Multiple buffer overflows in X.org libX11 1.5.99.901 (1.6 RC1) and earlier allow X servers to cause a denial of service (crash) and possibly execute arbitrary code via crafted length or index values to the (1) XAllocColorCells, (2) _XkbReadGetDeviceInfoReply, (3) _XkbReadGeomShapes, (4) _XkbReadGetGeometryReply, (5) _XkbReadKeySyms, (6) _XkbReadKeyActions, (7) _XkbReadKeyBehaviors, (8) _XkbReadModifierMap, (9) _XkbReadExplicitComponents, (10) _XkbReadVirtualModMap, (11) _XkbReadGetNamesReply, (12) _XkbReadGetMapReply, (13) _XimXGetReadData, (14) XListFonts, (15) XListExtensions, and (16) XGetFontPath functions.
http://www.x.org/wiki/Development/Security/Advisory-2013-05-23
http://www.ubuntu.com/usn/USN-1854-1
http://www.openwall.com/lists/oss-security/2013/05/23/3
http://www.debian.org/security/2013/dsa-2693
http://lists.fedoraproject.org/pipermail/package-announce/2013-May/106781.html