The ProcSetEventMask function in difs/events.c in the xfs font server for X.Org X11R6 through X11R6.6 and XFree86 before 3.3.3 calls the SendErrToClient function with a mask value instead of a pointer, which allows local users to cause a denial of service (memory corruption and crash) or obtain potentially sensitive information from memory via a SetEventMask request that triggers an invalid pointer dereference.
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.