Bug 921331 (CVE-2013-1855)

XSS vulnerability in sanitize_css in Action Pack

There is an XSS vulnerability in the `sanitize_css` method in Action Pack. This vulnerability has been assigned the CVE identifier CVE-2013-1855.

Versions Affected:  All.
Not affected:       None.
Fixed Versions:     3.2.13, 3.1.12

Impact 
------ 
Carefully crafted text can bypass the sanitization provided in the `sanitize_css` method in Action Pack.  Impacted code will look like this:

    sanitize_css(some_user_input)

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

Releases 
-------- 
The 3.2.13 and 3.1.12 releases are available at the normal locations. 

Workarounds 
----------- 
To work around this issue, you can apply the following monkey patch:

```
module HTML
  class WhiteListSanitizer
      # Sanitizes a block of css code. Used by #sanitize when it comes across a style attribute
    def sanitize_css(style)
      # disallow urls
      style = style.to_s.gsub(/url\s*\(\s*[^\s)]+?\s*\)\s*/, ' ')

      # gauntlet
      if style !~ /\A([:,;#%.\sa-zA-Z0-9!]|\w-\w|\'[\s\w]+\'|\"[\s\w]+\"|\([\d,\s]+\))*\z/ ||
          style !~ /\A(\s*[-\w]+\s*:\s*[^:;]*(;|$)\s*)*\z/
        return ''
      end

      clean = []
      style.scan(/([-\w]+)\s*:\s*([^:;]*)/) do |prop,val|
        if allowed_css_properties.include?(prop.downcase)
          clean <<  prop + ': ' + val + ';'
        elsif shorthand_css_properties.include?(prop.split('-')[0].downcase)
          unless val.split().any? do |keyword|
            !allowed_css_keywords.include?(keyword) &&
              keyword !~ /\A(#[0-9a-f]+|rgb\(\d+%?,\d*%?,?\d*%?\)?|\d{0,2}\.?\d{0,2}(cm|em|ex|in|mm|pc|pt|px|%|,|\))?)\z/
          end
            clean << prop + ': ' + val + ';'
          end
        end
      end
      clean.join(' ')
    end
  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. 

* 3-2-css_sanitize.patch - Patch for 3.2 series 
* 3-1-css_sanitize.patch - Patch for 3.1 series 
* 3-0-css_sanitize.patch - Patch for 3.0 series 
* 2-3-css_sanitize.patch - Patch for 2.3 series 

Please note that only the 3.1.x and 3.2.x series are supported at present.  Users of earlier unsupported releases are advised to upgrade as soon as possible as we cannot guarantee the continued availability of security fixes for unsupported releases.

Credits 
-------

Thanks to Charlie Somerville for reporting this!