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>>                                                      CERT-NL, 01-Mar-2000 <<
>> All CERT-NL information has been moved to http://cert.surfnet.nl.  Links  <<
>> to CERT-NL information contained in this advisory are therefore outdated. <<
>>                                                                           <<
>> CERT-NL also has stopped the CERT-CC-Mirror service.  Due to this the     <<
>> links to the CERT-CC mirror are obsolete. Visit the CERT-CC site for the  <<
>> complete CERT-CC advisory texts:                     http://www.cert.org  <<
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Security Advisory                                                       CERT-NL
===============================================================================
Author/Source : Erik-Jan Bos (CERT-NL)                      Index  :    S-92-24
Distribution  : SURFnet constituency                        Page   :          1
Classification: External                                    Version:      Final
Subject       : Cisco Access List Vulnerability             Date   :  11-dec-92
===============================================================================

CERT-NL has received information concerning a vulnerability with Cisco
routers when access lists are utilized.  This vulnerability is present
in Cisco software releases 8.2, 8.3, 9.0 and 9.1.

Cisco Systems and CERT-NL strongly recommend that sites using Cisco
routers for firewalls take immediate action to eliminate this
vulnerability from their networks.

This vulnerability is fixed in Cisco software releases 8.3 (update 5.10),
9.0 (update 2.5), 9.1 (update 1.1) and in all later releases.  Customers
who are using software release 8.2 and do not want to upgrade to a later
release should contact Cisco's Technical Assistance Center (TAC) at
800-553-2447 (Internet: tac@cisco.com) for more information.

The following interim releases are available via anonymous FTP from
ftp.cisco.com (131.108.1.111).

Note: this FTP server will not allow filenames to be listed or matched
with wildcards.  You also cannot request the file by its full pathname.
You must first cd to the desired directory (beta83_dir, beta90_dir, or
beta91_dir) and then request the file desired (gs3-bfx.83-5.10, etc.).

 Release (Update)  Filename                       Size       Checksum
     8.3 (5.10)    /beta83_dir/gs3-bfx.83-5.10    1234696    02465  1206
     9.0 (2.5)     /beta90_dir/gs3-bfx.90-2.5     1705364    47092  1666
     9.1 (1.1)     /beta91_dir/gs3-k.91-1.1       2005548    59407  1959

These releases are also available on Cisco's Customer Information On-Line
(CIO) service for those customers having a maintenance contract.
Other customers may obtain these releases through Cisco's Technical
Assistance Center or by contacting their local Cisco distributor; for the
Netherlands that is Geveke Electronics BV in Amsterdam, telephone number
020-5869711, faxnumber 020-5869777.

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

I.   Description

     A vulnerability in Cisco access lists allows some packets to be
     erroneously routed which one would expect to be filtered by the access
     list and vice-versa.  This vulnerability can allow unauthorized traffic
     to pass through the gateway and can block authorized traffic.

II.  Problem

     If a Cisco router is configured to use extended IP access lists for
     traffic filtering on an MCI, SCI, cBus or cBusII interface, and the IP
     route cache is enabled, and the "established" keyword is used in the
     access list, then the access list can be improperly evaluated.  This
     can permit packets which should be filtered and filter packets which
     should be permitted.

III. Workaround

     This vulnerability can be avoided by either rewriting the extended
     access list to not use the "established" keyword, or by configuring
     the interface to not use the IP route cache.  To disable the IP route
     cache, use the configuration command "no ip route-cache".

     Example for a serial interface:
        router>enable

        Password:
        router#configure terminal

        Enter configuration commands, one per line.
        Edit with DELETE, CTRL/W, and CTRL/U; end with CTRL/Z
        interface serial 0
        no ip route-cache
        ^Z
        router#write memory

IV.  Solution

     Obtain and install the appropriate interim release listed above.
     Sites which are not experienced at this installation process
     should contact the TAC center at 800-553-2447 for assistance.

- ---------------------------------------------------------------------------
The CERT Coordination Center wishes to thank Keith Reynolds of the
Santa Cruz Operation for his assistance in identifying this problem
and Cisco Systems for their assistance in providing technical information
for this advisory.
- ---------------------------------------------------------------------------

CERT-NL wishes to thank the CERT Coordination Center for bringing this
information to our attention.

==============================================================================
CERT-NL is the Computer Emergency Response Team for SURFnet customers. SURFnet
is the Dutch network for educational, research and related institutes. CERT-NL
is a member of the Forum of Incident Response and Security Teams (FIRST).

All CERT-NL material is available under:
  http://cert.surfnet.nl/

In case of computer or network security problems please contact your local
CERT/security-team or CERT-NL  (if your institute is NOT a SURFnet customer
please address the appropriate (local) CERT/security-team).

CERT-NL is one/two hour(s) ahead of UTC (GMT) in winter/summer,
i.e. UTC+0100 in winter and UTC+0200 in summer (DST).

   Email:     cert-nl@surfnet.nl     ATTENDED REGULARLY ALL DAYS
   Phone:     +31 302 305 305        BUSINESS HOURS ONLY
   Fax:       +31 302 305 329        BUSINESS HOURS ONLY
   Snailmail: SURFnet bv
              Attn. CERT-NL
              P.O. Box 19035
              NL - 3501 DA  UTRECHT
              The Netherlands

NOODGEVALLEN:    06 22 92 35 64      ALTIJD BEREIKBAAR
EMERGENCIES : +31 6 22 92 35 64      ATTENDED AT ALL TIMES
CERT-NL'S EMERGENCY PHONENUMBER IS ONLY TO BE USED IN CASE OF EMERGENCIES:
THE SURFNET HELPDESK OPERATING THE EMERGENCY NUMBER HAS A *FIXED*
PROCEDURE FOR DEALING WITH YOUR ALERT AND WILL IN REGULAR CASES RELAY IT
TO CERT-NL IN AN APPROPRIATE MANNER. CERT-NL WILL THEN CONTACT YOU.
===============================================================================

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