This indicates an attack attempt to exploit a Buffer Overflow vulnerability in Apple QuickTime.
The vulnerability is due to insufficient validation when parsing a MP4 file with a malformed atom. A remote attacker may be able to exploit this to execute arbitrary code within the context of the application, via a crafted QuickTime mp4 file.
Apple Computer Quicktime prior to 7.7.4
System Compromise: Remote attackers can gain control of vulnerable systems.
Apply the most recent upgrade or patch from the vendor.