The (1) GetDatabase and (2) _XimParseStringFile functions in X.org libX11 1.5.99.901 (1.6 RC1) and earlier do not restrict the recursion depth when processing directives to include files, which allows X servers to cause a denial of service (stack consumption) via a crafted file.
http://www.x.org/wiki/Development/Security/Advisory-2013-05-23
http://www.ubuntu.com/usn/USN-1854-1