Cisco Security Advisory: LDAP and VPN Vulnerabilities in PIX and ASA Appliances

Contents

Summary
Affected Products
Details
Vulnerability Scoring Details
Impact
Software Version and Fixes
Workarounds
Obtaining Fixed Software
Exploitation and Public Announcements
Status of this Notice: FINAL
Distribution
Revision History
Cisco Security Procedures

Vulnerable Products

Products Confirmed Not Vulnerable

The PIX is a firewall appliance that delivers user and application policy enforcement, multi-vector attack protection, and secure connectivity services.

The Adaptive Security Appliance (ASA) is a modular platform that provides security and VPN services. The ASA offers firewall, intrusion prevention (IPS), anti-X, and VPN services.

LDAP Authentication Bypass

Cisco ASA and PIX devices leveraging LDAP AAA servers for authentication of terminating L2TP IPSec tunnels or remote management sessions may be vulnerable to an authentication bypass attack. See the following bullets for more details:

• Layer 2 Tunneling Protocol (L2TP)
  Devices terminating L2TP IPSec tunnels must be configured to use LDAP in conjunction with CHAP, MS-CHAPv1, or MS-CHAPv2 authentication protocols to be vulnerable. If LDAP authentication is used in conjunction with PAP, the device is not vulnerable to the LDAP L2TP authentication bypass.
  
• Remote Management Access
  Cisco ASA and PIX devices leveraging LDAP AAA servers for authentication of management sessions (telnet, SSH and HTTP) may be vulnerable to an authentication bypass attack. Access for management sessions must be explicitly enabled and is limited to the defined source IP address within the device configuration.
  

This vulnerability is documented as bug ID CSCsh42793 ( registered customers only) .

Denial of Service in VPNs with Password Expiry

Cisco ASA and PIX devices terminating remote access VPN connections may be vulnerable to a DoS attack if the tunnel group is configured with password expiry. To exploit this vulnerability for IPSec VPN connections, an attacker would need to know the group name and group password. An attacker would not need this information for SSL VPN connections. A successful attack may result in a reload of the device.

This vulnerability is documented as software bug CSCsh81111 ( registered customers only) .

Denial of Service in SSL VPNs

Cisco ASAs using clientless SSL VPNs are vulnerable to a denial of service attack via the SSL VPN HTTP server. A successful attack must exploit a race condition in the processing non-standard SSL sessions and may result in a reload of the device.

More details are available in bug CSCsi16248 ( registered customers only) .

Cisco is providing scores for the vulnerabilities in this advisory based on the Common Vulnerability Scoring System (CVSS).

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

Cisco PSIRT will set the bias in all cases to normal. Customers are encouraged to apply the bias parameter when determining the environmental impact of a particular vulnerability.

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

Cisco has provided an FAQ to answer additional questions regarding CVSS at http://www.cisco.com/web/about/security/intelligence/cvss-qandas.html .

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at http://intellishield.cisco.com/security/alertmanager/cvss .

Successful exploitation of the LDAP Authentication bypass vulnerability may allow unauthorized users to access the device or internal resources. The DoS vulnerability in VPN password expiry and the DoS vulnerability in clientless SSL VPNs could be repeatedly exploited to cause an extended DoS condition.

When considering software upgrades, also consult http://www.cisco.com/go/psirt and 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") or your contracted maintenance provider for assistance.

More information on how and where to obtain fixed software can be found in the Obtaining Fixed Software section of this advisory.

This section of the advisory describes workarounds that may be useful in some environments. Additional mitigations that can be deployed on Cisco devices within the network are available in the Cisco Applied Mitigation Bulletin companion document for this advisory at the following link: http://www.cisco.com/warp/public/707/cisco-amb-20070502-asa.shtml

LDAP Authentication bypass

The following workarounds may be a useful reference for some customers to mitigate the LDAP authentication bypass vulnerabilities.

