Cisco Security Advisory: ICMP Unreachable vulnerability in Cisco 12000 Series

Type securityvulns
Reporter Securityvulns
Modified 2001-11-15T00:00:00



Cisco Security Advisory: ICMP Unreachable vulnerability in Cisco 12000 Series Internet Router

Revision 1.0

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


The performance of Cisco 12000 series routers can be degraded when they have to send a large number of ICMP Unreachable packets. This situation usually can occur during a heavy network scanning. This vulnerability is tracked by three different bug IDs: CSCdr46528, CSCdt66560, CSCds36541. Each bug ID is assigned to a different Engine the line card is based upon.

The rest of the Cisco routers and switches are not affected by this vulnerability. It is specific for Cisco 12000 Series.

No other Cisco product is vulnerable.

The workaround is to either prevent the router from sending Internet Control Message Protocol (ICMPs) unreachables at all or to rate limit them.

This advisory is available at

Affected Products

Only Cisco 12000 Series Internet Routers are affected with this vulnerability. No other routers or switches are affected. Not all line cards of a Cisco 12000 Series are affected by this vulnerability. Vulnerability is 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 correspoding number.

The following example shows the output for an Engine 2 based line card.

      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]

All line cards that are based on the Engines 0, 1 and 2 are vulnerable. Line cards based on the Engine 3 and 4 are not affected.

The following table depicts which Cisco IOS® release is vulnerable to a particular issue:

+--------------+------------------+------------------+--------------+ | DDTS | 12.0S | 12.0SC | 12.0ST | +--------------+------------------+------------------+--------------+ | CSCdr46528 | Vulnerable | | Vulnerable | +--------------+------------------+------------------+--------------+ | CSCds36541 | Vulnerable | Vulnerable | Vulnerable | +--------------+------------------+------------------+--------------+ | CSCdt66560 | Vulnerable | | Vulnerable | +--------------+------------------+------------------+--------------+


The received packet will be dropped when either there is no valid path to the destination or when the packet should be routerd to the Null0 interface. The packets are either fast dropped (Engine 0 Line Cards) or hardware dropped (all other application-specific integrated circuit (ASIC) based forwarding Line Cards). Given the fast and hardware drop capabilities of the Cisco 12000, a large volume of traffic can be dropped without impacting the capabilities of the router. Whenever a packet is dropped the router must send an ICMP Unreachable packet back to the source. That is mandated by the Internet Standards.

When a high volume of traffic is sent to the router that requires ICMP Unreachable replies, the processing of the replies can saturate the CPU. This condition can happen when the router is "Black Hole" filtering, dropping packets sent to it as the networks default path, or from a direct Denial of Service (DOS) against the router. For further information of "Black Hole" filtering consult the document: Essential IOS Features Every ISP Should Consider, section "Black Hole Routing as a Packet Filter".

The following table shows the relationship between the vulnerabilities and Engine the line card is based on.

+------------+---------------+--------------+----------------+---------+ | DDTS | Engine 0 Engine 1 Engine 2 Engine 4| +------------+---------------+--------------+----------------+---------+ |CSCdr46528 | Vulnerable | | | | +------------+---------------+--------------+----------------+---------+ |CSCds36541 | | Vulnerable | | | +------------+---------------+--------------+----------------+---------+ |CSCdt66560 | | | Vulnerable | | +------------+---------------+--------------+----------------+---------+


Exploitation of this vulnerabilities may lead to the Denial-of-Service. The router's performance will degrade and, in the worst case scenario, the router will stop forwarding packets.

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 contains 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 label).

When selecting a release, keep in mind the following definitions:

Maintenance Most heavily tested and highly recommended release of any label in a given row of the table.

Rebuild 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 repair.

Interim Built at regular intervals between maintenance releases and receives less testing. Interims 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

+-------+------------------------+---------------------------------------+ | Train| Description of Image or| Availability of Fixed Releases | | | Platform | | +-------+------------------------+---------------------------------------+ | Vulnerability CSCdr46528 | Rebuild | Interim | Maintenance| +-------+------------------------+-------------+------------+------------+ | 12.0S |Core/ISP support: GSR, |12.0(16)S1 |12.0(16.5)S | 12.0(17)S | | |RSP, c7200 | | | | | |Cisco IOS software | | | | | |Release 12.OST is an | | | | | |early deployment (ED) | | | | +-------+------------------------+-------------+------------+------------+ |12.0ST |release for the Cisco |12.0(15.6)ST3|12.0(16.5)ST| 12.0(17)ST | | |7200, 7500/7000RSP and | | | | | |12000 (GSR) series | | | | | |routers for Service | | | | | |Providers (ISPs). | | | | +-------+------------------------+-------------+------------+------------+ | Vulnerability CSCds36541 | Rebuild | Interim | Maintenance| +-------+------------------------+-------------+------------+------------+ | 12.0S |Core/ISP support: GSR, |12.0(13.6)S2 |12.0(14.1)S | 12.0(15)S | | |RSP, c7200 | | | | +-------+------------------------+-------------+------------+------------+ |12.0SC |Cable/broadband ISP: |12.0(13.6)SC1|12.0(14.6)SC| 12.0(15)SC | | |ubr7200 | | | | | |Cisco IOS software | | | | | |Release 12.OST is an | | | | | |early deployment (ED) | | | | +-------+------------------------+-------------+------------+------------+ |12.0ST |release for the Cisco | |12.0(14.3)ST| | | |7200, 7500/7000RSP and | | | | | |12000 (GSR) series | | | | | |routers for Service | | | | | |Providers (ISPs). | | | | +-------+------------------------+-------------+------------+------------+ | Vulnerability CSCdt66560 | Rebuild | Interim | Maintenance| +-------+------------------------+-------------+------------+------------+ | 12.0S |Core/ISP support: GSR, |12.0(16)S1 |12.0(16.6)S | 12.0(17)S | | |RSP, c7200 | | | | | |Cisco IOS software | | | | | |Release 12.OST is an | | | | | |early deployment (ED) | | | | +-------+------------------------+-------------+------------+------------+ |12.0ST |release for the Cisco |12.0(15.6)ST3|12.0(16.6)ST| 12.0(17)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

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:

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 "" or "" for software upgrades.


There are two workarounds for this issue. The first one is to prevent the router from sending ICMP unreachables at all. That behavior is governed with the "no ip unreachables" command. This command sould be applied on an interface, like in this example:

router(config)#interface ethernet 0 router(config-if)#no ip unreachables

It is possible to mitigate the problem by rate limiting number of ICMP unreachables packets that are sent. Here is the example:

router(config)#ip icmp rate-limit unreachable n

Where n is the number of milliseconds between two consecutive ICMP ureachable packets. The default value is 500. That menas that one ICMP unreachable packet is send every 500 ms.

Exploitation and Public Announcements

The Cisco PSIRT is aware that some ISPs have experiencing difficulties due to this vulnerability.

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 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:

 * (includes CERT/CC)
 * 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-20 08:00 GMT -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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