ZF2009-02: XSS vector in Zend_Filter_StripTags
Piwik 0.2.33 (released Mar. 2, 2009) and earlier versions are not affected by this security advisory (disclosed Mar. 2, 2009) because Piwik uses a subset of ZF which does not include Zend_Filter.
Piwik users are, however, encouraged to upgrade to take advantage of new features and bug fixes.