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.