The sse2_composite_src_x888_8888 function in Pixman, as used in Cairo in Mozilla Firefox 28.0 and SeaMonkey 2.25 on Windows, allows remote attackers to execute arbitrary code or cause a denial of service (out-of-bounds write and application crash) by painting on a CANVAS element.
Improper Restriction of Operations within the Bounds of a Memory Buffer The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary.