Original issue date: December 10,1992<BR>
Last revised: September 19, 1997<BR>
Attached copyright statement

<P>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.

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

<P>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: 
	<A HREF=mailto:tac@cisco.com>tac@cisco.com</A> 
) for more information.

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

<P>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.

<H2>I. Description</H2>

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.

<H2>II.  Problem</H2>

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 &quot;established&quot; 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.

<H2>III. Workaround</H2>

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

<P>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

<H2>IV.  Solution</H2>

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.


<P>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.


<!--#include virtual="/include/footer_nocopyright.html" -->
<P>Copyright 1992 Carnegie Mellon University.</P>


Revision History
September 19,1997  Attached copyright statement