Summary

• 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 https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20040609-catos.

Affected Products

• This section provides details on affected products.

  Vulnerable Products

  Hardware

  • 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
    

  Software

  CatOS Release Train	Affected Releases
  8.xGLX	earlier than 8.3(2)GLX
  8.x	earlier than 8.2(2)
  7.x	earlier than 7.6(6)
  6.x	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.

Details

• 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 http://www.cisco.com/univercd/cc/td/doc/cisintwk/.

  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) .

Workarounds

• 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 http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note09186a0080094713.shtml.

  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 http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00801a1a55.shtml 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 http://www.cisco.com/en/US/docs/switches/lan/catalyst6500/catos/6.x/configuration/guide/acc_list.html.

  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 http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080120f48.shtml and http://www.ietf.org/rfc/rfc2827.txt. 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 http://www.cisco.com/en/US/docs/ios/12_2/security/configuration/guide/scfrpf.html .

  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.

Fixed Software

• CatOS Release Train	Fixed Releases
  8.xGLX	8.3(2)GLX and later
  8.x	8.2(2) and later
  7.x	7.6(6) and later
  6.x	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 http://www.cisco.com/en/US/products/hw/switches/ps708/products_configuration_example09186a0080116ff0.shtml. 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.

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.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20040609-catos

Revision History

• Revision 1.0

  2004-Jun-9

  Initial public release.

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