TUCoPS :: Linux :: Red Hat/Fedora :: ciach067.txt

Redhat Linux X11 Libraries Buffer Overflow



                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
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                             INFORMATION BULLETIN

                    Red Hat Linux X11 Libraries Buffer Overflow

May 30, 1997 22:00 GMT                                             Number H-67
PROBLEM:       A problem has been identified in the X11 libraries that allows
               a buffer overflow condition.
PLATFORM:      This problem affects all Red Hat Linux machines with X Windows
DAMAGE:        This vulnerability allows local users to gain unauthorized root
               access to a system.
SOLUTION:      Apply the necessary patches indicated below.
VULNERABILITY  Information involving this vulnerability has been made publicly
ASSESSMENT:    available.

[  Start Linux Security Alert ]

     Buffer overflow in the resource handling code of the libXt (X11R6)

                              Thu May 29, 1997

                 Distribution of this document is unlimited
              Copyright (C) Alexander O. Yuriev (alex@yuriev.com)
                                 Net Access


     A buffer overflow was found in the resource handling section of the X11
     system (libXt). As this is a problem with libXt iself, every program
     using libXt is affected, including core programs such as xterm and
     programs derived from it. Of course only suid and sgid programs can be
     exploited to gain access to gain extra priviledges.

Permanent Solution

     The permanent solution requires fixing the libXt. It is recommended
     that you utilize temporary solution. This buffer overflow does not
     exist in XFree86 3.3 code. It is recommended that you upgrade to
     XFree86 3.3 as soon as it becomes available.

     Currently fixed versions of fixed libraries are available for:

        o Red Hat Linux 4.0, 4.1, 4.2 from Red Hat Software

Temporary solution

     The workaround requires identifying and temporary disabling suid
     programs in the X11R6 tree. The following sequence of commands can be
     used to find all suid and sgid programs of the X11 tree:

          $ cd /usr/X11/bin
          $ find . -type f -a \( -perm -2000 -o -perm -4000 \) -print

     As the output, these commands would produce a list of suid or sgid
     programs in directories starting from the current working directory
     (/usr/X11/bin) to the end of the tree. A typical output would look


     Determine if you use every program in question. Look at the manual
     pages to see if you really need it at this time. I personally have no
     idea why Red Hat did not remove the dga(1) program of the XFree86
     distribution shipped with Red Hat 4.1 as dga(1) manual page states:

          dga - test program for the Xfree86-DGA extension

     Assuming that the DGA extension is required, the test program should be
     used only by "root". Therefore, the suid bit is not needed and should
     be removed. Same logic should be applied to other suid/sgid programs.
     At this time you probably should remove suid bit from the dga(1) or at
     least make it non-world executable. Use commands:

          # chmod 111 dga
          # chattr +I dga

     to disable suid bit on a dga binary and make it immutable. Use the same
     method to evaluate all other suid programs.

     Programs that should be run by root only, should never be suid to root,
     or at least should not be world executable. X Display Manager, xdm(1),
     falls into this category.

     If you need the functionality provided by the vulnerable program,
     disable execution for that program and add trusted accounts that need
     to run the program into the group which own the program. You must
     realize that by doing this you are allowing those who have access to
     the trusted accounts exploit the vulnerability and gain access to the
     euid of the program.

XTERM(1) and xterm derived programs

     Unfortunately, you cannot remove suid bit from the xterm(1) and
     programs derived from it withot losing part of functionality. The
     advice by authors of exploits from bugtraq to squash suid bit prevents
     xterm(1) from changing ownerships of tty devices allowing any user on a
     system to read information from terminal devices.

     This looks like a lose-lose situation unless you are willing to disable
     xterm(1) program completely (and leave with it being disabled ) until
     the fixed version becomes available. Basically, you should consider
     risks of someone from your system running xterm(1) and gaining root
     access to a system vs. not being able to run xterm(1) at all and vs.
     running xterm(1) as non-suid application which would allow one user to
     intercept keystrokes of another. It is your choice but no matter what
     you decide to do, think about the consequences first.

Vendor fixes

   * Red Hat Linux from Red Hat Software

        o Red Hat Linux/Alpha 4.1, 4.2


        o Red Hat Linux/Intel 4.0, 4.1, 4.2


        o Red Hat Linux/SPARC 4.0, 4.1, 4.2


          Please verify the signature of RPMs using the rpm --checksig
          command. The RPMs are signed with the PGP key of Red Hat Software:

          pub 1024/CBA29BF9 1996/02/20 Red Hat Software, Inc.


     The exploits were posted in bugtraq mailing list by Ming Zhang. Erik
     Troan (ewt@redhat.com) from Red Hat Software provided information about
     the XFree86 3.3, as well as fixes for the Red Hat Linux

[ End Linux Security Alert ]


CIAC wishes to acknowledge the contributions of Alexander O. Yuriev and others
for the information contained in this bulletin.

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