Vulnerabilities in the Internet Key Exchange Xauth Implementation

Summary

• Cisco Internetwork Operating System (IOS) Software release trains 12.2T, 12.3 and 12.3T may contain vulnerabilities in processing certain Internet Key Exchange (IKE) Xauth messages when configured to be an Easy VPN Server.

  Successful exploitation of these vulnerabilities may permit an unauthorized user to complete authentication and potentially access network resources.

  This advisory will be posted to http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050406-xauth

  Cisco has made free software available to address this vulnerability for affected customers.

Affected Products

• This section provides details on affected products.

  Vulnerable Products

  This issue affects all Cisco devices running any unfixed version of Cisco IOS that supports, and is configured for, Cisco Easy VPN Server Xauth version 6 authentication.

  A Cisco device running Easy VPN Server and configured for Xauth authentication will have the following line in the configuration:

    crypto map  client authentication list 
  

  The Easy VPN Server XAUTH feature may also be enabled underneath an ISAKMP profile via a configuration similar to:

    crypto isakmp profile  
      match identity group 
      client authentication list 
      isakmp authentication list 
      client configuration address respond
      qos-group 2
  

  To determine the software running on a Cisco product, log in to the device and issue the 'show version' command to display the system banner. Cisco IOS Software will identify itself as "Internetwork Operating System Software" or simply "IOS." On the next line of output, the image name will be displayed between parentheses, followed by "Version" and the IOS release name. Other Cisco devices will not have the show version command or will give different output.

  The following example identifies a Cisco product running IOS release 12.3(6) with an installed image name of C3640-I-M:

    Cisco Internetwork Operating System Software
    IOS (tm) 3600 Software (C3640-I-M), Version 12.3(6), RELEASE SOFTWARE (fc3)
  

  The next example shows a product running IOS release 12.3(11)T3 with an image name of C3845-ADVIPSERVICESK9-M:

    Cisco IOS Software, 3800 Software (C3845-ADVIPSERVICESK9-M), Version 12.3(11)T3, RELEASE SOFTWARE (fc4)
    Technical Support: http://www.cisco.com/techsupport
    Copyright (c) 1986-2005 by Cisco Systems, Inc.
  

  Additional information about Cisco IOS release naming can be found at http://www.cisco.com/warp/public/620/1.html.

  Products Confirmed Not Vulnerable

  Cisco Easy VPN Server is an IOS-only feature. Devices that do not run IOS are not vulnerable.

  No other Cisco products are currently known to be affected by these vulnerabilities.

Details

• IPSec is a framework of open standards that provides data confidentiality, data integrity, and data authentication between participating peers. IPSec provides these security services at the IP layer allowing for data to be transmitted across a public network without fear of observation, modification, or spoofing, thus enabling applications such as Virtual Private Networks (VPNs). IPSec uses the Internet Key Exchange (IKE) protocol to provide authentication of the IPSec peers, negotiate IPSec security associations (SA), and establish IPSec keys.

  Extended Authentication (XAUTH) is an extension to IKE defined in an expired Internet Engineering Task Force (IETF) Internet Draft, draft-ietf-ipsec-isakmp-xauth-06.txt, which allows for organizations to utilize existing legacy authentication methods in order to manage remote access.

  Successful VPN establishment consists of two levels of SA's known as phases. Phase 1 authentication establishes session keys. Using the Xauth feature, the client waits for a "username/password" challenge after the IKE Phase 1 SA has been established. When the end user responds to the challenge, the response is forwarded to the IPsec peers for an additional level of authentication.

  The Cisco IOS Easy VPN Server feature introduced in IOS 12.2(8)T allows an IOS device to act as a VPN concentrator, providing authentication and encrypted access to network resources.

