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Security Advisory                                                       CERT-NL
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Author/Source : CERT-NL (Gert Meijerink)                    Index  :    S-94-07
Distribution  : World                                       Page   :          1
Classification: External                                    Version:      Final
Subject       : Writable /etc/utmp Vulnerability            Date   :  01-Apr-94
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CERT-NL received information on a vulnerability that exists on systems
where the file /etc/utmp is writable by any user on the system

The vulnerability is being actively exploited, so take care.

All honours to CERT/CC as provider of the following information 
(slightly abridged version):

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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-NL Advisory S-94-02 "Ongoing Network Monitoring Attacks".

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

Patches can be obtained from local Sun Answer Centers worldwide as well as
through anonymous FTP from:
ftp.nic.surfnet.nl in the surfnet/net-security/cert-nl/patches/sun-fixes
directory.

CERT 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. 

CERT recommends 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.

     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

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CERT-NL is the Computer Emergency Response Team, located in The
Netherlands. CERT-NL is a Full Member of the Forum of Incident Response
and Security Teams (FIRST). The constituency of CERT-NL are the SURFnet
connected institutions.
 
Past CERT-NL Security Bulletins and other CERT-NL related material can
be found on the anonymous FTP server of SURFnet bv:
"ftp.nic.surfnet.nl" [192.87.46.3], in the directory
"surfnet/net-security/cert-nl/docs/bulletin".  This information is also
available using email. Send an email saying "help" to
"mailserv@nic.surfnet.nl".
 
In case of computer or network security problems please contact CERT-NL
or the CERT of your own constituency. Please be aware of the fact that
we are one (when DST is in effect two) hour(s) ahead of Universal Time
Coordinated (i.e. UTC+0100 (UTC+0200)).
Email:     cert-nl@surfnet.nl
Phone:     +31 30 310290
Fax:       +31 30 340903
Snailmail: SURFnet bv
           Attn. CERT-NL
           P.O. Box 19035
           NL - 3501 DA  UTRECHT
           The Netherlands
A 7 * 24 hours phone number is available to SURFnet SSC's and FIRST
members on request.
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