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Re: libX11
From: szabo_p () MATHS SU OZ AU (Paul Szabo)
Date: Fri, 28 Feb 1997 08:16:50 +1100

A few days ago SNI released an advisory concerning buffer overrun problems
in libX11. Their "fix advice" was to upgrade to X11R6.3, or to remove
setuid/setgid privileges from vulnerable programs (e.g. xload and xterm).

I do not think I can upgrade to the current release of X11: how would I
integrate that into Digital Unix (a.k.a. OSF/1)? And I could not give up the
functionality of xterm...

So instead I wrote the following wrapper, and used it to wrap xload, xterm
and xconsole. My wrapper, and the SNI advisory, included below.

Paul Szabo - System Manager   //        School of Mathematics and Statistics
psz () maths usyd edu au         //   University of Sydney, NSW 2006, Australia


    sec_wrapper.c -- Wrap setuid program to prevent command line or
                     environment variable buffer overrun.

Only tested on DUnix V4.0

        Does NOT check the values of the arguments and/or environment
        (other than checking their lengths), though that functionality
        could (should?) be added.

        This program is based loosely on the lpr wrapper released with
        AUSCERT Advisory AA-96.12. Any errors are my own.

DISCLAIMER:     Anything you do with this is at your own risk.

    V1.0  26 Feb 97  PSz

Installation instructions

  1.    su to root

  2.    Determine the full path of the prog you want to wrap, and
        its permissions, owner, and group:

                # ls -lg /usr/bin/badprog

  3.    Copy badprog to badprog.real, and change the permissions on it:

                # cd /usr/bin
                # cp -ip badprog badprog.real
                # chmod 711 badprog.real

  4.    Edit (a copy of) this program and define REAL_PROG (an absolute
        pathname), check the settings of MAX_ and GOOD_ variables, then

                # cc -o badprog badprog_wrapper.c

  5.    Copy this new wrapper program into the directory originally
        containing badprog, replacing the existing badprog program.

        Use the information found in step #2 and set the same
        owner, group, permissions on the new badprog program:

                # cp badprog /usr/bin
                # cd /usr/bin
                # chown root badprog
                # chgrp daemon badprog
                # chmod 6711 badprog

        Check that the permissions, owner and group exactly
        match those noted in step #2:

                # ls -lg /usr/bin/badprog

  6.    Check that badprog still works!

/* Make sure REAL_PROG points to the right location */
/* The MAX_ values should be appropriate to the program being wrapped. */
/* The GOOD_ values should be appropriate, leave undefined to skip check. */
/* (Beware: $TERMCAP often contains weird characters.) */

/* #define REAL_PROG    "/usr/bin/badprog.real" */

#define MAX_ARGC        50
#define MAX_ARG_LENGTH  1000
#define MAX_ENV_LENGTH  1000
/* #define GOOD_ARG_CHARS       "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ/_.-" */
/* #define GOOD_ENV_CHARS       "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ/_.,:+-=" */

#define SYSLOG 1

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#ifdef SYSLOG
#include <syslog.h>

#ifdef SYSLOG
#define sysl(msg)       syslog(LOG_ERR,"Possible %s attack by uid %d: %s\n",REAL_PROG,getuid(),msg)
#define sysl(msg)

#define ZAP(x,err)      if (x) { printf("Sorry: %s\n",err); sysl(err); exit(-1); }

#ifndef REAL_PROG
        Cannot compile: you do not have REAL_PROG defined
#if MAX_ARGC > 1000 || MAX_ARGC < 1
        Cannot compile: you have a crazy value for MAX_ARGC
        Cannot compile: you have a crazy value for MAX_ARG_LENGTH
        Cannot compile: you have a crazy value for MAX_ENV_LENGTH

void main( int argc, char* argv[] /* , char *envp[] */ )
        int     iii, jjj;
        char    **ccc, *ddd;

        extern char **environ;

        ZAP( argc<1, "too few arguments" );
        ZAP( argc>MAX_ARGC, "too many arguments" );

        for( iii=1; iii<argc; iii++ )
                jjj = strlen(argv[iii]);
                ZAP( jjj<1, "argument too short" );
                ZAP( jjj>MAX_ARG_LENGTH, "argument too long" );
                ZAP( strspn(argv[iii],GOOD_ARG_CHARS) != jjj, "bad char in argument" );

        /* Which one: environ and/or envp ?? */
        for( ccc = environ; *ccc; ccc++ )
                jjj = strlen(*ccc);
                ZAP( jjj<2, "environment component too short" );
                ZAP( jjj>MAX_ENV_LENGTH, "environment component too long" );
                ZAP( !strncmp(*ccc,"LD_",3), "suspicious environment component LD_" );
                ddd = strchr(*ccc,'=');
                ZAP( (int)ddd < (int)*ccc || (int)ddd >= ((int)*ccc)+jjj || *ddd != '=',
                    "badly formed environment component" );
                ZAP( strspn(*ccc,GOOD_ENV_CHARS) != jjj, "bad char in environment component" );