• L2TP
  For Cisco ASA or PIX devices configured to use a LDAP authentication server for L2TP over IPSec connections, configuring the device to use PAP as an authentication protocol may mitigate this vulnerability. It is important to note that PAP transmits passwords in clear-text. PAP authentication is encrypted via IPSec when it is used for the L2TP connection. Communications between the security appliance and the LDAP server are not encrypted by default and can be secured with SSL using the ldap-over-ssl command. Configuration of PAP authentication can be done using the following example as a guide or by referring to the security appliance configuration guides listed:
  

  ciscoasa#configure terminal
  ciscoasa(config)#tunnel-group l2tp_group ppp-attributes
  ciscoasa(config-ppp)#authentication pap
  ciscoasa(config-ppp)#no authentication ms-chap-v1
  ciscoasa(config-ppp)#no authentication ms-chap-v2
  ciscoasa(config-ppp)#no authentication chap
  

  Information on configuring L2TP over IPSEC using the CLI is available at the following link: http://www.cisco.com/en/US/partner/products/ps6120/products_configuration_guide_chapter09186a008066ebb6.html
  Information on configuring L2TP over IPSEC using the ADSM can be found at: http://www.cisco.com/en/US/partner/products/ps6121/products_configuration_guide_chapter09186a00806a81bc.html
  
• Remote Management
  Cisco ASA or PIX devices that authenticate remote management sessions with either the local database or an AAA server other than a LDAP server are not affected by this vulnerability. More information on changing the AAA server protocol used with remote management sessions is available at the following link: http://www.cisco.com/univercd/cc/td/doc/product/multisec/asa_sw/v_7_2/conf_gd/sysadmin/mgaccess.htm.
  Remote management sessions must be explicitly enabled before the Cisco ASA or PIX will accept sessions. The source IP addresses are defined within the command that enables remote management access. Below are examples of enabling remote management sessions (Note that other commands are required, but these commands control the source IP address of the device that is allowed access to the Cisco ASA or PIX device):
  For remote telnet, ssh and http access:
  

  ciscoasa#configure terminal
  ciscoasa(config)#telnet source_IP_address mask source_interface 
  ciscoasa(config)#ssh source_IP_address mask source_interface
  ciscoasa(config)#http source_IP_address mask source_interface
  

Denial of Service in VPNs with Password Expiry

Disabling password expiry for remote access users until a device can be updated with non-vulnerable code can prevent the exposure of this vulnerability. This can be accomplished by removing the password management entry in the general attributes of the tunnel group, as shown in the following example:

ciscoasa#configure terminal
ciscoasa(config)#tunnel-group remote_access_group general-attributes
ciscoasa(config-tunnel-general)#no password-management

Implementing this workaround will disable the password expiry feature, and users will not be forced to change their passwords.

More information on the password-management command is available in the Security Appliance Command reference at the following link: http://www.cisco.com/en/US/products/ps6120/products_command_reference_chapter09186a008063f0f8.html#wp1725278

Denial of Service in SSL VPNs

If clientless SSL VPNs are used, there is no workaround for the SSL VPN vulnerability. Client-based VPNs are not affected, and may be used as an alternative to the clientless VPN connections.

More information on configuring clientless SSL VPNs on the ASA is available in the configuration example at the following link: http://www.cisco.com/en/US/products/ps6120/products_configuration_example09186a00806ea271.shtml

Cisco will make free software available to address this vulnerability for affected customers. This advisory will be updated as fixed software becomes available. 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 http://www.cisco.com/public/sw-license-agreement.html, or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml.

Registered users can download the latest ASA and PIX releases at:

http://www.cisco.com/pcgi-bin/tablebuild.pl/asa-interim?psrtdcat20e2

http://www.cisco.com/pcgi-bin/tablebuild.pl/pix-interim?psrtdcat20e2

Do not contact either "psirt@cisco.com" or "security-alert@cisco.com" for software upgrades.

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 http://www.cisco.com.

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 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: tac@cisco.com
  

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.

Refer to http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.

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

These vulnerabilities were reported to Cisco by customers that experienced these issues during normal operation of their equipment.

This advisory is posted on Cisco's worldwide website at : http://www.cisco.com/warp/public/707/cisco-sa-20070502-asa.shtml 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.

• cust-security-announce@cisco.com
  
• first-teams@first.org
  
• bugtraq@securityfocus.com
  
• vulnwatch@vulnwatch.org
  
• cisco@spot.colorado.edu
  
• cisco-nsp@puck.nether.net
  
• full-disclosure@lists.grok.org.uk
  
• comp.dcom.sys.cisco@newsgate.cisco.com
  

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.

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 http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at http://www.cisco.com/go/psirt.