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Home : Advisories : Writable /etc/utmp Vulnerability

Title: Writable /etc/utmp Vulnerability
Released by: CERT
Date: 21st March 1994
Printable version: Click here

Hash: SHA1


CERT(*) Advisory CA-94:06

Original issue date: March 21, 1994

Last revised:  September 19,1997

                updated copyright statement   

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 ftp.uu.net in the /systems/sun/sun-dist

directory, and in Europe from ftp.eu.net 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: cert@cert.org

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 info.cert.org.

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

Copyright 1994 Carnegie Mellon University. Conditions for use, disclaimers,

and sponsorship information can be found in

http://www.cert.org/legal_stuff.html and http://ftp.cert.org/pub/legal_stuff .

If you do not have FTP or web access, send mail to cert@cert.org with

"copyright" in the subject line.

CERT is registered in the U.S. Patent and Trademark Office.


Revision history

Sep. 19,1997   Updated copyright statement

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: PGP for Personal Privacy 5.0

Charset: noconv





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