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TUCoPS :: Cisco :: ciacf025.txt

Cisco IOS Soft Vulnerability



                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                             INFORMATION BULLETIN

		   Cisco IOS Router Software Vulnerability

June 2, 1995 1000 PDT                                               Number F-25

PROBLEM:	A vulnerability exists in Cisco's IOS software when the 
		keyword 'established' is used with the extended IP access 
		control list feature.  This vulnerability can, under
		limited circumstances, allow unauthorized packets to 
		circumvent a filtering router.  This may permit unintended
		IP traffic to pass through a firewall setup.	
PLATFORM:	IOS software levels:
			10.3 (1) through 10.3 (2)
			10.2 (1) through 10.2 (5)
			10.0 (1) through 10.0 (9)
DAMAGE:		This vulnerability may permit unintended IP traffic to pass 
		through a firewall setup.  Networks protected from the 
		firewall could be at risk and/or compromised.        
SOLUTION:	Obtain and install the appropriate release of IOS software.
AVAILABILITY:	IOS software upgrades are available now.

VULNERABILITY	Due to the fact that unintended IP traffic may be able to 
ASSESSMENT:	pass through a router used as part of a firewall, internal 
		networks may be at risk and/or compromised.  We urge all 
		owners of Cisco routers that are configured to filter IP 
		packets based upon the 'established' mechanism be upgraded 
		as soon as possible.
		In addition, this problem may also exist in other vendors 
		routers who support IP packet filtering and "extended 
		access lists" features.   We urge users who are using a 
		router (configured with IP packet filtering and extended 
		access list mechanisms) as part of a firewall, to contact 
		their vendors to confirm that this vulnerability does not 

	     CRITICAL Information for Cisco IOS Router Software

CIAC has obtained information from Cisco, describing a vulnerability in
Cisco's IOS software.  Specific details of the vulnerability are provided in
the included Cisco Security Advisory.


			  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 or via the
World-Wide-Web at  In Europe you can contact TAC by
phone at 32-2-778-42-42 or via E-Mail to

- - ----------------------------------------------------------------------------

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

    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 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.


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

	    router(config)#interface ethernet 0

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

    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.

- - ----------------------------------------------------------------------------

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:

     and select the 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 and at the initial (username) prompt, enter:


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

         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

        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:


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

- - ----------------------------------------------------------------------------

CIAC wishes to thank Paul Traina of Cisco for the information provided
in this bulletin.

CIAC, the Computer Incident Advisory Capability, is the computer security 
incident response team for the U.S. Department of Energy. CIAC is located at 
the Lawrence Livermore National Laboratory in Livermore, California. CIAC is 
also a founding member of FIRST, the Forum of Incident Response and Security 
Teams, a global organization established to foster cooperation and 
coordination among computer security teams worldwide. 

CIAC services are available to DOE and DOE contractors, and CIAC can be 
contacted at:
    Voice:    510-422-8193
    FAX:      510-423-8002
    STU-III:  510-423-2604

For emergencies and off-hour assistance, DOE and DOE contractor sites may 
contact CIAC 24-hours a day. During off hours (5PM - 8AM PST), call the CIAC 
voice number 510-422-8193 and leave a message, or call 800-759-7243 
(800-SKY-PAGE) to send a Sky Page. CIAC has two Sky Page PIN numbers, the 
primary PIN number, 8550070, is for the CIAC duty person, and the secondary 
PIN number, 8550074 is for the CIAC Project Leader.

Previous CIAC notices, anti-virus software, and other information are 
available from the CIAC Computer Security Archive. 

   World Wide Web:
   Anonymous FTP: (
   Modem access:  	(510) 423-4753 (14.4K baud)
                  	(510) 423-3331 (9600 baud)

CIAC has several self-subscribing mailing lists for electronic publications:
1. CIAC-BULLETIN for Advisories, highest priority - time critical information 
   and Bulletins, important computer security information;
2. CIAC-NOTES for Notes, a collection of computer security articles;
3. SPI-ANNOUNCE for official news about Security Profile Inspector (SPI) 
   software updates, new features, distribution and availability;
4. SPI-NOTES, for discussion of problems and solutions regarding the use of 
   SPI products.

Our mailing lists are managed by a public domain software package called 
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yourself) to one of our mailing lists, send the following request as the 
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SPI-NOTES for list-name and valid information for LastName FirstName and 
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E-mail to
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  e.g.,	subscribe ciac-notes OUHara, Scarlett W. 404-555-1212 x36

You will receive an acknowledgment containing address, initial PIN, and 
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get help. 

PLEASE NOTE: Many users outside of the DOE and ESnet computing communities
receive CIAC bulletins.  If you are not part of these communities, please
contact your agency's response team to report incidents. Your agency's team
will coordinate with CIAC. The Forum of Incident Response and Security Teams
(FIRST) is a world-wide organization. A list of FIRST member organizations and
their constituencies can be obtained by sending email to
with an empty subject line and a message body containing the line: send

This document was prepared as an account of work sponsored by an agency of the
United States Government. Neither the United States Government nor the
University of California nor any of their employees, makes any warranty,
express or implied, or assumes any legal liability or responsibility for the
accuracy, completeness, or usefulness of any information, apparatus, product,
or process disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial products, process,
or service by trade name, trademark, manufacturer, or otherwise, does not
necessarily constitute or imply its endorsement, recommendation or favoring by
the United States Government or the University of California. The views and
opinions of authors expressed herein do not necessarily state or reflect those
of the United States Government or the University of California, and shall not
be used for advertising or product endorsement purposes.

CIAC BULLETINS ISSUED IN FY95 (Previous bulletins available from CIAC)
(F-01)	SGI IRIX serial_ports Vulnerability
(F-02)	Summary of HP Security Bulletins
(F-03)	Restricted Distribution
(F-04)	Security Vulnerabilities in DECnet/OSI for OpenVMS
(F-05)	SCO Unix at, login, prwarn, sadc, and pt_chmod 
          Patches Available
(F-06)	Novell UnixWare sadc, urestore, and suid_exec Vulnerabilities
(F-07)	New and Revised HP Bulletins
(F-08)	Internet Address Spoofing and Hijacked Session Attacks
(F-09)	Unix /bin/mail Vulnerabilities
(F-10)	HP-UX Remote Watch
(F-11)	Unix NCSA httpd Vulnerability
(F-12)	Kerberos Telnet Encryption Vulnerability
(F-13)	Unix sendmail vulnerabilities
(F-14)	HP-UX Malicious Code Sequences
(F-15)	HP-UX "at" and "cron" vulnerabilities
(F-16)	SGI IRIX Desktop Permissions Tool Vulnerability
(F-17)	Limited Distribution
(F-18)	MPE/iX Vulnerabilities
(F-19)	Protecting HP-UX Systems Against SATAN
(F-20)	Security Administrator Tool for Analyzing Networks (SATAN)
(F-21)  Protecting SUN OS Systems Against SATAN
(F-22)  SATAN Password Disclosure
(F-23)	Protecting IBM AIX Systems Against SATAN
(F-24)  Protecting SGI IRIX Systems Against SATAN

CIAC NOTES ISSUED IN FY1995 (Previous Notes available from CIAC)
04c	December 8, 1994
05d	January 11, 1995
06	March 22, 1995
07	March 29, 1995
08	April 4, 1995
09	April 24, 1995

Version: 2.6.2


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