The SProcXCMiscGetXIDList function in the XC-MISC extension in X.Org X Window System (aka X11 or X) X11R6.0 and X.Org Server (aka xserver and xorg-server) before 1.16.3 allows remote authenticated users to cause a denial of service (out-of-bounds read or write) or possibly execute arbitrary code via a crafted length or index value.