CERT Advisory - Cisco Access List Vulnerability

CERT Advisory cert-advisory-request at cert.org
Thu Dec 10 19:44:59 UTC 1992

CA-92:20                         CERT Advisory
                               December 10, 1992
                        Cisco Access List Vulnerability

The CERT Coordination Center 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 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 at cisco.com) for more information.

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

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.


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

Internet E-mail: cert at cert.org
Telephone: 412-268-7090 (24-hour hotline)
           CERT personnel answer 7:30 a.m.-6:00 p.m. EST(GMT-5)/EDT(GMT-4),
           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 cert.org (

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