Cisco Security Advisory

Advisory ID: cisco-sa-20071017-asa

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20071017-asa

Revision 1.1

For Public Release 2007 October 17 16:00  UTC (GMT)

Summary

Two crafted packet vulnerabilities exist in the Cisco PIX 500 Series Security Appliance (PIX) and the Cisco 5500 Series Adaptive Security Appliance (ASA) that may result in a reload of the device. These vulnerabilities are triggered during processing of Media Gateway Control Protocol (MGCP) packets, or during processing of Transport Layer Security (TLS) traffic that terminates on the PIX or ASA security appliance.

Note: These vulnerabilities are independent of each other; a device may be affected by one and not by the other.

This advisory is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20071017-asa.

Affected Products

Products Confirmed Not Vulnerable

This Security Advisory describes two distinct vulnerabilities that are independent of each other.

1. Crafted MGCP Packet

A PIX or ASA security appliance with the Media Gateway Control Protocol (MGCP) application layer protocol inspection feature enabled may reload when the device processes a crafted MGCP packet. MGCP application layer protocol inspection is not enabled by default.

MGCP messages are transmitted over the User Datagram Protocol (UDP), which does allow the crafted MGCP messages to be sourced from a spoofed address. Only the MGCP for gateway application (MGCP traffic on UDP port 2427) is affected.

To determine whether MGCP inspection is configured on the PIX or ASA, log in to the device and issue the CLI command show service-policy | include mgcp. If the output contains the text Inspect: mgcp and some statistics, then the device has a vulnerable configuration. The following example shows a vulnerable Cisco ASA Security Appliance:

ASA# show service-policy | include mgcp
      Inspect: mgcp, packet 15, drop 0, reset-drop 0
ASA#

This vulnerability is documented in Cisco Bug ID CSCsi90468. The corresponding Cisco Bug ID for the FWSM, included in the companion FWSM Security Advisory, is CSCsi00694.

2. Crafted TLS Packet

Transport Layer Security (TLS) is the replacement for the Secure Socket Layer (SSL) protocol. It is a protocol that provides, via cryptography, secure communications between two end-points.

The PIX and ASA security appliances rely on TLS to protect the confidentiality of communications in a variety of scenarios. In all these scenarios, the PIX and ASA may be affected by a vulnerability in the handling of the TLS protocol that may lead to a reload of the device when it processes specially crafted TLS packets.

The scenarios affected by this vulnerability are clientless WebVPN connections, HTTPS management sessions, cut-through proxy for network access, and TLS proxy for encrypted voice inspection.

Clientless WebVPN, SSL VPN Client, and AnyConnect Connections

Clientless WebVPN connections are enabled via the webvpn command. For example, the following configuration shows an ASA running 8.0 software with clientless WebVPN configured and enabled. In this case the ASA will listen for WebVPN connections on the default port, TCP port 443:

http server enable                                                              
!
webvpn                                                                          
 enable outside

Note that with this particular configuration, the device is vulnerable to attacks coming from the outside interface.

HTTPS Management Sessions

HTTPS management sessions are enabled via the http server enable and http commands. For example, the following configuration shows an ASA configured for remote HTTPS management:

http server enable
http 192.168.0.0 255.255.255.0 inside

Note that with this particular configuration the device is vulnerable to attacks coming from the inside interface and from the 192.168.0.0/24 IP sub-network.

Cut-Through Proxy for Network Access

The cut-through proxy feature is used to authenticate users before they can access the network. A configuration that requires users to authenticate before they can be granted network access looks like the following example:

access-list auth-proxy extended permit tcp any any eq www
access-list auth-proxy extended permit tcp any any eq telnet
access-list auth-proxy extended permit tcp any any eq https
!
aaa authentication match auth-proxy inside LOCAL
aaa authentication secure-http-client
aaa authentication listener https inside port https

A configuration affected by this vulnerability will contain the command aaa authentication secure-http-client or aaa authentication listener https inside port <port number>.

Note that with the particular configuration in the preceding example, the device is vulnerable to attacks coming from the inside interface.

TLS Proxy for Encrypted Voice Inspection

This feature allows the security appliance to decrypt, inspect and modify (as needed, for example, performing NAT fixup), and re-encrypt voice signaling traffic while all of the existing VoIP inspection functions for Skinny and Session Initiation Protocol (SIP) protocols are preserved. Once voice signaling is decrypted, the plain-text signaling message is passed to the existing inspection engines. The security appliance accomplishes this by acting as a TLS proxy between the IP phone and Cisco Unified CallManager, which implies that TLS sessions are terminating on the security appliance.

