The decode_ihdr_chunk function in libavcodec/pngdec.c in FFMpeg before 2.1.6, 2.2.x through 2.3.x, and 2.4.x before 2.4.4 allows remote attackers to cause a denial of service (out-of-bounds heap access) and possibly have other unspecified impact via an IDAT before an IHDR in a PNG file.
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.