The jpeg2000_read_main_headers function in libavcodec/jpeg2000dec.c in FFmpeg before 2.6.5, 2.7.x before 2.7.3, and 2.8.x through 2.8.2 does not enforce uniqueness of the SIZ marker in a JPEG 2000 image, which allows remote attackers to cause a denial of service (out-of-bounds heap-memory access) or possibly have unspecified other impact via a crafted image with two or more of these markers.
The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.