An input validation issue exists in the QuickTime ActiveX control. An optional parameter '_Marshaled_pUnk' may be passed to the ActiveX control to specify an arbitrary integer that is later treated as a pointer. Visiting a maliciously crafted website may lead to an unexpected application termination or arbitrary code execution. This issue is addressed by ignoring the '_Marshaled_pUnk' parameter. This issue does not affect Mac OS X systems. Credit to HBelite working with TippingPoint's Zero Day Initiative for reporting this issue.

More: http://support.apple.com/kb/HT4339