Products & Services

Contents

Summary
Affected Products
Details
Vulnerability Scoring Details
Impact
Software Versions 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

Data-link switching (DLSw) provides a means of transporting IBM Systems Network Architecture (SNA) and network basic input/output system (NetBIOS) traffic over an IP network. Cisco implementation of DLSw also uses UDP port 2067 and IP Protocol 91 for Fast Sequenced Transport (FST).

Multiple vulnerabilities exists in Cisco IOS when processing UDP and IP protocol 91 packets. These vulnerabilities do not affect TCP packet processing. A successful exploitation may result in a reload of the system or a memory leak on the device, leading to a denial of service (DoS) condition.

Cisco IOS devices configured for DLSw with dlsw local-peer automatically listen for IP protocol 91 packets. A Cisco IOS device that is configured for DLSw with the dlsw local-peer peer-id <IP-address> command listen for IP protocol 91 packets and UDP port 2067.

Cisco IOS devices listen to IP protocol 91 packets when DLSw is configured. However, it is only used if DLSw is configured for Fast Sequenced Transport (FST). A DLSw FST peer configuration will contain the following line:

dlsw remote-peer 0 fst <ip-address>

It is possible to disable UDP processing in DLSw with the dlsw udp-disable command. However, disabling UDP only prevents the sending of UDP packets, it does not prevent the device from receiving and processing incoming UDP packets.

These vulnerabilities are documented in Cisco Bug ID CSCsk73104 ( registered customers only) and have been assigned Common Vulnerabilities and Exposures (CVE) ID CVE-2008-1152.

Cisco has provided 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.

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 these vulnerabilities may result in the reload of the device or memory leaks, leading to a 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.

Each row of the Cisco IOS software table (below) names a Cisco IOS release train. If a given release train is vulnerable, then the earliest possible releases that contain the fix (along with the anticipated date of availability for each, if applicable) are listed in the "First Fixed Release" column of the table. The "Recommended Release" column indicates the releases which have fixes for all the published vulnerabilities at the time of this Advisory. 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. Cisco recommends upgrading to a release equal to or later than the release in the "Recommended Releases" column of the table.

A special patch for Cisco IOS Software Modularity is also available for 12.2(18)SXF11 and can be downloaded from the Cisco IOS Software Modularity Patch Navigator at http://tools.cisco.com/swdf/ionpn/jsp/result.jsp?s_tarballWild=mp001-p.122-18.SXF11&reqType=cWork.

The workaround consists of filtering UDP packets to port 2067 and IP protocol 91 packets. Filters can be applied at network boundaries to filter all IP protocol 91 packets and UDP packets to port 2067 or can be applied on individual affected devices to permit such traffic only from trusted peer IP addresses. However, since both of the protocols are connectionless, it is possible for an attacker to spoof malformed packets from legitimate peer IP addresses.

As soon as DLSw is configured, the Cisco IOS device begins listening on IP protocol 91. However, this protocol is only used if DLSw is configured for Fast Sequenced Transport (FST). A DLSw FST peer configuration will contain the following line:

dlsw remote-peer 0 fst <ip-address>

If FST is used, filtering IP protocol 91 will break the operation, so filters need to permit protocol 91 traffic from legitimate peer IP addresses.

It is possible to disable UDP processing in DLSw with the dlsw udp-disable command. However, disabling UDP only prevents the sending of UDP packets, it does not prevent the receiving and processing of incoming UDP packets. To protect a vulnerable device from malicious packets via UDP port 2067, both of the following actions must be taken:

1. Disable UDP outgoing packets with the "dlsw udp-disable" command, AND
   
2. Filter UDP 2067 in the vulnerable device using infrastructure ACL.
   

Additional mitigation techniques that can be deployed on Cisco devices within the network are available in the Cisco Applied Mitigation Bulletin companion document for this advisory:

http://www.cisco.com/warp/public/707/cisco-amb-20080326-dlsw.shtml

Using Control Plane Policing on Affected Devices

Control Plane Policing (CoPP) can be used to block untrusted DLSw traffic to the device. Cisco IOS software releases 12.0S, 12.2SX, 12.2S, 12.3T, 12.4, and 12.4T support the CoPP feature. CoPP may be configured on a device to protect the management and control planes to minimize the risk and effectiveness of direct infrastructure attacks by explicitly permitting only authorized traffic sent to infrastructure devices in accordance with existing security policies and configurations. The following example, which uses 192.168.100.1 to represent a trusted host, can be adapted to your network. If FST is not used, protocol 91 may be completely filtered. Additionally, if UDP is disabled with the dlsw udp-disable command, UDP port 2067 may also be completely filtered.