  To determine if Cisco's Easy VPN Server XAUTH feature is enabled, check the device's configuration for the following line:

    crypto map  client authentication list 
  

  The Easy VPN Server XAUTH feature may also be enabled underneath an ISAKMP profile via a configuration similar to:

    crypto isakmp profile  
      match identity group 
      client authentication list 
      isakmp authentication list 
      client configuration address respond
      qos-group 2
  

  Certain packets sent to the IOS Easy VPN Server listening on User Datgram Protocol (UDP) port 500 may permit an unauthorized user to complete Xauth authentication and thereby gain access to network resources.

  In order for the attack to succeed, an attacker must know the shared group key to complete the IKE Phase 1 negotiation before the Xauth negotiation takes place.

  This malformed packet vulnerability is documented as Cisco Bug ID CSCin82407. ( registered ) customers only.

  A second vulnerability exists in a feature introduced in IOS 12.3(8)T where an ISAKMP profile can be assigned to a remote access peer based on the certificate the peer uses during IKE negotiation. If the ISAKMP profile mandates XAUTH (AAA authentication and authorization lists are configured in the profile), then the peer must perform XAUTH authentication after Phase 1 negotiation.

  A vulnerability exists where the ISAKMP profile is assigned but the attributes that are configured in the ISAKMP profile are not processed. This can result in a situtation where both the VPN client and VPN server are expecting to hear something from the other end of the connection. Normally this deadlock will be broken by idle timers tearing down the SA, but it is possible for a malicious client to propose Phase 2 negotiation during this time which may allow for the IPSec SA to be fully established.

  This issue only affects ISAKMP profiles matched by certificate maps. Configurations with certificate maps configured will contain the commands:

    crypto isakmp profile  
      match certificate  
  

  This vulnerability is documented as Cisco Bug ID CSCeg00277. ( registered ) customers only.

Workarounds

• The effectiveness of any workaround is dependent on specific deployment scenarios 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 is the most appropriate for use in the intended network before it is deployed.

  Using Strong Group Passwords

  Because the preshared group password (also referred to as the group key) must be known by an attacker, the use of a best practice to deploy strong preshared group keys may mitigate a brute-force attack against this group key.

  The preshared key can be changed by using the following configuration commands:

        Router(config)#crypto isakmp client configuration group 
        Router(config)#key 
        

Fixed Software

• Each row of the Cisco IOS software table (below) 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 contain the fix (the "First Fixed Release") and the anticipated date of availability for each are listed in the "Rebuild" and "Maintenance" columns. A device running a release in the given train that is earlier than the release in a specific column (less than the First Fixed Release) is known to be vulnerable. The release should be upgraded at least to the indicated release or a later version (greater than or equal to the First Fixed Release label).

  For further information on the terms "Rebuild" and "Maintenance" please consult the following URL:

  http://www.cisco.com/warp/public/620/1.html

  When considering software upgrades, please also consult http://www.cisco.com/en/US/products/products_security_advisories_listing.htmland any subsequent advisories to determine exposure and a complete upgrade solution.

  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 Technical Assistance Center ("TAC") for assistance.

