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AA-97.19                        AUSCERT Advisory
                    SGI IRIX df Buffer Overrun Vulnerability
                                 26 May 1997

Last Revised: 17 December 1997

              Changed Section 3 to include vendor patch and bulletin
              information.

              A complete revision history is at the end of this file.

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AUSCERT has received information that a vulnerability exists in df(1),
distributed under IRIX versions 5.x and 6.x.  Other IRIX versions may also
be vulnerable.

This vulnerability may allow local users to gain root privileges.

Exploit information involving this vulnerability has been made publicly
available.

Vendor patches have been released addressing this vulnerability.

AUSCERT recommends that sites take the steps outlined in section 3 as soon
as possible.

This advisory will be updated as more information becomes available.

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1.  Description

    df(1) is a program used to display statistics about the amount of used
    and free disc space on file systems.

    Due to insufficient bounds checking on arguments which are supplied
    by users, it is possible to overwrite the internal stack space of the
    df program while it is executing.  By supplying a carefully designed
    argument to the df program, intruders may be able to force df to
    execute arbitrary commands.  As df is setuid root, this may allow
    intruders to run arbitrary commands with root privileges.

    Sites can determine if this program is installed by using:

  % ls -l /sbin/df

    df is installed by default in /sbin.  Sites are encouraged to check
    for the presence of this program regardless of the version of IRIX
    installed.

    Specific versions of IRIX which are vulnerable to this problem are
    listed in Section 3.3.

    Exploit information involving this vulnerability has been made publicly
    available.

2.  Impact

    Local users may gain root privileges.

3.  Workarounds/Solution

    Official vendor patches have been released by Silicon Graphics which
    address this vulnerability (Section 3.3).

    If the patches recommended by Silicon Graphics cannot be applied,
    AUSCERT recommends that sites prevent the exploitation of this
    vulnerability by immediately applying the workaround given in Section
    3.1.  If the df functionality is required by non-root users and patches
    can not be installed, AUSCERT recommends applying the workaround given
    in Section 3.2.

3.1 Remove setuid and non-root execute permissions

    To prevent the exploitation of the vulnerability described in this
    advisory, AUSCERT recommends that the setuid permissions be removed
    from the df program immediately.  As df will no longer work for non-root
    users, it is recommended that the execute permissions for them also
    be removed.

  # ls -l /sbin/df
  -r-sr-xr-x   1 root   sys     23136 Nov 22  1994 /sbin/df

  # chmod 500 /sbin/df
  # ls -l /sbin/df
        -r-x------   1 root   sys     23136 Nov 22  1994 /sbin/df

3.2 Install df wrapper

    AUSCERT has developed a wrapper to help prevent programs from being
    exploited using the vulnerability described in this advisory.  Sites
    which have a C compiler can obtain the source, compile and install
    the wrapper.

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

        ftp://ftp.auscert.org.au/pub/auscert/tools/overflow_wrapper/
                                               overflow_wrapper.c

    This wrapper replaces the df 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 df command.  The wrapper program can also be configured
    to syslog any failed attempts to execute df 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 df, AUSCERT recommends
    defining MAXARGLEN to be 32.

    The MD5 checksum for the current version of overflow_wrapper.c can be
    retrieved from:

        ftp://ftp.auscert.org.au/pub/auscert/tools/overflow_wrapper/CHECKSUM

    The CHECKSUM file has been digitally signed using the AUSCERT PGP key.

3.3 Install vendor patches

    Silicon Graphics has released a security bulletin containing
    information on this vulnerability including patch details. The original
    release of this bulletin can be retrieved from:

        ftp://sgigate.sgi.com/security/19970505-02-PX

    Information on patches which address the vulnerability described in
    this advisory has been extracted from the SGI bulletin and is listed
    below.

      OS Version     Vulnerable?     Patch #      Other Actions
      ----------     -----------     -------      -------------    

      IRIX 3.x          unknown      not avail    Note 1
      IRIX 4.x          unknown      not avail    Note 1
      IRIX 5.0.x        yes          not avail    Note 1
      IRIX 5.1.x        yes          not avail    Note 1
      IRIX 5.2          yes          not avail    Note 1
      IRIX 5.3          yes          2224
      IRIX 6.0.x        yes          not avail    Note 1
      IRIX 6.1          yes          not avail    Note 1
      IRIX 6.2          yes          2177
      IRIX 6.3          yes          2232
      IRIX 6.4          yes          2233

      NOTES

        1) upgrade operating system or see "Temporary Solution" section.

    "Temporary Solution" refers to the SGI bulletin but is equivalent to
    the suggestions given in our AUSCERT advisory in section 3.1.

    These patches can be retrieved from:

        http://www.sgi.com/Support/Secur/security.html

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The AUSCERT team have made every effort to ensure that the information
contained in this document is accurate.  However, the decision to use the
information described is the responsibility of each user or organisation.
The appropriateness of this document for an organisation or individual
system should be considered before application in conjunction with local
policies and procedures.  AUSCERT takes no responsibility for the
consequences of applying the contents of this document.

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

AUSCERT is located at The University of Queensland within the Prentice
Centre.  AUSCERT is a full member of the Forum of Incident Response and
Security Teams (FIRST).

AUSCERT maintains an anonymous FTP service which is found on:
ftp://ftp.auscert.org.au/pub/.  This archive contains past SERT and AUSCERT
Advisories, and other computer security information.

AUSCERT also maintains a World Wide Web service which is found on:
http://www.auscert.org.au/.

Internet Email: auscert@auscert.org.au
Facsimile:  (07) 3365 4477
Telephone:  (07) 3365 4417 (International: +61 7 3365 4417)
    AUSCERT personnel answer during Queensland business hours
    which are GMT+10:00 (AEST).
    On call after hours for emergencies.

Postal:
Australian Computer Emergency Response Team
Prentice Centre
Brisbane
Qld.  4072.
AUSTRALIA

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Revision History

17 Dec 1997     Silicon Graphics has released a security bulletin
                addressing the vulnerability described in this advisory.
                Section 3 has been modified to include vendor patch
                information.

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