
Apple issued a security update for Mac OS X today. The update specifically addresses a possible security flaw in Quicktime:
Impact: Visiting malicious websites may lead to arbitrary code execution
Description: A buffer overflow exists in QuickTime's handling of RTSP URLs. By enticing a user to access a maliciously-crafted RTSP URL, an attacker can trigger the buffer overflow, which may lead to arbitrary code execution.
http://docs.info.apple.com/article.html?artnum=304989
A proof of concept exploting this bug was published at the Month of Apple Bugs site.