xterm, Eterm, and rxvt allow an attacker to cause a denial of service by embedding certain escape characters which force the window to be resized.
XFree86 3.3.x and 4.0 allows a user to cause a denial of service via a negative counter value in a malformed TCP packet that is sent to port 6000.
libICE in XFree86 allows remote attackers to cause a denial of service by specifying a large value which is not properly checked by the SKIP_STRING macro.