MediaWiki before 1.15.2 does not prevent wiki editors from linking to images from other web sites in wiki pages, which allows editors to obtain IP addresses and other information of wiki users by adding a link to an image on an attacker-controlled web site, aka "CSS validation issue."
http://www.vupen.com/english/advisories/2010/1001
http://www.vupen.com/english/advisories/2010/0685
http://www.debian.org/security/2010/dsa-2022
http://secunia.com/advisories/39656
http://secunia.com/advisories/39022
http://lists.wikimedia.org/pipermail/mediawiki-announce/2010-March/000088.html
http://lists.opensuse.org/opensuse-security-announce/2010-04/msg00006.html