Software Engineering Institute

Apple QuickTime is a media player that includes a browser plugin. QuickTime can use atom containers to organize movies and music tracks.

From Apple Article ID: 306896 "About the security content of QuickTime 7.3":
A memory corruption issue exists in QuickTime's handling of image description atoms. By enticing a user to open a maliciously crafted movie file, an attacker may cause an unexpected application termination or arbitrary code execution. This update addresses the issue by performing additional validation of QuickTime image descriptions.
To exploit this vulnerability, an attacker would have to convince a user to open a specially crafted QuickTime movie which could be hosted on a web page or sent in an email.

A remote, unauthenticated attacker may be able to execute arbitrary code or cause QuickTime to crash.

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Apple has addressed this issue in QuickTime 7.3.