Ruby on Rails: Potential XSS on sanitize/Rails::Html::WhiteListSanitizer

Possible XSS vulnerability in rails-html-sanitizer

There is a possible XSS vulnerability in the white list sanitizer in the
rails-html-sanitizer gem. This vulnerability has been assigned the CVE
identifier CVE-2015-7580.

Versions Affected: All.
Not affected: None.
Fixed Versions: v1.0.3

Impact

Carefully crafted strings can cause user input to bypass the sanitization in
the white list sanitizer which will can lead to an XSS attack.

Vulnerable code will look something like this:

<%= sanitize user_input, tags: %w(em) %>

All users running an affected release should either upgrade or use one of the
workarounds immediately.

Releases

The FIXED releases are available at the normal locations.

Workarounds

Putting the following monkey patch in an initializer can help to mitigate the
issue:

class Rails::Html::PermitScrubber
  alias :old_scrub :scrub
  alias :old_skip_node? :skip_node?

  def scrub(node)
    if node.cdata?
      text = node.document.create_text_node node.text
      node.replace text
      return CONTINUE
    end
    old_scrub node
  end

  def skip_node?(node); node.text?; end
end

Patches

To aid users who aren’t able to upgrade immediately we have provided patches for
the two supported release series. They are in git-am format and consist of a
single changeset.

• 1-0-whitelist_sanitizer_xss.patch - Patch for 1.0 series

Credits

Thanks to Arnaud Germis, Nate Clark, John Colvin and Yassine ABOUKIR for reporting this issue.