The wikitext parser in MediaWiki 1.17.x before 1.17.3 and 1.18.x before 1.18.2 allows remote attackers to cause a denial of service (infinite loop) via certain input, as demonstrated by the padleft function.
lists.wikimedia.org/pipermail/mediawiki-announce/2012-March/000109.html
lists.wikimedia.org/pipermail/mediawiki-announce/2012-March/000110.html
secunia.com/advisories/48504
www.openwall.com/lists/oss-security/2012/03/22/9
www.openwall.com/lists/oss-security/2012/03/24/1
www.securityfocus.com/bid/52689
bugzilla.wikimedia.org/show_bug.cgi?id=22555
bugzilla.wikimedia.org/show_bug.cgi?id=35315