Summary

• When an 802.1x frame is received by an affected Catalyst 5000 series switch on a STP blocked port it is forwarded in that VLAN instead of being dropped. This causes a performance impacting 802.1x frames network storm in that part of the network, which is made up of the affected Catalyst 5000 series switches. This network storm only subsides when the source of the 802.1x frames is removed or one of the workarounds in the workaround section is applied. This vulnerability can be exploited to produce a denial of service (DoS) attack.

  This vulnerability is documented as Cisco bug id CSCdt62732.

  This notice will be posted at https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20010413-cat5k-8021x.

Affected Products

• This section provides details on affected products.

  Vulnerable Products

  Cisco Catalyst 5000 series switches based on any of the following EARL (Encoded Address Recognition Logic) hardware revisions:

  • EARL 1
    
  • EARL 1+
    
  • EARL 1++
    

  and running any of the following switch software revisions:

  • 4.5 (11) or earlier
    
  • 5.5 (6) or earlier
    
  • 6.1 (2) or earlier
    

  are affected by this vulnerability. This series includes the Catalyst models 5000, 5002, 5500, 5505, 5509, 2901, 2902 and 2926 switches.

  To determine your hardware and software revision type sh mod on the console prompt of the switch.

  Additional information can be found in the document, "Identifying Catalyst 5000 EARL Version and Other Common EARL Questions."

  Products Confirmed Not Vulnerable

  Catalyst 5000 series switches based on EARL 2 or later hardware revisions are not affected by this vulnerability.

  Catalyst 5000 series switches regardless of the EARL hardware revision, running the following switch software revisions

  • 4.5 (12) or later - expected general availability before 2001, May 1
    
  • 5.5 (7) or later
    
  • 6.1 (3) or later
    

  are not affected by this vulnerability.

  No other Cisco product is currently known to be affected by this vulnerability. This includes the Catalyst 6000, 4000, 3500XL, 2900XL and 2948G switches.

Details

• When an 802.1x (IEEE standard for port based network access control) frame is received by an affected Catalyst 5000 series switch on a STP (Spanning Tree Protocol) blocked port it is forwarded in that VLAN (Virtual Local Area Network) instead of being dropped. This causes a performance impacting 802.1x frames network storm in that part of the network, which is made up of the affected Catalyst 5000 series switches. This network storm only subsides when the source of the 802.1x frames is removed or one of the workarounds in the workaround section is applied.

  This vulnerability is documented as Cisco bug id CSCdt62732, which requires a CCO account to view.

Workarounds

• The following workarounds will prevent the 802.1x frames from causing an 802.1x frames network storm in an affected Catalyst 5000 series switch network.

  These workarounds can also be applied to a network experiencing an 802.1x frames network storm.

  1. Configure permanent MAC address entries for the entire reserved STP range 01-80-c2-00-00-02 to 01-80-c2-00-00-0f to be directed out an unused port for each VLAN on each affected switch in the network.
     The commands to configure are given below.
     

     set cam permanent 01-80-c2-00-00-02 <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-03 <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-04 <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-05 <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-06 <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-07 <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-08 <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-09 <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-0a <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-0b <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-0c <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-0d <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-0e <mod#>/<port#> <VLAN> 
     set cam permanent 01-80-c2-00-00-0f <mod#>/<port#> <VLAN>

  2. Break the STP loop by either
     
     1. Disabling the redundant (STP blocked ports) or
        
     2. Disconnecting the cable from these ports
        
     Remove all the sources of 802.1x frames before re-enabling the ports or reconnecting the cables.
     
  3. Power down the Catalyst 5000 switch(es) that create the spanning-tree loop (any switch with STP blocked ports).
     Remove all the sources of 802.1x frames before powering up the switches.
     

Fixed Software

• This vulnerability has been fixed in the following switch software revisions

  • 4.5 (12) or later - expected availability before 2001, May 1
    
  • 5.5 (7) or later
    
  • 6.1 (3) or later
    

  and the fix will be carried forward in all future releases.

  Software upgrade can be performed via the console interface.

Exploitation and Public Announcements

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

  A customer who discovered this vulnerability while using Microsoft Windows XP BETA software reported this vulnerability to Cisco. Microsoft Windows XP attempts 802.1x authentication during its boot-up phase. Following these configuration steps can disable this:

  1. Click on the associated Local Area Connection under Network Connections
     
  2. Click on the Authentication Tab at the top right.
     
  3. Uncheck "Network Access Control using IEEE 802.1x"
     

  This issue has been discussed in news articles regarding issues with Microsoft Windows XP BETA program and the Cisco Catalyst 5000 series switches.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20010413-cat5k-8021x

Revision History

• Revision 1.1	2002-August-07
  Revision 1.0	2001-April-13	Initial public release.

  Show Less