The make_clickable function in wp-includes/formatting.php in WordPress before 3.1.1 does not properly check URLs before passing them to the PCRE library, which allows remote attackers to cause a denial of service (crash) via a comment with a crafted URL that triggers many recursive calls.
http://www.openwall.com/lists/oss-security/2012/04/19/6
http://www.openwall.com/lists/oss-security/2012/04/19/17
http://www.debian.org/security/2012/dsa-2470
http://wordpress.org/news/2011/04/wordpress-3-1-1/
http://secunia.com/advisories/49138