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Home : Advisories : Cisco Access List Vulnerability

Title: Cisco Access List Vulnerability
Released by: CERT
Date: 10th December 1992
Printable version: Click here

Hash: SHA1


Laste Revised:  September 19,1997

                Attached copyright statement

                              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@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@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 (

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

Copyright 1992 Carnegie Mellon University. Conditions for use, disclaimers,

and sponsorship information can be found in

http://www.cert.org/legal_stuff.html and http://ftp.cert.org/pub/legal_stuff .

If you do not have FTP or web access, send mail to cert@cert.org with

"copyright" in the subject line.

CERT is registered in the U.S. Patent and Trademark Office.


Revision History:

September 19,1997  Attached Copyright Statement


Version: PGP for Personal Privacy 5.0

Charset: noconv





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