  Major Release	Availability of Repaired Releases
  Affected 12.2-Based Release	Rebuild	Maintenance
  12.2B	Migrate to 12.3(4)T8 or later
  12.2BC	12.2(15)BC1f
  	12.2(15)BC2e
  12.2BX	Vulnerable, contact TAC
  12.2BY	Migrate to 12.3(4)T8 or later
  12.2BZ	Vulnerable, contact TAC
  12.2CX	Migrate to 12.3(9a)BC
  12.2CY	Migrate to 12.2(15)BC1f or 12.2(15)BC2e
  12.2CZ	12.2(15)CZ1
  12.2JK	12.2(15)JK2
  12.2SU	12.2(14)SU2
  12.2SX	Migrate to 12.2(17d)SXB5
  12.2SXA	Migrate to 12.2(17d)SXB5
  12.2SXB	12.2(17d)SXB5
  12.2SXD	12.2(18)SXD1
  12.2SY	Migrate to 12.2(17d)SXB5
  12.2T	Migrate to 12.3 or later
  12.2XJ	Migrate to 12.3 or later
  12.2XK	Migrate to 12.3 or later
  12.2XL	Migrate to 12.3 or later
  12.2XM	Migrate to 12.3 or later
  12.2XW	Migrate to 12.3 or later
  12.2XZ	Migrate to 12.3 or later
  12.2YA	12.2(4)YA8
  12.2YB	Migrate to 12.3 or later
  12.2YD	Migrate to 12.3(8)T5 or later
  12.2YF	Migrate to 12.3 or later
  12.2YG	Migrate to 12.3 or later
  12.2YH	Migrate to 12.3 or later
  12.2YJ	Migrate to 12.3 or later
  12.2YL	Migrate to 12.3T or later
  12.2YM	Migrate to 12.3T or later
  12.2YN	Migrate to 12.3T or later
  12.2YP	Migrate to 12.3 or later
  12.2YQ	Migrate to 12.3(4)T8 or later
  12.2YR	Migrate to 12.3(4)T8 or later
  12.2YT	Migrate to 12.3 or later
  12.2YU	Migrate to 12.3T or later
  12.2YV	Migrate to 12.3(4)T8 or later
  12.2YW	Migrate to 12.3T or later
  12.2YX	Migrate to 12.2(14)SU2 or later
  12.2YY	Migrate to 12.3T or later
  12.2ZB	Migrate to 12.3T or later
  12.2ZC	Migrate to 12.3T or later
  12.2ZD	Migrate to 12.3(14)T
  12.2ZE	Migrate to 12.3 or later
  12.2ZF	Migrate to 12.3(4)T8 or later
  12.2ZG	Migrate to 12.3(4)T8 or later
  12.2ZH	12.2(13)ZH5
  12.2ZJ	Migrate to 12.3T or later
  12.2ZK	Contact TAC
  12.2ZL	12.2(15)ZL2 available TBD
  12.2ZN	Migrate to 12.3T or later
  12.2ZP	Contact TAC
  Affected 12.3-Based Release	Rebuild	Maintenance
  12.3	12.3(6e)
  	12.3(9c)
  	12.3(10a)
  		12.3(12)
  12.3B	12.3(5a)B3
  12.3BC	12.3(9a)BC
  12.3BW	Migrate to 12.3(7)T6 or later
  12.3T	12.3(2)T9
  	12.3(4)T8
  	12.3(7)T7
  	12.3(8)T5
  	12.3(11)T2
  		12.3(14)T
  12.3XA	12.3(2)XA3 available TBD
  12.3XB	Migrate to 12.3(8)T5 or later
  12.3XC	12.3(2)XC3 available TBD
  12.3XD	12.3(4)XD4
  12.3XE	12.3(2)XE1
  12.3XF	Migrate to 12.3(11)T2 or later
  12.3XG	12.3(4)XG2
  12.3XH	Migrate to 12.3(11)T2 or later
  12.3XI	Contact TAC
  12.3XJ	Contact TAC
  12.3XK	Migrate to 12.3(11)T2 or later
  12.3XL		12.3(11)XL
  12.3XM	Migrate to 12.3(14)T or later
  12.3XN	Migrate to 12.3(14)T or later
  12.3XQ	12.3(4)XQ1
  12.3XR	12.3(7)XR3
  12.3XS	12.3(7)XS2
  12.3XT	Contact TAC
  12.3XU	12.3(8)XU3
  12.3XW	Contact TAC
  12.3XX	12.3(8)XX1
  12.3XY	Migrate to 12.3(14)T
  12.3YA	12.3(8)YA1
  12.3YC	12.3(8)YC1
  12.3YD		12.3(8)YD
  12.3YF		12.3(11)YF
  12.3YG		12.3(8)YG1
  12.3YH		12.3(8)YH
  12.3YI		12.3(8)YI available TBD
  12.3YJ		12.3(11)YJ
  12.3YK		12.3(11)YK

Exploitation and Public Announcements

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

  Both vulnerabilities were reported by Cisco customers

URL

• http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050406-xauth

Revision History

• Revision 1.0

  2005-April-6

  Initial public release.

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