Cisco Security Advisory: Denial-of-Service of TCP-based Services in CatOS

Type securityvulns
Reporter Securityvulns
Modified 2003-07-10T00:00:00



Cisco Security Advisory: Denial-of-Service of TCP-based Services in CatOS

Revision 1.0

For Public Release 2003 July 09 at 16:00 UTC (GMT)


After receiving eight TCP connection attempts using a non-standard TCP flags combination, a Catalyst switch will stop responding to further TCP connections to that particular service. In order to re-establish functionality of that service, the switch must be rebooted. There is no workaround. This vulnerability affects only CatOS. No other Cisco products are affected.

This advisory is available at

Affected Products

The CatOS for the following Catalyst models are affected:

  • Catalyst 4000 Series including models 2948G and 2980G/2980G-A

  • Catalyst 5000 Series including models 2901, 2902 and 2926

  • Catalyst 6000

No other Cisco products are affected.


After receiving eight connection attempts on any TCP service, the switch will stop responding to any further connection attempts to that service. These attempts must use a non-standard combination of TCP flags. The switch will continue to pass other switched traffic normally and the console is also not affected. Only the service to which connections were made will become unresponsive. Standard TCP services include HTTP, Telnet, and SSH.

This vulnerability is documented as Cisco Bug ID CSCdw52219 (registered customers only) .


By exploiting this vulnerability, an attacker can prevent further use of the specified TCP-based service. Depending on the configuration of the device, if SSH or Telnet are enabled and exploited, the availability of those services could be affected, possibly resulting in a loss of management capability using those same services. However, UDP-based services such as Simple Network Management Protocol (SNMP) would still be available and unaffected.

Software Versions and Fixes

The vulnerability is fixed in the following releases.

+-------------------------------------------------------+ | | Description | | | Train | of Image or | Availability of Fixed Releases | | | Platform | | |-------+-------------+---------------------------------| | | | Rebuild | Interim | Maintenance | | | | | * | | |-------+-------------+---------+---------+-------------| | | Catalyst | | 5.5 | | | 5.5 | 4000, 5000, | | (13.5) | 5.5(14) | | | 6000 | | | | |-------+-------------+---------+---------+-------------| | | Catalyst | | 6.3 | | | 6.3 | 4000, 5000, | | (5.10) | 6.3(6) | | | 6000 | | | | |-------+-------------+---------------------------------| | | Catalyst | | | 6.4 | 4000, 5000, | Not affected | | | 6000 | | |-------+-------------+---------------------------------| | 7.2 | Catalyst | | 7.2 | 7.2(1) | | | 4000, 6000 | | (0.65) | | |-------+-------------+---------+---------+-------------| | | Catalyst | | 7.4 | | | 7.4 | 4000, 6000 | | (0.2) | 7.4(1) | | | | | CLR | | |-------+-------------+---------------------------------| | 8.1 | Catalyst | Not affected | | | 6000 | | +-------------------------------------------------------+

  • 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 address these vulnerabilities for all affected customers. Customers may only install and expect support for the feature sets they have purchased. By installing, downloading, accessing or otherwise using such software upgrades, Customers agree to be bound by the terms of Cisco software license terms found at, or as otherwise set forth at the Cisco Connection Online Software Center at

Customers with service contracts should contact their regular update channels to obtain the free software upgrade identified via this advisory. For most customers with service contracts, this means that upgrades should be obtained through the Software Center on the Cisco worldwide website at To access the software download URL, you must be a registered user and you must be logged in.

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 is no workaround. In order to continue using an affected TCP service, the switch must be rebooted.

It is possible to mitigate the exposure by configuring VLAN Access Control Lists (VACLs) on the switch (where they are supported) that will allow only legitimate hosts to connect to the desired services. This must be combined with Unicast Reverse Path Forwarding (uRPF), or some other anti-spoofing technique, on the network edge to protect against spoofed packets from the outside of the network.

Exploitation and Public Announcements

This vulnerability has been reported to Cisco by a customer. The Cisco PSIRT has received no reports of malicious exploitation of this vulnerability and we are not aware of any public discussion.

Status of This Notice: FINAL

This is a final advisory. Although Cisco cannot guarantee the accuracy of all statements in this advisory, all of the facts have been checked to the best of our ability. Cisco does not anticipate issuing updated versions of this advisory unless there is some material change in the facts. Should there be a significant change in the facts, Cisco will update this advisory.

A stand-alone 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 advisory will be posted on the Cisco 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 advisory, if any, will be placed on the Cisco 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 | 2003-July-09 | Initial | | 1.0 | 16:00 UTC | public | | | (GMT) | 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 the Cisco 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 2003 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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