xrdb.c in xrdb before 1.0.9 in X.Org X11R7.6 and earlier allows remote attackers to execute arbitrary commands via shell metacharacters in a hostname obtained from a (1) DHCP or (2) XDMCP message.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.