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From: Jan Vroonhof (janXEMACS.ORG)
Date: Fri Feb 02 2001 - 08:13:35 CST

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    All currently available versions of gnuserv for unix prior to 3.12 are
    vulnerable to remote exploit due to a buffer overflow and weak
    security. Gnuserv is a remote control facility for Emacsen. Gnuserv
    ships with XEmacs but is also available stand-alone from various
    sources for use with GNU Emacs.


    An attacker can excute remote commands with the uid of the user that
    is running gnuserv.


    This problem was discovered by Klaus Frank
    <klausfi3.informatik.rwth-aachen.de>. Klaus provided a fix as
    well. Many thanks to Klaus for bringing this to attention of the
    XEmacs and gnuserv maintainers.

    I also with to thank Vince Shelton and Martin Schwenke for putting
    fixed releases of XEmacs and gnuserv on the net quickly.


    gnuserv/gnuclient is a pair of utility programs used to
    sent commands to an already running Emacs process. gnuserv
    is the helper binary used by the running Emacs to listen
    for commands. It must be started explicitly using the gnuserv-start

    gnuserv can use several different communication mechanisms, one of
    them being a tcp port. This can be switched off at compile time, but
    defaults to on. If enabled gnuserv binds to a user specified TCP port,
    with the default being (21490 + uid). Note that (if enabled) gnuserv
    _always_ listens for TCP connections, even if one of the other
    mechanisms is normally used by the user.

    Connections on the gnuserv port are authenticated either against a
    list of trusted hosts or using a MIT-MAGIC-COOKIE based system.
    (MIT-MAGIC_COOKIE authentication can be switched of, but again is the

    The problem lies in the fact that the gnuserv program trusts the
    remote sides specification for the lenght of the cookie without
    any sanity checking. This allows the attacker to

    1. Overflow the buffer used to hold a copy of the cookie.
    2. Force the comparison of the cookies to be restricted to
       a prefix of a length chosen by him, e.g. 1 byte, making
       bruteforcing the authentication trivial.

    Both problems are sufficient to give any attacker easy access
    to running arbitrary commands under the uid of the user running


    Unfortunately gnuserv has rather a complicated history:

    gnuserv was origionally written by Andy Norman (ange). The problematic
    Xauth based authentication was later added by somebody else. As ange
    effectively stopped maintaining his version (gnuserv-2.1alpha.tar.gz)
    various people have put up their own modified copies. That includes
    among others the version shipped with XEmacs and fgnuserv by Noah
    Friedman, which is an easier to compile stand-alone version.

    After a recent rewrite we made the XEmacs version the official
    verion[2], and bumped the version number to the 3.x range. Martin
    Schwenke has made a backport of this version for use with Emacs
    using fgnuserv's build mecahnism.

    All of the above versions should be assumbed vulnerable, including
    those shipped with XEmacs 21.1.x for x < 14. As a test run

    strings gnuserv | grep "gnuserv version"

    If this gives either nothing or a version below 3.12, then you are


    There is a seperate fork for gnuserv on windows for use with NTEmacs.
    This is not vulnerable as it uses a completely different communication
    channel. This is, however, unconfirmed.


    A fix by Klaus Frank is in gnuserv 3.12.

    If you are using XEmacs we suggest upgrading to XEmacs 21.1.14
    that contains this version (or 21.2.35 if you are running betas).
    This version can be had from
    or mirrors.

    If you are using a standalone gnuserv with GNU Emacs on unix we
    suggest getting Martin Schwenkes fixed version from

    This is the vulnerability indicated in Mandrake Advisory MDKSA-2001:019

    Jan Vroonhof <janxemacs.org>
    Gnuserv feel-responsible-for-person

    [1] However I have seen many icons for XEmacs in UI's start
         "xemacs -f gnuserv" so it is not always obvious to the
         user he is running gnuserv.

    [2] With permission form Andy Norman