CERT Vendor-Initiated Bulletin VB-95:06 - Cisco

CERT Bulletin cert-advisory at cert.org
Thu Aug 10 20:36:37 UTC 1995

CERT Vendor-Initiated Bulletin VB-95:06
August 10, 1995

Topic: Vulnerability in Cisco's IOS software
Source: Cisco Systems

To aid in the wide distribution of essential security information,
the CERT Coordination Center is forwarding the following information
from Cisco Systems. 


If you have already seen this and are certain that you have correctly
configured your routers, you do not need to take any further action.
However, if you have not seen this Cisco Security Advisory or are
not certain that your routers are configured correctly, we urge you
to review this bulletin and check the configuration of your router.

Cisco contact information is included in the forwarded text below;
please contact them if you have any questions or need further information.

========================FORWARDED TEXT STARTS HERE============================

Cisco Security Advisory

Thu Jun 1 16:27:08 PDT 1995

The following describes a vulnerability in Cisco's IOS software when the
'established' keyword is used in extended IP access control lists. This bug
can, under very specific circumstances and only with certain IP host
implementations, allow unauthorized packets to circumvent a filtering router.
This vulnerability is present in the following IOS software versions:

    10.3(1) through 10.3(2)
    10.2(1) through 10.2(5)
    10.0(1) through 10.0(9)

...and all previous versions of Cisco software.

If you are running any of these IOS versions on a product that uses IP
extended access lists, and you are using the 'established' keyword in these
lists, then Cisco strongly recommends that you take immediate action to remove
the vulnerability. You can determine what version of IOS you are running by
issuing the following command:

    show version

The recommended action is to upgrade to a more recent version of IOS, or take
one of the immediate workaround actions described below. The vulnerability is
fixed by in the following official software releases:

     10.0(10) or later
     10.2(6) or later
     10.3(3) or later

(For reference, the Cisco update identifier for this fix is "CSCdi34061".)

Customers may obtain software upgrades without going through the Cisco's
Technical Assistance Center (TAC) via the Cisco Information Online (CIO)
service. Instructions for downloading are available at the end of this

You may also contact your Cisco distributor or contact Cisco's TAC for more
information. The TAC can be reached by phone at 800-553-2447, by e-Mail at
tac at cisco.com or via the World Wide Web at http://www.cisco.com (for
registered CIO users only). In Europe, you can contact the TAC by phone at
32-2-778-42-42 or via e-Mail to euro-tac at cisco.com.



  1. Description

     A bug in Cisco's extended IP access list implementation can, under very
     specific circumstances, allow a user to bypass IP packet filtering. This
     may permit unintended IP traffic to pass through your firewall setup.

     To determine if you are vulnerable, look through your configuration. The
     configuration can be displayed by enabling and then entering the command
     "write term".

     If you see an access list line using a list number in the range of 100
     through 199 that permits or denies TCP traffic and contains the word
     'established' near the end of the line, you may be vulnerable.

     An example line might look like:

     In IOS 10.3:

             access-list 100 permit tcp any any established

     In IOS 10.2 or earlier:

             access-list 100 permit tcp

     If you do not meet this test, then you are not vulnerable. You do not
     need to do anything.

  2. Workaround

     The following actions will remove the vulnerability:

        o Rewrite the access list parameters so the 'established' keyword is
          not necessary. This does not simply mean that you may remove the
          'established' keyword, but rather that you will need to re-design
          your access lists to provide similar functionality without using the
          established mechanism.


        o Disable the interfaces to which the access list is applied using the
          'shutdown' interface subcommand.


                      router(config)#interface ethernet 0

  3. Solution

     Obtain and install the appropriate release of IOS software as described
     above. For assistance, contact Cisco's TAC.

  4. Technical Comments

     This problem is caused by an obscure but common design flaw that we
     believe exists in many router/firewall vendor's packet filtering

     Owners of non-Cisco hardware who use IP packet filtering features similar
     to Cisco's "extended access lists" as part of a firewall system may wish
     to contact their vendor to confirm that this vulnerability does not exist
     in their system. (Technical discussions about the problem have already
     occurred in the appropriate forum.)

     This vulnerability can only be exploited with certain IP host
     implementations (we do not have information on which implementations are
     susceptible). Cisco suggests that all routers configured to filter IP
     packets based upon the 'established' mechanism be upgraded.


Software Upgrades

Software upgrades may be obtained via any of the following mechanisms:

   * World Wide Web (WWW):

     For registered CIO users please open a URL to:


     and select the version of software to download.

     For non-registered users open a URL to:


     When prompted for a code, please enter:


     for a list of available files to download.

   * FTP:

     ftp cio.cisco.com and at the initial (username) prompt, enter:


     At the password prompt, enter your e-mail address. Then:

              get README.certjun2

     This file contains a list of files available that close this
     vulnerability. Please examine this list to determine which files you need
     and then download them.

   * Character-based "CIO Classic":

     For access, the following connection options are offered:

        o telnet:

        o Dial-up modem:
               In Europe: +33 1 64 46 40 82
               In the US: (408) 526 8070

               Settings: vt100, N81, up to 14.4Kbps

     Enter either as a guest or registered user and navigate to the topic:

              Software Updates
                Special Files

     At the prompt for a code, please enter:


     A list of files will be displayed for you to select and download.


=========================FORWARDED TEXT ENDS HERE=============================

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