        /* execve( REAL_PROG, argv, envp ); */
        execv( REAL_PROG, argv );
        perror( REAL_PROG );
        exit( 1 );


From owner-bugtraq () NETSPACE ORG Tue Feb 25 11:22:38 1997
Date:         Mon, 24 Feb 1997 16:06:43 -0700
From: David Sacerdote <davids () SECNET COM>
Subject:      libX11

                        ######    ##   ##    ######
                        ##        ###  ##      ##
                        ######    ## # ##      ##
                            ##    ##  ###      ##
                        ###### .  ##   ## .  ######.

                            Secure Networks Inc.

                             Security Advisory
                             February 24, 1997

                    Environment Variable Problems in X11

While examining the differences between X11R6.1 and X11R6.3, it has come to
our attention that a number of serious security problems in libX11 were fixed
between releases.  These problems permit unprivileged users to obtain elevated
access, including group sys, group kmem, and root privileges, depending on the
operating system and the X11 release.  Administrators should be aware that
these problems are actively being exploited, and should take the precautions
outlined below to ensure they are not susceptible to these problems.

Technical Details

In X11R6.1 and earlier, there are many places where libX11 looks at
environment variables, and then performs string operations on them.  X11R6.1
and earlier, however, perform no bounds checking when doing these string
operations.  Setuid and setgid programs which use functions provided by libX11
may allow users to obtain elevated privileges.

One of the many examples of flawed code in X11R6.1, in this case from
GetDflt.c reads:

if (ptr = getenv("HOME"))
        (void) strcpy(dest, ptr);

While the corrected code for this particular exammple in X11R6.3 reads:

if (ptr = getenv("HOME")) {
        (void) strncpy(dest, ptr, len);
        dest[len-1] = '\0';

Note that this code correctly adds a null character at the end of the
string after the strncpy.

Depending on platform and X11 release, individuals with shell access can
obtain elevated access, including group sys, group kmem, and root

Vulnerable Systems
Any system which is running X11R6.1 or earlier, and has at least one setuid or
setgid program which uses libX11 is vulnerable.

You can perform a simple test to determine whether your system is vulnerable.
First, set the HOME environment variable to a string at least 2500 characters
long.  Using a sh compatible shell, do this by issuing the commands:

$ HOME=jjjjjjj...jjjjj                  (2500 repititions of 'j')
$ export HOME

Using csh or tcsh, use the command:

% setenv HOME jjjjj...jjjjjjj           (2500 repititions of 'j')

Then, run a setuid or setgid X program, such as xload.  If you are running a
vulnerable release of X11, you will get an error message including either the
words "Segmentation Fault" or "Bus error."  If the words "Segmentation Fault"
and "Bus Error" do not appear, and the program operates correctly, you are not
vulnerable to this problem.

Be aware that if you use a string much shorter than 2500 characters, this test
will not produce meaningful results, because the length of the buffer in
question is 2048 characters.  Also, if your DISPLAY environment variable does
not point to a display which you have authorization to connect to, the test
will not be able to connect to a valid display and therefore will not work.

Fix Information
To fix these problems without loss of functionality, upgrade to the current
release of X11.  You can obtain X11R6.3 by referring to

As an alternative workaround, administrators may want to remove setuid and
setgid bits from vulnerable programs.  To find all setuid and setgid programs,
in the X11 distribution, the following command can be executed:

% cd /usr/X11/bin
% find . \( -perm -02000 -o -perm -04000 \) -exec ls -l {} \;
% find . \( -perm -02000 -o -perm -04000 \) -exec chmod ug-s {} \;

Remember to perform the same command if you wish to remove permissions from
programs stored in other system directories.  Keep in mind that that the
use of this workaround will result in reduced functionality for non-root

Additional Information

If you have any questions about this advisory, feel free to contact me,
David Sacerdote, at davids () secnet com   If you should wish to encrypt
traffic for me, my pgp key is:

Version: 2.6.2


Many thanks to the unknown individual who undertook to fix this set
of holes in the final days of the X Consortium.

Additional information about the X Windowing System can be found at

You can find Secure Networks papers at ftp://ftp.secnet.com/pub/papers
and advisories at ftp://ftp.secnet.com/advisories

You can browse our web site at http://www.secnet.com

You can subscribe to our security advisory mailing list by sending mail to
majordomo () secnet com with the line "subscribe sni-advisories" in the body of
the message.

Copyright Notice
The contents of this advisory are Copyright (C) 1997 Secure Networks Inc,
and may be distributed freely provided that no fee is charged for
distribution, and that proper credit is given.

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