[nsp] Cisco Security Advisory: Multiple Vulnerabilities in Access Control List Implementation for Cisco 12000 Series Internet Router

From: Cisco Systems Product Security Incident Response Team (psirt@cisco.com)
Date: Wed Nov 14 2001 - 10:30:00 EST


   Cisco Security Advisory: Multiple Vulnerabilities in Access Control List
             Implementation for Cisco 12000 Series Internet Router
Revision 1.0

  For Public Release 2001 November 14 08:00 (UTC -0800)

   Six vulnerabilities involving Access Control List (ACL) has been
   discovered in multiple releases of Cisco IOSŪ Software Release for
   Cisco 12000 Series Internet Routers. Not all vulnerabilities are
   present in all IOS releases and only line cards based on the Engine 2
   are affected by them.
   No other Cisco product is vulnerable.
   The workarounds are described in the Workarounds section.
   This advisory is available at
Affected Products

   Only Cisco 12000 Series Internet Routers with line cards based on
   Engine 2 are affected with these vulnerabilities. Not all line cards
   of a Cisco 12000 Series are affected by all vulnerabilities.
   Vulnerabilities are present in the underlying technology an individual
   line card is based upon. That technology is called "Engine". Currently
   Cisco is shipping line cards based on the following Engines: 0, 1, 2,
   3 and 4.
   To determine what Engine your card is based on, you need to log on the
   Cisco 12000 router and issue "sh diag" command while in enable mode.
   The engine type will be displayed as "L3 Engine: x" where x will be
   the corresponding number.
   The following example shows the output for an Engine 2 based line
          c12000#sh diag
          SLOT 1 (RP/LC 1 ): 1 Port Packet Over SONET OC-48c/STM-16
          Single Mode/SR SC-SC connector
                MAIN: type 41, 800-5271-01 rev A0 dev 0
                      HW config: 0x04 SW key: 00-00-00
                PCA: 73-3295-05 rev A0 ver 5
                      HW version 1.1 S/N SDK034004AY
                MBUS: Embedded Agent
                      Test hist: 0x00 RMA#: 00-00-00 RMA hist: 0x00
                DIAG: Test count: 0x00000000 Test results: 0x00000000
                L3 Engine: 2 - Backbone OC48 (2.5 Gbps)
                ^^^^^^^^^^^ <- Note the engine type
                [further output truncated]
   These vulnerabilities are affecting line cards based on Engine 2.

   Six vulnerabilities were found in IOS releases that are supporting
   Cisco 12000 platforms. Only line cards based on Engine 2 are affected.
          ACL will not block non initial fragments of a packet. This
          Cisco bug ID is adding a support for "fragment" keyword in the
          ACL. The White Paper Access Control Lists and IP Fragments
          describes how keyword fragment modifies behavior of ACL
          The keyword "fragment" in the compiled ACL (Turbo ACL) is
          ignored if a packet is destined to the router itself.
          The implicit "deny ip any any" rule at the end of an ACL is
          ignored if an ACL of exactly 448 entries is applied on an
          interface as an outgoing ACL. An ACL with any other number of
          rules, greater or less than 448, is unaffected by this
          A support for "fragment" keyword in an outgoing ACL is added.
          Previously, only incoming ACL supported this keyword and
          outgoing ACL was ignoring it.
          An outbound Access Control List (ACL) may not block all
          intended traffic on a router when an input ACL is configured on
          some, but not all, interfaces of a multi port Engine 2 line
          card. The prerequisite is that, the traffic in question, was
          not filtered by an inbound ACL on the ingress port. An ACL
          applied at the ingress point will work as expected and block
          desired traffic.
          Packet fragments are not filtered by the ACL despite using
          "fragment" keyword. The White Paper Access Control Lists and
          IP Fragments describes how keyword fragment modifies behavior
          of ACL (http://www.cisco.com/warp/public/105/acl_wp.html).

          The router will not block all traffic. By sending an offending
          traffic in packet fragments it is possible to circumvent the
          protection offered by ACL and cause Denial-of-Service for the
          protected IP address.
          It is possible to cause the Denial-of-Service on the router
          itself if sufficient amount of traffic is sent to the router.
          This offending traffic should be send as packet fragments.
          If an outgoing ACL contains exactly 448 entries and if explicit
          rule "deny ip any any" is not present as the last statement,
          the ACL will fail to drop packets. Our tests shows that only
          50% of packets are dropped. This may allow some undesired
          traffic to pass into the protected network thus violating
          security policy.
          Fragmented packets may be allowed into the protected network if
          the keyword "fragment" was applied to an outgoing ACL.
          This vulnerability can cause unwanted traffic to be allowed in
          and out of the protected network. The security based on an ACL
          will be breached completely.
          This vulnerability can be exploited to attack systems that are
          supposed to be protected by the ACL on the router.
Software Versions and Fixes

