Apple QuickTime before 7.5.5 does not properly handle (1) MDAT atoms in MP4 video files within QuickTimeH264.qtx, (2) MDAT atoms in mov video files within QuickTimeH264.scalar, and (3) AVC1 atoms in an unknown media type within an unspecified component, which allows remote attackers to execute arbitrary code or cause a denial of service (heap corruption and application crash) via a crafted, H.264 encoded movie file.