viewtopic.php in phpBB 2.x before 2.0.11 improperly URL decodes the highlight parameter when extracting words and phrases to highlight, which allows remote attackers to execute arbitrary PHP code by double-encoding the highlight value so that special characters are inserted into the result, which is then processed by PHP exec, as exploited by the Santy.A worm.
https://security.gentoo.org/glsa/200411-32
https://exchange.xforce.ibmcloud.com/vulnerabilities/18052
http://www.us-cert.gov/cas/techalerts/TA04-356A.html
http://www.securityfocus.com/bid/10701
http://www.securityfocus.com/archive/1/385208
http://www.phpbb.com/phpBB/viewtopic.php?t=240513
http://www.kb.cert.org/vuls/id/497400
http://secunia.com/advisories/13239/
http://marc.info/?t=110079440800004&r=1&w=2