Cisco Access List Vulnerability
Original issue date: December 10,1992
Last revised: September 19, 1997
Attached copyright statement
A complete revision history is at the end of this file.
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.
Systems and CERT strongly recommend that sites using Cisco routers
for firewalls take immediate action to eliminate this vulnerability from
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
) for more information.
The following interim releases are available via anonymous FTP from
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.
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.
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.
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:
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
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.
This document is available from:
CERT/CC Contact Information
Phone: +1 412-268-7090 (24-hour hotline)
Fax: +1 412-268-6989
CERT Coordination Center
Software Engineering Institute
Carnegie Mellon University
Pittsburgh PA 15213-3890
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Copyright 1992 Carnegie Mellon University.
September 19,1997 Attached copyright statement