Klaus Frank has found a vulnerability in the way gnuserv handled remote connections. Gnuserv is a remote control facility for Emacsen which is available as standalone program as well as included in XEmacs21. Gnuserv has a buffer for which insufficient boundary checks were made. Unfortunately this buffer affected access control to gnuserv which is using a MIT-MAGIC-COOCKIE based system. It is possible to overflow the buffer containing the cookie and foozle cookie comparison.
Gnuserv was derived from emacsserver which is part of GNU Emacs. It was reworked completely and not much is left over from its time as part of GNU Emacs. Therefore the versions of emacsserver in both Emacs19 and Emacs20 doesn't look vulnerable to this bug, they don't even provide a MIT-MAGIC-COOKIE based mechanism.
This could lead into a remote user issue commands under the UID of the person running gnuserv.