   Each row of the table describes a release train and the platforms or
   products for which it is intended. If a given release train is
   vulnerable, then the earliest possible releases that contain the fix
   and the anticipated date of availability for each are listed in the
   "Rebuild", "Interim", and "Maintenance" columns. A device running any
   release in the given train that is earlier than the release in a
   specific column (less than the earliest fixed release) is known to be
   vulnerable, and it should be upgraded at least to the indicated
   release or a later version (greater than the earliest fixed release
   When selecting a release, keep in mind the following definitions:
                Most heavily tested and highly recommended release of any
                label in a given row of the table.
                Constructed from the previous maintenance or major
                release in the same train, it contains the fix for a
                specific defect. Although it receives less testing, it
                contains only the minimal changes necessary to effect the
                Built at regular intervals between maintenance releases
                and receives less testing. Interim releases should be
                selected only if there is no other suitable release that
                addresses the vulnerability, and interim images should be
                upgraded to the next available maintenance release as
                soon as possible. Interim releases are not available via
                manufacturing, and usually they are not available for
                customer download from CCO without prior arrangement with
                the Cisco TAC.
   In all cases, customers should exercise caution to be certain the
   devices to be upgraded contain sufficient memory and that current
   hardware and software configurations will continue to be supported
   properly by the new release. If the information is not clear, contact
   the Cisco TAC for assistance as shown in the following section.
   More information on Cisco IOS software release names and abbreviations
   is available at http://www.cisco.com/warp/public/620/1.html.
| Train |Description of Image or | Availability of Fixed Releases* |
| | Platform | |
| Vulnerability CSCdm4476 | Rebuild | Interim** |Maintenance |
| 12.0S |Core/ISP support: GSR, | |12.0(10.1)S | |
| |RSP, c7200 | | | |
| Vulnerability CSCdu57417 | Rebuild | Interim** |Maintenance |
| 12.0S |Core/ISP support: GSR, | |12.0(19.3)S |12.0(19)S |
| |RSP, c7200 | | | |
| |Early deployment (ED) | | | |
| |release for the Cisco | | | |
|12.0ST |7200, 7500/7000RSP and |12.0(18.6)ST1|12.0(19.3)ST | |
| |12000 series routers | | | |
| |for Service Providers | | | |
| |(ISPs). | | | |
| Vulnerability CSCdu03323 | Rebuild | Interim** |Maintenance |
| 12.0S |Core/ISP support: GSR, |12.0(16)S2 |12.0(17.5)S |12.0(17)S |
| |RSP, c7200 | | | |
| |Early deployment (ED) | | | |
| |release for the Cisco | | | |
|12.0ST |7200, 7500/7000RSP and |12.0(16.6)ST1|12.0(17.5)ST | |
| |12000 series routers | | | |
| |for Service Providers | | | |
| |(ISPs). | | | |
| Vulnerability CSCdu35175 | Rebuild | Interim** |Maintenance |
| 12.0S |Core/ISP support: GSR, | |12.0(19.6)S | |
| |RSP, c7200 | | | |
| |Early deployment (ED) | | | |
| |release for the Cisco | | | |
|12.0ST |7200, 7500/7000RSP and | |12.0(19.6)ST | |
| |12000 series routers | | | |
| |for Service Providers | | | |
| |(ISPs). | | | |
| Vulnerability CSCdt96370 | Rebuild | Interim** |Maintenance |
| 12.0S |Core/ISP support: GSR, |12.0(16)S1 |12.0(17.1)S |12.0(18)S |
| |RSP, c7200 | | | |
| |Early deployment (ED) | | | |
|12.0ST |release for the Cisco |12.0(15.6)ST3|12.0(17.1)ST |12.0(18)ST |
| |7200, 7500/7000RSP and | | | |
| |12000 (GSR) series | | | |
| |routers for Service | | | |
| |Providers (ISPs). | | | |
| Vulnerability CSCdt69741 | Rebuild | Interim** |Maintenance |
| 12.0S |Core/ISP support: GSR, |12.0(16.6)S2 |12.0(17.3)S |12.0(18)S |
| |RSP, c7200 | | | |
| |Early deployment (ED) | | | |
|12.0ST |release for the Cisco | |12.0(17.3)ST |12.0(18)ST |
| |7200, 7500/7000RSP and | | | |
| |12000 (GSR) series | | | |
| |routers for Service | | | |
| |Providers (ISPs). | | | |
| Notes |
| |
|* All dates are estimates and subject to change. |
| |
|** Interim releases are subjected to less rigorous testing than regular |
|maintenance releases, and may have serious bugs. |

