Cisco IP packet filtering vulnerablility

[pst () cisco com (Paul Traina)]

Does anyone have any more info?

-Carson

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                          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 via Cisco's Customer Information On-Line
service,  instructions for downloading are available at the end
of this message.

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

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A)  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 0.0.0.0 255.255.255.255 0.0.0.0 255.255.255.255 established
                                                
    If you do not meet this test, then you are not vulnerable.  You
    do not need to do anything.

B)  Workaround

    The following actions will remove the vulnerability:

    -   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.

    or

    -   Disable the interfaces to which the access list is applied
        using the 'shutdown' interface subcommand:

        example:
            router(config)#interface ethernet 0
            router(config-if)#shutdown

C)  Solution

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

D)  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
    implementations.

    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
    occured 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.

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

A) World Wide Web (WWW):

     For registered CIO users please open a URL to:

         http://cio.cisco.com/kobayashi/Library_root.shtml

     and select the the version of software to download.

     For non-registered users open a URL to:

         http://cio.cisco.com/public/library/spc_req.shtml

     When prompted for a code, please enter:

         certjun2
 
     for a list of available files to download.
     
B) FTP:

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

         certjun2

     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.

C) Character-based "CIO Classic":

     For access, the following connection options are offered:

        o telnet
               cio.cisco.com 

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

         certjun2

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

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