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TUCoPS :: Unix :: General :: cert0078.txt

CERT Advisory CA-94:06 utmp vulnerability


CERT(sm) Advisory CA-94:06
Original issue date: March 21, 1994
Last revised: August 30, 1996
              Information previously in the README was inserted
              into the advisory.

              A complete revision history is at the end of this file.
Topic:  Writable /etc/utmp Vulnerability

The CERT Coordination Center has received information concerning a
vulnerability that exists on systems where the file /etc/utmp is writable
by any user on the system.

This vulnerability is being actively exploited; please review CERT Advisory
CA-94:01 "Ongoing Network Monitoring Attacks."

The problem is known to affect Sun Microsystems, Inc. SunOS 4.1.X and
Solaris 1.1.1 operating systems. Solbourne Computer, Inc. and other Sparc
products using SunOS 4.1.X or Solaris 1.1.1 are also affected. Solaris 2.x
and SunOS 4.1.3_U1 (Solaris 1.1.1) are not affected by this problem.

Patches can be obtained from Sun Answer Centers worldwide.  They are also
available via anonymous FTP from in the /systems/sun/sun-dist 
directory, and in Europe from in the /sun/fixes directory.

We queried several vendors in addition to Sun.  The following vendors
reported that their operating systems, as distributed by the vendor, are
not affected by this problem:

Convex Computer Corporation             Digital Equipment Corporation
Data General Corporation                Hewlett-Packard Company
IBM                                     Intergraph
Motorola, Inc.                          NeXT, Inc.
Pyramid Technology Corporation          Sequent Computer Systems
Sony Corporation

Currently, we are not aware of /etc/utmp being writable on other systems.
If your operating system is not explicitly mentioned above, and if you 
determine that /etc/utmp is writable by someone other than root, we 
encourage you to contact your vendor.

If /etc/utmp on your system is writable only by the root account, you need
not be concerned about the vulnerability. 

We recommend that sites check their /etc/utmp file to be sure it is not
writable by users other than root.  If it is generally writable, you should
obtain patches from the system vendor or protect /etc/utmp as described below.

- -----------------------------------------------------------------------------

I.   Description

     If the file /etc/utmp is writable by users other than root,
     programs that trust the information stored in that file can
     be subverted.

II.  Impact

     This vulnerability allows anyone with access to a user account
     to gain root access.

III. Solution

     The solutions to this vulnerability are to either (a) protect the file,
     or (b) patch all the programs that trust it.

     Note that SunOS 4.1.3_U1 (Solaris 1.1.1) is _not_ vulnerable to this

     A.  To protect the file, make /etc/utmp writable only by root:

                 # chown root /etc/utmp
                 # chmod 644 /etc/utmp

     B.  Patches from Sun Microsystems

     Program     Patch ID    Patch File Name
     -------     ---------   ---------------
     in.comsat   100272-07   100272-07.tar.Z
     dump        100593-03   100593-03.tar.Z
     syslogd     100909-02   100909-02.tar.Z
     in.talkd    101480-01   101480-01.tar.Z
     shutdown    101481-01   101481-01.tar.Z
     write       101482-01   101482-01.tar.Z

     Program     BSD         SVR4        MD5 Digital Signature
                 Checksum    Checksum
     -------     ---------   ---------   --------------------------------
     in.comsat   26553  39   64651  78   912ff4a0cc8d16a10eecbd7be102d45c
     dump        52095 242   41650 484   cdba530226e8735fae2bd9bcbfa47dd0
     syslogd     61539 108   38239 216   b5f70772384a3e58678c9c1f52d81190
     in.talkd    47917  44   32598  88   5c3dfd6f90f739100cfa4aa4c97f01df
     shutdown    46562  80   56079 159   bfc257ec795d05646ffa733d1c03855b
     write       61148  41   48636  81   f93276529aa9fc25b35679ebf00b2d6f

     C. Clarifications added April 1, 1994

        1) If you make /etc/utmp writable only by root, this should only affect
           programs that allocate pseudo terminal interfaces and want to add
           an appropriate entry to the /etc/utmp file.  Such programs include
           script(1), cmdtool(1), gfxtool(1), shelltool(1), and tektool(1).
           These programs will no longer be able to add an entry to /etc/utmp
           which means that programs such as who(1), syslogd(1), and others
           that use /etc/utmp will not know that an account is using that
           pseudo tty. 

        2) No program should be made setuid root just to workaround this
           problem. Setuid programs must be written very carefully to avoid
           creating yet more vulnerabilities.

        3) The installation instructions on the syslogd patch do not point out
           that, until you stop and restart syslogd (or reboot the system),
           the old version is still running and the security hole has not been

- ---------------------------------------------------------------------------

If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in Forum of Incident
Response and Security Teams (FIRST).

Internet E-mail:
Telephone: 412-268-7090 (24-hour hotline)
           CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4),
           and are on call for emergencies during other hours.

CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh, PA 15213-3890

Past advisories, information about FIRST representatives, and other
information related to computer security are available for anonymous
FTP from

Copyright 1994, 1996 Carnegie Mellon University
This material may be reproduced and distributed without permission provided
it is used for noncommercial purposes and the copyright statement is

CERT is a service mark of Carnegie Mellon University.

Revision history

Aug. 30, 1996  Information previously in the README was inserted
               into the advisory.
Apr. 01, 1994  Intro. and Sec. III - added note that SunOS 4.1.3_U1 is not
Apr. 01, 1994  Sec. III.C - added this new section, which contains

Version: 2.6.2


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