Vulnerability Instance

Possible XSS vulnerability with certain configurations of rails-html-sanitizer
## Summary

There is a possible XSS vulnerability with certain configurations of Rails::Html::Sanitizer.

- Versions affected: ALL
- Not affected: NONE
- Fixed versions: 1.4.4


## Impact

A possible XSS vulnerability with certain configurations of Rails::Html::Sanitizer may allow an attacker to inject content if the application developer has overridden the sanitizer's allowed tags in either of the following ways:

- allow both "math" and "style" elements,
- or allow both "svg" and "style" elements

Code is only impacted if allowed tags are being overridden. Applications may be doing this in four different ways:

1. using application configuration:

  ```ruby
  # In config/application.rb
  config.action_view.sanitized_allowed_tags = ["math", "style"]
  # or
  config.action_view.sanitized_allowed_tags = ["svg", "style"]
  ```

  see https://guides.rubyonrails.org/configuring.html#configuring-action-view

2. using a `:tags` option to the Action View helper `sanitize`:

  ```
  <%= sanitize @comment.body, tags: ["math", "style"] %>
  <%# or %>
  <%= sanitize @comment.body, tags: ["svg", "style"] %>
  ```

  see https://api.rubyonrails.org/classes/ActionView/Helpers/SanitizeHelper.html#method-i-sanitize

3. using Rails::Html::SafeListSanitizer class method `allowed_tags=`:

  ```ruby
  # class-level option
  Rails::Html::SafeListSanitizer.allowed_tags = ["math", "style"]
  # or
  Rails::Html::SafeListSanitizer.allowed_tags = ["svg", "style"]
  ```

4. using a `:tags` options to the Rails::Html::SafeListSanitizer instance method `sanitize`:

  ```ruby
  # instance-level option
  Rails::Html::SafeListSanitizer.new.sanitize(@article.body, tags: ["math", "style"])
  # or
  Rails::Html::SafeListSanitizer.new.sanitize(@article.body, tags: ["svg", "style"])
  ```

All users overriding the allowed tags by any of the above mechanisms to include (("math" or "svg") and "style") should either upgrade or use one of the workarounds immediately.


## Workarounds

Remove "style" from the overridden allowed tags, or remove "math" and "svg" from the overridden allowed tags.


## References

- [CWE - CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') (4.9)](https://cwe.mitre.org/data/definitions/79.html)
- https://hackerone.com/reports/1656627


## Credit

This vulnerability was responsibly reported by Dominic Breuker.