Cisco Security Advisory



Thu Jun 1 16:27:08 PDT 1995

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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

message.



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@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@cisco.com.



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Details



  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 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.



  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.



     or



        o 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



  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

     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

     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.



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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:



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



     and select the version of software to download.



     For non-registered users open a URL to:



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



     When prompted for a code, please enter:



              certjun2



     for a list of available files to download.



   * 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.



   * 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



               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:



              certjun2



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