CERT Advisory 053

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Published in 

CERT Advisory

 · 4 years ago

  

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

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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. 

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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. 
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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 (192.88.209.5). 

 
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