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CERT* Advisory CA-97.19
Original issue date: June 25, 1997
Last revised: --
Topic: lpr Buffer Overrun Vulnerability
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            The technical content of this advisory was 
            originally published by AUSCERT (AA-96.12), 
            who last updated the information on June 19, 1997.
            We use it here with their permission.

- ---------------------------------------------------------------------------
There is a vulnerability in the BSD-based printing software, lpr, available on
a variety of Unix platforms. This vulnerability may allow local users to gain
root privileges.

Exploit information involving this vulnerability has been publicly available
for some time. Recently, the CERT/CC has received reports that the
vulnerability is being actively exploited.

We recommend installing a vendor patch if one is available. Until you can do
so, we recommend using the wrapper described in Section III.B.

We will update this advisory as we receive additional information.
Please check our advisory files regularly for updates that relate to your
- ----------------------------------------------------------------------------

I.  Description

    A vulnerability exists in the BSD-based lpr printing package found on many
    Unix systems.

    Due to insufficient bounds checking on arguments that are supplied by
    users, it is possible to overwrite the internal stack space of the lpr
    program while it is executing. This can allow an intruder to cause lpr to
    execute arbitrary commands by supplying a carefully designed argument to
    lpr. These commands will be run with the privileges of the lpr program.
    When lpr is installed setuid or setgid, it may allow intruders to gain
    those privileges.

    When lpr is setuid root, it may allow intruders to run arbitrary commands
    with root privileges.

    For information from vendors relating to this vulnerability, please check
    Appendix A of this advisory. In addition to the products mentioned, be
    aware that platforms using the BSD-based lpr systems, in which lpr is
    installed setuid or setgid, may also be vulnerable.

    Note also that the vulnerability described in this advisory is not present
    in the LPRng printing package.

II. Impact

    Local users may gain root privileges. It is necessary to have access to an
    account on the system to exploit this vulnerability.

III. Solution

    The lpr printing package is available on many different systems. As vendor
    patches are made available sites are encouraged to install them. Until
    vendor patches are available, we recommend applying the workaround
    referred to in III.B.

    A. Install vendor patches

       Specific vendor information has been placed in Appendix A. If the BSD-
       based lpr printing software is used and your vendor is not listed in
       Appendix A, please contact your vendor directly.

    B. Install lpr wrapper

       Until you can install a vendor patch, we encourage you install a
       wrapper developed by AUSCERT to help prevent lpr being exploited using
       this vulnerability. 

       The source for the wrapper, including installation instructions, can be
       found at

       This wrapper replaces the lpr program and checks the length of the
       command line arguments which are passed to it. If an argument exceeds a
       certain predefined value (MAXARGLEN), the wrapper exits without
       executing the lpr command. The wrapper program can also be configured
       to syslog any failed attempts to execute lpr with arguments exceeding
       MAXARGLEN. For further instructions on using this wrapper, please read
       the comments at the top of overflow_wrapper.c.  

       When compiling overflow_wrapper.c for use with lpr, AUSCERT recommends
       defining MAXARGLEN to be 32.  

       The MD5 checksum for the current version of overflow_wrapper.c can be
       retrieved from
       The CHECKSUM file has been digitally signed using the AUSCERT PGP key.

Appendix A  Vendor information

Below is a list of the vendors who have provided information. We will update
this appendix as we receive additional information. If you do not see your
vendor's name, please contact the vendor directly.

Berkeley Software Design, Inc. (BSDI)
   BSD/OS 3.0 is not vulnerable to the problem.

   BSDI have issued a patch which addresses this vulnerability under
   BSD/OS 2.1.  This patch is available from:

Digital Equipment Corporation
   Digital Equipment Corporation                         
   Software Security Response Team
   Copyright (c) Digital Equipment Corporation 1997. All rights reserved.
    This reported problem is not present for Digital's ULTRIX or
    Digital UNIX Operating Systems Software.
                        - DIGITAL EQUIPMENT CORPORATION  06/19/97

   This problem was fixed prior to the release of FreeBSD 2.1.6 and 2.2.
   Users running older versions of the OS should review the security
   advisory describing this vulnerability (SA-96.18) at:

   Patches can be found in the directory:

IBM Corporation
   AIX is not vulnerable to the lpr buffer overflow.  The version of lpr
   shipped with AIX is not installed with the setuid bit turned on.

   IBM and AIX are registered trademarks of International Business Machines

   The Linux Emergency Response Team have released a Linux Security FAQ
   Update which addresses this vulnerability.  This Update contains
   information regarding various Linux distributions.  

   It is available from:

   The NeXT group has addressed the vulnerability described in this advisory
   in release 4.2 of OpenStep/Mach.

The Santa Cruz Operation, Inc. (SCO)
   SCO has determined that the following SCO operating systems
   are not vulnerable:
   - SCO CMW+ 3.0
   - SCO Open Desktop/Open Server 3.0, SCO UNIX 3.2v4
   - SCO OpenServer 5.0
   - SCO UnixWare 2.1
Sun Microsystems, Inc.
   All versions of Solaris are not affected. SunOS 4.1.3_U1 and SunOS 4.1.4
   are vulnerable. Sun recommends that sites using SunOS 4.1.3_U1 and SunOS
   4.1.4 apply the workaround provided in this advisory.

- ---------------------------------------------------------------------------
The CERT Coordination Center staff thanks AUSCERT for permission to republish
the information in their advisory AA-96.12. AUSCERT originally thanked
Alexander O. Yuriev, the FreeBSD security team, IBM, and the CERT/CC for their
assistance in the production of their advisory.
- ---------------------------------------------------------------------------

If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in the Forum of Incident Response
and Security Teams (see
CERT/CC Contact Information
- ----------------------------
Email    This email address is being protected from spambots. You need JavaScript enabled to view it.
Phone    +1 412-268-7090 (24-hour hotline)
                CERT personnel answer 8:30-5:00 p.m. EST(GMT-5) / EDT(GMT-4)
                and are on call for emergencies during other hours.
Fax      +1 412-268-6989
Postal address
         CERT Coordination Center
         Software Engineering Institute
         Carnegie Mellon University
         Pittsburgh PA 15213-3890
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   support a shared DES key or PGP. Contact the CERT/CC for more information.
   Location of CERT PGP key
Getting security information
   CERT publications and other security information are available from
   CERT advisories and bulletins are also posted on the USENET newsgroup
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* Registered U.S. Patent and Trademark Office.
Copyright 1997 Carnegie Mellon University
This material may be reproduced and distributed without permission provided
it is used for noncommercial purposes and the copyright statement is
The CERT Coordination Center is part of the Software Engineering Institute
(SEI). The SEI is sponsored by the U.S. Department of Defense.
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