X.Org server (xorg-server) 1.0.0 and later, X11R6.9.0, and X11R7.0 inadvertently treats the address of the geteuid function as if it is the return value of a call to geteuid, which allows local users to bypass intended restrictions and (1) execute arbitrary code via the -modulepath command line option or (2) overwrite arbitrary files via -logfile.