Security Advisory

ZF2009-02: XSS vector in Zend_Filter_StripTags

Executive Summary

Zend_Filter_StripTags is a filtering class analogous to PHP's strip_tags() function. In addition to stripping HTML tags and selectively keeping those provided in a whitelist, it also provides the ability to whitelist specific attributes to retain per whitelisted tag.

The reporter discovered that attributes that contained whitespace, and in paricular, line breaks, surrounding the attribute assignment operator would not be stripped, regardless of whether or not they were whitelisted. As examples of input affected: 


<!-- newlines before and/or after assignment: -->
<a href="http://framework.zend.com/issues" onclick
=
"alert('Broken'); return false;">Issues</a>

When passed to the following code: 


$filter = new Zend_Filter_StripTags(array('a' => array('href')));
$value  = $filter->($html);

then the "onclick" attribute would remain, even though it was not specified in the tag's whitelist. This could open potential cross-site scripting attack (XSS) vectors.

Action Taken

The functionality was patched, and the patch released starting in Zend Framework version 1.7.6.

Recommendations

If you were using Zend_Filter_StripTags and utlizing the attribute whitelisting functionality, you should immediately upgrade to Zend Framework 1.7.6 or above; regardless, it is always best to run the most current version of the framework.

Also, if relying on Zend_Filter_StripTags to prevent XSS, the only way to reliably do so is to strip all tags, and never to whitelist. If you are whitelisting, you should consider finding a reliable XSS filter through which to run your output; we recommend HTML Purifier.

Other Information

Acknowledgments

The Zend Framework team thanks the following for working with us to help protect its users:

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