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CERT Advisory 078
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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 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.
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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
problem.
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
closed.
- ---------------------------------------------------------------------------
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, 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
included.
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
vulnerable.
Apr. 01, 1994 Sec. III.C - added this new section, which contains
clarifications.
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