Four holes have been found and patched in Apple's Quicktime media player that could to be used for denial of service attacks.
Secunia has warned in an advisory that the holes were "highly critical" and could potentially compromise a system.
The holes can be exploited by hackers building video clips containing executable code. Apple patched the holes last month in QuickTime 7.0.3, but this is the first time details about what those holes are have been revealed.
All users are advised to upgrade as soon as possible but those that have bought keys for QuickTime Pro 6 will be forced to buy a new licence to use QuickTimes 7.