Obtaining Fixed Software

   Cisco is offering free software upgrades to eliminate this
   vulnerability for all affected customers.
   Customers with contracts should obtain upgraded software through their
   regular update channels. For most customers, this means that upgrades
   should be obtained through the Software Center on Cisco's Worldwide
   Web site at http://www.cisco.com.
   Customers whose Cisco products are provided or maintained through
   prior or existing agreement with third-party support organizations
   such as Cisco Partners, authorized resellers, or service providers
   should contact that support organization for assistance with the
   upgrade, which should be free of charge.
   Customers who purchase direct from Cisco but who do not hold a Cisco
   service contract and customers who purchase through third party
   vendors but are unsuccessful at obtaining fixed software through their
   point of sale should get their upgrades by contacting the Cisco
   Technical Assistance Center (TAC). TAC contacts are as follows:

     * +1 800 553 2447 (toll-free from within North America)
     * +1 408 526 7209 (toll call from anywhere in the world)
     * e-mail: tac@cisco.com
   Please have your product serial number available and give the URL
   of this notice as evidence of your entitlement to a free upgrade. Free
   upgrades for non-contract customers must be requested through the TAC.
   Please do not contact either "psirt@cisco.com" or
   "security-alert@cisco.com" for software upgrades.

          There is no direct workaround for this vulnerability. If
          feasible, packet fragments may be filtered before they reach
          the GSR.
          There is no direct workaround for this vulnerability. If
          feasible, packet fragments may be filtered before they reach
          the GSR.
          The workaround is to either shorten the ACL to be less than 448
          lines in length or to explicitly add rule "deny ip any any" as
          the last statement.
          The workaround is to transform an ACL to an incoming ACL
          instead of the outgoing one.
          Apply an ACL on all ports on the ingress line card. If a
          particular port is supposed to not block any traffic, then
          apply the ACL of the form access-list xy permit ip any any.
          There is no direct workaround for this vulnerability. It is
          possible to block the fragments on an intermediate router, if
          such exists, that should be placed between the affected Cisco
          12000 and the final target. The intermediate router must not be
          another Cisco 12000 affected by the same vulnerability.
Exploitation and Public Announcements

   The Cisco PSIRT is not aware of any public announcements or malicious
   use of the vulnerabilities described in this advisory.
Status of This Notice: FINAL

   This is a final notice. Although Cisco cannot guarantee the accuracy
   of all statements in this notice, all of the facts have been checked
   to the best of our ability. Cisco does not anticipate issuing updated
   versions of this notice unless there is some material change in the
   facts. Should there be a significant change in the facts, Cisco may
   update this notice.
   A standalone copy or paraphrase of the text of this security advisory
   that omits the distribution URL in the following section is an
   uncontrolled copy, and may lack important information or contain
   factual errors.

   This notice will be posted on Cisco's Worldwide Web site at
   http://www.cisco.com/warp/public/707/GSR-ACL-pub.shtml. In
   addition to Worldwide Web posting, a text version of this notice is
   clear-signed with the Cisco PSIRT PGP key and is posted to the
   following e-mail and Usenet news recipients:

     * cust-security-announce@cisco.com
     * bugtraq@securityfocus.com
     * first-teams@first.org (includes CERT/CC)
     * cisco@spot.colorado.edu
     * comp.dcom.sys.cisco
     * firewalls@lists.gnac.com
     * Various internal Cisco mailing lists
   Future updates of this notice, if any, will be placed on Cisco's
   Worldwide Web server, but may or may not be actively announced on
   mailing lists or newsgroups. Users concerned about this problem are
   encouraged to check the URL given above for any updates.
Revision History

   Revision 1.0 2001-November-14 08:00 UTC -0800 Initial public release
Cisco Security Procedures

   Complete information on reporting security vulnerabilities in Cisco
   products, obtaining assistance with security incidents, and
   registering to receive security information from Cisco, is available
   on Cisco's Worldwide Web site at
   This includes instructions for press inquiries regarding Cisco
   security notices.
   All Cisco Security Advisories are available at
   This notice is Copyright 2001 by Cisco Systems, Inc. This notice may
   be redistributed freely after the release date given at the top of the
   text, provided that redistributed copies are complete and unmodified,
   and include all date and version information.

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