Products & Services

Cisco CatOS Telnet, HTTP and SSH Vulnerability

Advisory ID: cisco-sa-20040609-catos

Revision 1.0

For Public Release 2004 June 9 16:00  UTC (GMT)


Cisco CatOS is susceptible to a TCP-ACK Denial of Service (DoS) attack on the Telnet, HTTP and SSH service. If exploited, the vulnerability causes the Cisco CatOS running device to stop functioning and reload.

This vulnerability is documented as Cisco bug IDs CSCec42751, CSCed45576, and CSCed48590. There are techniques available to mitigate the potential effects of this vulnerability in the workaround section of this advisory. Cisco is providing fixed software, and recommends that customers upgrade to it.

This advisory will be posted at

Affected Products

This section provides details on affected products.

Vulnerable Products


  • Catalyst 6000 series
  • Catalyst 5000 series
  • Catalyst 4500 series
  • Catalyst 4000 series
  • Catalyst 2948G, 2980G, 2980G-A, 4912G - use Catalyst 4000 series code base
  • Catalyst 2901, 2902, 2926[T,F,GS,GL], 2948 - use Catalyst 5000 series code base


CatOS Release Train

Affected Releases


earlier than 8.3(2)GLX


earlier than 8.2(2)


earlier than 7.6(6)


earlier than 6.4(9)

5.x and earlier

earlier than 5.5(20)

Products Confirmed Not Vulnerable

The following Catalyst switches do not run Cisco CatOS.

  • Catalyst 8500 series
  • Catalyst 4800 series
  • Catalyst 4200 series
  • Catalyst 4840G
  • Catalyst 4908G-l3
  • Catalyst 4224 Access Gateway Switch
  • Catalyst 3750
  • Catalyst 3750 Metro
  • Catalyst 3560
  • Catalyst 3550
  • Catalyst 3500 XL
  • Catalyst 2948G-l3
  • Catalyst 2970
  • Catalyst 2955
  • Catalyst 2950
  • Catalyst 2950 LRE
  • Catalyst 2940
  • Catalyst 2900 XL
  • Catalyst 2900 LRE XL
  • Catalyst 2820
  • Catalyst 1900

Cisco IOS is not vulnerable to this issue.

No other Cisco products are currently known to be affected by this vulnerability.

To determine your software revision, type show version at the command line prompt of the network device.


A TCP-ACK DoS attack is conducted by not sending the regular final ACK required for a 3-way TCP handshake to complete, and instead sending an invalid response to move the connection to an invalid TCP state. This attack can be initiated from a remote spoofed source.

This vulnerability is currently known to be exploitable only if you have the Telnet, HTTP or SSH service configured on a device which is running Cisco CatOS.

CatOS release 5.4 was the first CatOS release which incorporated the HTTP feature. Software releases that contain a "cv" in the image filename support the HTTP feature. The HTTP server is disabled by default. It is typically enabled to allow web based management of the switch using CiscoView. To disable the HTTP server on the switch type set ip http server disable.

CatOS K9 (crypto) release 6.1 was the first CatOS release which incorporated the SSH feature. The SSH server is disabled by default. To verify if SSH has been configured on the switch type show crypto key. If this shows you the RSA key then SSH has been configured and enabled on the switch. To remove the crypto key type clear crypto key RSA and this will disable the SSH server on the switch.

To check if the HTTP or SSH services are enabled one can also do the following: For HTTP, try and connect to the default HTTP port, TCP 80, using Telnet. telnet ip_address_of_device 80. If the session connects, the service is enabled and accessible. Similarly, for SSH try and connect to the SSH port, TCP 22.

The Internetworking Terms and Cisco Systems Acronyms online guides can be found at

This vulnerability is documented in the Cisco Bug Toolkit as Bug IDs CSCec42751 ( registered customers only) , CSCed45576 ( registered customers only) , and CSCed48590 ( registered customers only) .

Vulnerability Scoring Details

Cisco has provided scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS). The CVSS scoring in this Security Advisory is done in accordance with CVSS version 2.0.

CVSS is a standards-based scoring method that conveys vulnerability severity and helps determine urgency and priority of response.

Cisco has provided a base and temporal score. Customers can then compute environmental scores to assist in determining the impact of the vulnerability in individual networks.

Cisco has provided an FAQ to answer additional questions regarding CVSS at .

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at .


When exploited, the vulnerability causes the Cisco CatOS running device to stop functioning and reload.

Software Versions and Fixes

CatOS Release Train

Fixed Releases


8.3(2)GLX and later


8.2(2) and later


7.6(6) and later


6.4(9) and later

5.x and earlier

5.5(20) 1

The procedure to upgrade to the fixed software version is described at Some Catalyst platforms may also require a boot ROM upgrade in conjunction with a software upgrade, please refer to the appropriate software release notes.

1If a device running fixed Cisco CatOS 5.5(20) version of code is being actively attacked for this vulnerability, network management access over the Ethernet interface may not be possible during that period; console access would still be possible. This is due to the software architectural limitations in 5.x. Customers are recommended to upgrade to 6.4(9) or later.


Implement the best practice of assigning all switch management interfaces to a dedicated VLAN and apply appropriate access controls on routers switching between the switch management interface VLAN and the rest of the network. To read more about best practices for Catalyst 4500/4000, 5500/5000, and 6500/6000 Series Switches running CatOS configuration and management, refer to

Apply ACLs on routers / switches / firewalls in front of the vulnerable switches such that traffic destined for the Telnet TCP port 23, HTTP TCP port 80 and SSH TCP port 22 on the vulnerable switches is only allowed from the network management workstations. Refer to for examples on how to apply access control lists (ACLs) on Cisco routers.

On the Catalyst 6000 series switches, if the VLAN Access Control List (VACL) feature is available in the code base, you can use VACLs to enable Telnet, HTTP and SSH access to the switch's management interface only from the network management workstations, refer to

Please note, these workarounds will not prevent spoofed IP packets with the source IP address set to that of the network management station from reaching the switch's management interface. For more information on anti-spoofing refer to and The Unicast Reverse Path Forwarding (Unicast RPF) feature helps to mitigate problems that are caused by malformed or forged IP source addresses that are passing through a router, refer to .

IP Permit Lists will not provide any mitigation against this vulnerability.

The Cisco PSIRT recommends that affected users upgrade to a fixed software version of code.

Obtaining Fixed Software

Cisco has made free software available to address this vulnerability for affected customers. Prior to deploying software, customers should consult their maintenance provider or check the software for feature set compatibility and known issues specific to their environment.

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's software license terms found at, or as otherwise set forth at Downloads at

Do not contact either "" or "" for software upgrades.

Customers with Service Contracts

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 website at

Customers using Third Party Support Organizations

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 guidance and assistance with the appropriate course of action in regards to this advisory.

The effectiveness of any workaround or fix is dependent on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround or fix is the most appropriate for use in the intended network before it is deployed.

Customers without Service Contracts

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 in obtaining fixed software through their point of sale should obtain software patches and bug fixes 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:

Have your product serial number available and provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.

Refer to for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.

Exploitation and Public Announcements

The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability described in this advisory.

This vulnerability was found internally during testing.

Status of this Notice: Final


A stand-alone copy or Paraphrase of the text of this document 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 Cisco's worldwide website at .

In addition to worldwide web posting, a text version of this notice is clear-signed with the Cisco PSIRT PGP key having the fingerprint 8C82 5207 0CA9 ED40 1DD2 EE2A 7B31 A8CF 32B6 B590 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 Cisco's worldwide website, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the above URL for any updates.

Revision History

Revision 1.0


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 website at This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at