Buffer overflow in the compNewPixmap function in compalloc.c in the Composite extension for the X.org X11 server before 1.4 allows local users to execute arbitrary code by copying data from a large pixel depth pixmap into a smaller pixel depth pixmap.
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.