To determine whether the Cisco PIX or ASA security appliance is configured to support inspection of encrypted voice, log in to the device and issue the CLI command show service-policy | include tls. If the output contains the text tls-proxy: active and some statistics, then the device has a vulnerable configuration. The following example shows a vulnerable Cisco ASA Security Appliance:

ASA# show service-policy | include tls
      Inspect: sip tls-proxy myproxy, packet 0, drop 0, reset-drop 0
               tls-proxy: active sess 0, most sess 0, byte 0
      Inspect: skinny tls-proxy myproxy, packet 0, drop 0, reset-drop 0
               tls-proxy: active sess 0, most sess 0, byte 0
ASA#

This vulnerability is documented in Cisco Bug IDs CSCsg43276 and CSCsh97120. This vulnerability does not affect the FWSM.

Cisco is providing 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.

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.

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 vulnerabilities described in this advisory will result in a reload of the affected device. Repeated exploitation can result in a sustained denial of service (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.

The following list contains the first fixed software release of each vulnerability:

The following maintenance software releases are the first software releases that contain the fixes for the two vulnerabilities mentioned in this Security Advisory: 7.0(7), 7.1(3), 7.2(3), and 8.0(2).

Fixed PIX software can be downloaded from http://www.cisco.com/pcgi-bin/tablebuild.pl/pix?psrtdcat20e2.

Fix ASA software can be downloaded from http://www.cisco.com/pcgi-bin/tablebuild.pl/asa?psrtdcat20e2.

Filters that deny TLS packets using TCP port 443 and MGCP packets on UDP port 2427 should be deployed throughout the network as part of a transit ACL (tACL) policy for protection of traffic which enters the network at ingress access points. This policy should be configured to protect the network device where the filter is applied and other devices behind it. Filters for TLS packets using TCP port 443 and MGCP packets on UDP port 2427 should also be deployed in front of vulnerable network devices so that traffic is only allowed from trusted clients.

Additional information about tACLs is available in "Transit Access Control Lists : Filtering at Your Edge": http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00801afc76.shtml.

Additional mitigations techniques that can be deployed on Cisco devices within the network are available in the Cisco Applied Intelligence companion document for this advisory: http://www.cisco.com/warp/public/707/cisco-air-20071017-asafwsm.shtml.

1. Crafted MGCP Packet

There is no workaround for this vulnerability other than disabling MGCP application layer protocol inspection on the device.

Leveraging anti-spoofing techniques will help mitigate spoofed packets from triggering this vulnerability.

Limiting MGCP application layer inspection to traffic between MGCP gateways may help to mitigate this vulnerability since it would require an attacker to have additional information (the addresses of the MGCP gateways) to launch a successful attack. To limit MGCP application layer inspection to traffic between certain devices, a class map that matches only traffic between the gateways must be created. Then, MGCP inspection must be performed on traffic in that class. The following example shows how to accomplish this:

ASA(config)# access-list mgcp_traffic permit udp host 192.168.0.1 host 172.16.0.1 eq 2427
ASA(config)# access-list mgcp_traffic permit udp host 172.16.0.1 host 192.168.0.1 eq 2427
ASA(config)# class-map MGCP
ASA(config-cmap)# match access-list mgcp_traffic
ASA(config-cmap)# exit
ASA(config)# policy-map global_policy
ASA(config-pmap)# class inspection_default
ASA(config-pmap-c)# no inspect mgcp
ASA(config-pmap-c)# exit
ASA(config-pmap)# class MGCP
ASA(config-pmap-c)# inspect mgcp
ASA(config-pmap-c)# exit
ASA(config-pmap)# exit
ASA(config)# 

Note that MGCP inspection is applied only to UDP traffic between hosts 192.168.0.1 and 172.16.0.1

See the Cisco Applied Intelligence companion document for additional mitigation possibilities.

2. Crafted TLS Packet

ASDM is used to manage the Cisco PIX or ASA security appliance. Access to ASDM should be allowed only on trusted interfaces and only from authorized hosts. Restricting ASDM access to trusted hosts limits the ability of an attacker to conduct these attacks.

For example, to limit ASDM access to a single host on the inside interface with an address of 192.168.1.2, enter the following command:

hostname(config)# http 192.168.1.2 255.255.255.255 inside

Additional information is available at:

Cisco Security Appliance Command Line Configuration Guide, Version 7.2 ( http://www.cisco.com/en/US/docs/security/asa/asa72/configuration/guide/mgaccess.html#wp1047288)

There are no workarounds if the clientless WebVPN, client connections with AnyConnect and the SSL VPN client, cut-through proxy for network access, and TLS proxy for encrypted voice inspection features are in use.

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.

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

Customers with Service Contracts

Customers Using Third-Party Support Organizations

Customers Without Service Contracts

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

These vulnerabilities were discovered by Cisco internal testing and customer service requests.

THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS AND DOES NOT IMPLY ANY KIND OF GUARANTEE OR WARRANTY, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.

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 is posted on Cisco's worldwide website at:

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20071017-asa

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.