Cisco Security Advisory

Advisory ID: cisco-sa-20050126-bgp

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050126-bgp

Revision 1.5

For Public Release 2005 January 26 16:00  UTC (GMT)

Summary

A Cisco device running IOS Border Gateway Protocol (BGP) is vulnerable to a Denial of Service (DoS) attack from a malformed BGP packet. Only devices with either the command bgp log-neighbor-changes configured or the command snmp-server enable traps bgp are vulnerable. The BGP protocol is not enabled by default, and must be configured in order to accept traffic from an explicitly defined peer. Unless the malicious traffic appears to be sourced from a configured, trusted peer, it would be difficult to inject a malformed packet.

Cisco has made free software available to address this problem.

This issue is tracked by CERT/CC VU#689326.

This advisory will be posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050126-bgp.

Affected Products

Products Confirmed Not Vulnerable

The Border Gateway Protocol (BGP) is a routing protocol defined by RFC 1771, and designed to manage IP routing in large networks. An affected Cisco device running a vulnerable version of Cisco IOS software with the BGP protocol enabled will reload if a malformed BGP packet is already queued on the interface when a BGP neighbor change is logged. The device is not vulnerable unless either bgp log-neighbor-changes is configured or snmp-server traps enable bgp is configured.

Malformed packets can not only come from malicious sources. A valid peering device such as another BGP speaking router that produces the specific malformed packet in error can trigger this behavior.

In all cases, however, the packets must be sourced from an IP address that is explicitly configured.

BGP runs over the Transport Control Protocol (TCP), a reliable transport protocol which requires a valid three way handshake before any further messages will be accepted. The Cisco IOS implementation of BGP requires the explicit definition of a neighbor before a connection can be established, and traffic must appear to come from that neighbor. These implementation details make it very difficult to maliciously send a BGP packet to a Cisco IOS device from an unauthorized source.

This bug may also be triggered by other means which are not considered remotely exploitable. The use of the commands show ip bgp neighbors or debug ip bgp can cause a router to reload if the device has previously received a malformed packet.

Additionally, using an SNMP management station to poll the SNMP Object Identifier (OID) bgpPeerEntry.bgpPeerLastError (1.3.6.1.2.1.15.3.1.14) may cause the device to reset. This may only happen after the device has received the malformed packet and the BGP session is reset.

A Cisco IOS device receiving an invalid BGP packet will reset and may take several minutes to become fully functional. This vulnerability may be exploited repeatedly resulting in an extended DoS attack. This issue is documented in bug ID CSCee67450 ( registered customers only) .

A Cisco IOS XR device will restart the BGP process. All the sessions will be dropped and BGP process will have to re-establish sessions with its peers. No other routing protocols will be affected.

If graceful restart is enabled, forwarding should not be impacted.

In IOS XR, the BGP process is not mandatory by default. Unless the user has explicitly configured the BGP process to be mandatory, the IOS XR device will not reload.

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

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

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 this vulnerability on an IOS device results in a reload of the device. Repeated exploitation could result in a sustained DoS attack.

Successful exploitation of this vulnerability on an IOS XR device results in a restart of the BGP process. Repeated exploitation could result in a sustained DoS attack against BGP functionality. Other services on the device are not affected.

When considering software upgrades, please also consult http://www.cisco.com/en/US/products/products_security_advisories_listing.html 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") for assistance.

Cisco customers running IOS XR software should contact Cisco's TAC for repaired software.

The effectiveness of any workaround 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 is the most appropriate for use in the intended network before it is deployed.

• Remove the configuration command bgp log-neighbor-changes. This feature is used to monitor BGP peer status and its removal may reduce network monitoring capabilities. More information on this command is available here: http://www.cisco.com/en/US/docs/ios/12_3/iproute/command/reference/ip2_a1g.html#wp1040601
  

The use of networking best practices techniques can greatly reduce the probability of a network infrastructure attack. Best practices that may reduce risk in this case include:

BGP MD5

Under normal circumstances, due to inherent security factors in the TCP protocol, such as sequence number checks, it is difficult, but possible to forge an appropriate packet to exploit this problem. Configuring your Cisco IOS device for BGP MD5 authentication greatly increases the work necessary to forge a valid packet from a remote peer. This will not protect your peering session if a valid BGP peer generates an invalid packet.

This can be configured as shown in the following example:

router(config)# router bgp 
 router(config-router)# neighbor <IP_address> password <enter_your_secret_here>

It is necessary to configure the same shared MD5 secret on both peers and at the same time. Failure to do so will break the existing BGP session and the new session will not get established until the exact same secret is configured on both devices. For a detailed discussion on how to configure BGP, refer to the following document:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_configuration_guide_chapter09186a00800ca571.html

Once the secret is configured, it is prudent to change it periodically. The exact period must fit within your company security policy but it should not be longer than a few months. When changing the secret, again it must be done at the same time on both devices. Failure to do so will break your existing BGP session. The exception is if your Cisco IOS software release contains the integrated CSCdx23494 ( registered customers only) fix on both sides of the connection. With this fix, the BGP session will not be terminated when the MD5 secret is changed only on one side. The BGP updates, however, will not be processed until either the same secret is configured on both devices or the secret is removed from both devices.

Infrastructure Access Control Lists (iACLs)

Although it is often difficult to block traffic transiting your network, it is possible to identify traffic that should never be allowed to target your infrastructure devices and block that traffic at the border of your network. Infrastructure ACLs are considered a network security best practice and should be considered as a long-term addition to good network security as well as providing some added protection for this specific vulnerability. The white paper entitled "Protecting Your Core: Infrastructure Protection Access Control Lists" presents guidelines and recommended deployment techniques for infrastructure protection ACLs:

http://www.cisco.com/warp/public/707/iacl.html

Workarounds for IOS XR

Users of Cisco IOS XR can avoid exposure to this vulnerability by not enabling 'debug bgp' at a level of 'warning' or anything more verbose and not logging bgp neighbor state changes.

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

This vulnerability was discovered by Cisco during internal testing.

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 will be posted on Cisco's worldwide website at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20050126-bgp .

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 (includes CERT/CC)
  
• bugtraq@securityfocus.com
  
• vulnwatch@vulnwatch.org
  
• cisco@spot.colorado.edu
  
• cisco-nsp@puck.nether.net
  
• full-disclosure@lists.netsys.com
  
• 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.