March 18, 2013: XSS vulnerability in sanitize_css in Action Pack [CVE-2013-1855]

This vulnerability is app specific and does not pertain to all environments. If you have a specific question or need assistance with this issue, please open a Support Ticket. 

Issue

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, 2.3.18 

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.

For a link to the above information and patches for download, please see the following Google Document: https://groups.google.com/forum/?fromgroups=#!topic/rubyonrails-security/4_QHo4BqnN8

March 20, 2013 5:21am PDT (12:22pm UTC) - While a minority of users are likely affected by this vulnerability, we recommend everyone upgrade.