foreach ($attributes as $name => $value) {
    // Names are compared against literal strings below and emitted without escaping,
    // so anything that isn't a plain attribute name must not get through.
    if (\preg_match('/^' . RegexHelper::PARTIAL_ATTRIBUTENAME . '$/i', $name) !== 1) {
        unset($attributes[$name]);
        continue;
    }

    $attrNameLower = \strtolower($name);
    // ... existing logic unchanged
}

As defence in depth, HtmlElement::__toString() could validate or escape $key. It currently trusts its callers to supply safe attribute names, and filterAttributes() is the only thing standing between that method and user-supplied input.

Impact

Affected versions of league/commonmark perform super-linear work on three independent parsing paths, all of which are reachable on a stock new CommonMarkConverter() with default configuration and no extensions registered. Each trigger fits on a single line of input, so no complex Markdown structure is required.

The three paths were introduced at different times. This advisory's version range is their union; the individual ranges are:

| Path | Affected from | Affected through | |---|---|---| | 1. Fenced code block detection | 0.6.0 | 2.9.0 | | 2. Reference link label lookup | 0.6.0 | 2.9.0 | | 3. Emphasis / strikethrough delimiters (*, _, ~) | 2.6.0 | 2.9.0 | | 3. Highlight delimiters (=) | 2.9.0 | 2.9.0 |