Opera before 11.64 does not properly allocate memory for URL strings, which allows remote attackers to execute arbitrary code or cause a denial of service (memory corruption and application crash) via a crafted string.
http://www.opera.com/support/kb/view/1016/
http://www.opera.com/docs/changelogs/windows/1164/
http://www.opera.com/docs/changelogs/unix/1164/