!--- Deny DLSw traffic from trusted hosts to all IP addresses
!--- configured on all interfaces of the affected device so that
!--- it will be allowed by the CoPP feature


access-list 111 deny udp host 192.168.100.1 any eq 2067
access-list 111 deny 91 host 192.168.100.1 any


!--- Permit all other DLSw traffic sent to all IP addresses
!--- configured on all interfaces of the affected device so that it
!--- will be policed and dropped by the CoPP feature


access-list 111 permit udp any any eq 2067
access-list 111 permit 91 any any 


!--- Permit (Police or Drop)/Deny (Allow) all other Layer 3 and Layer 4
!--- traffic in accordance with existing security policies and
!--- configurations for traffic that is authorized to be sent
!--- to infrastructure devices
!--- Create a Class-Map for traffic to be policed by
!--- the CoPP feature


class-map match-all drop-DLSw-class
 match access-group 111


!--- Create a Policy-Map that will be applied to the
!--- Control-Plane of the device.


policy-map drop-DLSw-traffic
 class drop-DLSw-class
  drop


!--- Apply the Policy-Map to the Control-Plane of the
!--- device


control-plane
 service-policy input drop-DLSw-traffic

In the above CoPP example, the access control entries (ACEs) which match the potential exploit packets with the "permit" action result in these packets being discarded by the policy-map "drop" function, while packets that match the "deny" action (not shown) are not affected by the policy-map drop function. Please note that in the Cisco IOS 12.2S and 12.0S trains the policy-map syntax is different:

policy-map drop-DLSw-traffic
 class drop-DLSw-class
  police 32000 1500 1500 conform-action drop exceed-action drop

Additional information on the configuration and use of the CoPP feature is available at http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6586/ps6642/prod_white_paper0900aecd804fa16a.html and http://www.cisco.com/en/US/docs/ios/12_3t/12_3t4/feature/guide/gtrtlimt.html.

Using Infrastructure ACLs at Network Boundary

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. iACLs are a network security best practice and should be considered as a long-term addition to good network security as well as a workaround for this specific vulnerability. The iACL example shown below should be included as part of the deployed infrastructure access-list that will protect all devices with IP addresses in the infrastructure IP address range. If FST is not used, protocol 91 may be completely filtered. Additionally, if UDP is disabled with the dlsw udp-disable command, UDP port 2067 may also be completely filtered.

!--- Permit DLSw (UDP port 2067 and IP protocol 91) packets
!--- from trusted hosts destined to infrastructure addresses.


access-list 150 permit udp TRUSTED_HOSTS MASK INFRASTRUCTURE_ADDRESSES MASK eq 2067

access-list 150 permit 91 TRUSTED_HOSTS MASK INFRASTRUCTURE_ADDRESSES MASK 


!--- Deny DLSw (UDP port 2067 and IP protocol 91) packets from 
!--- all other sources destined to infrastructure addresses.


access-list 150 deny udp any INFRASTRUCTURE_ADDRESSES MASK eq 2067
access-list 150 deny 91 any INFRASTRUCTURE_ADDRESSES MASK


!--- Permit/deny all other Layer 3 and Layer 4 traffic in accordance
!--- with existing security policies and configurations
!--- Permit all other traffic to transit the device.


access-list 150 permit ip any any

interface serial 2/0
 ip access-group 150 in

The white paper entitled "Protecting Your Core: Infrastructure Protection Access Control Lists" presents guidelines and recommended deployment techniques for infrastructure protection access lists. This white paper can be obtained at the following link:

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_white_paper09186a00801a1a55.shtml

Cisco has released free software updates that address these vulnerabilities. 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/en/US/docs/general/warranty/English/EU1KEN_.html, or as otherwise set forth at Cisco.com Downloads at http://www.cisco.com/public/sw-center/sw-usingswc.shtml.

Do not contact 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.

These vulnerabilities were found internally.

This advisory is posted on Cisco's worldwide website at :

http://www.cisco.com/warp/public/707/cisco-sa-20080326-dlsw.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.