Cisco Security Advisory

Cisco Nexus 5000 and 3000 Series Switches Access Control List Bypass Vulnerability

Advisory ID: cisco-sa-20110907-nexus

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20110907-nexus

Revision 1.0

For Public Release 2011 September 7 16:00  UTC (GMT)


Summary

A vulnerability exists in Cisco Nexus 5000 and 3000 Series Switches that may allow traffic to bypass deny statements in access control lists (ACLs) that are configured on the device.

Cisco has released free software updates that address this vulnerability.

A workaround is available to mitigate this vulnerability.

This advisory is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20110907-nexus.

Affected Products

Cisco Nexus 5000 and 3000 Series Switches are affected by this vulnerability when a remark is configured before a deny statement on an ACL.

Vulnerable Products

All Cisco Nexus 5000 NX-OS Software Releases 5.0(2) and 5.0(3) prior to 5.0(3)N2(1) are affected by this vulnerability.

Note: Cisco Nexus 5000 NX-OS Software Releases 4.x are not affected by this vulnerability.

All Cisco Nexus 3000 NX-OS Software Releases prior to 5.0(3)U1(2a) or 5.0(3)U2(1) are affected by this vulnerability.

The effects of this vulnerability are experienced when an ACL remark is configured prior to any deny statement on the ACL. A remark is a comment about the configured access control entry (ACE).

The following example shows how to create a remark in an IPv4 ACL and display the results:

ip access-list acl-ipv4-01
 remark this ACL denies the 10.1.1.0/24 access to the 10.1.2.0/24 network 
 deny ip 10.1.1.0 0.0.0.255 10.1.2.0 0.0.0.255

Note: All the ACEs after a remark are affected. This includes the default implicit deny at the end of the ACL. IPv4, IPv6 and MAC ACLs are affected. Quality of service (QoS) classification and route-map ACLs are not affected by this vulnerability.

Determining Software Version

To determine the Cisco NX-OS Software release that is running on a Cisco product, administrators can log in to the device and issue the show version command to display the system banner. The following example shows how to display the version information for the kickstart and system image running on a device that runs Cisco NX-OS Release 5.0(2)N2(1):

switch# show version 
Cisco Nexus Operating System (NX-OS) Software
TAC support: http://www.cisco.com/tac
Copyright (c) 2002-2010, Cisco Systems, Inc. All rights reserved.
The copyrights to certain works contained herein are owned by
other third parties and are used and distributed under license.
Some parts of this software are covered under the GNU Public
License. A copy of the license is available at
http://www.gnu.org/licenses/gpl.html.

Software
  BIOS:      version 1.3.0
  loader:    version N/A
  kickstart: version 5.0(2)N2(1) [build 5.0(2)N2(1)]
  system:    version 5.0(2)N2(1) [build 5.0(2)N2(1)]


!--- output truncated

Products Confirmed Not Vulnerable

The following Cisco products are confirmed not to be affected by this vulnerability.

  • Cisco Nexus 7000 Series Switches
  • Cisco Nexus 4000 Series Switches
  • Cisco Nexus 2000 Series Switches
  • Cisco Nexus 1000V Series Switches
  • Cisco MDS 9000 Software
  • Cisco Unified Computing System

No other Cisco products are currently known to be affected by this vulnerability.

Details

An ACL is an ordered set of rules that filter traffic. Each rule specifies a set of conditions that a packet must satisfy to match the rule. When the device determines that an ACL applies to a packet, it tests the packet against the conditions of all rules. The first matching rule determines whether the packet is permitted or denied. If there is no match, the device applies the applicable implicit rule. The device continues processing packets that are permitted and drops packets that are denied.

A vulnerability in Cisco Nexus 5000 and 3000 Series Switches may allow traffic to bypass deny statements in IP, VLAN, or MAC ACLs that are configured in the device. This behavior is experienced when an ACL remark is configured prior to any deny statement on such ACL.

Note: All the ACEs after a remark are affected. This includes the default implicit deny at the end of the ACL. IPv4, IPv6 and MAC ACLs are affected. QoS classification and route-map ACLs are not affected by this vulnerability.

This vulnerability is documented in Cisco bug IDs CSCto09813 ( registered customers only) and CSCtr61490 ( registered customers only) ; and has been assigned CVE ID CVE-2011-2581.

Vulnerability Scoring Details

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 .

CSCto09813 and CSCtr61490 - Access Control List Bypass Vulnerability

Calculate the environmental score of CSCto09813 and CSCtr61490

CVSS Base Score - 5

Access Vector

Access Complexity

Authentication

Confidentiality Impact

Integrity Impact

Availability Impact

Network

Low

None

Partial

None

None

CVSS Temporal Score - 4.1

Exploitability

Remediation Level

Report Confidence

Functional

Official-Fix

Confirmed

Impact

Successful exploitation of the vulnerability may allow an attacker to access resources that should be protected by the ACL configured in Cisco Nexus 5000 and 3000 Series Switches.

Software Versions and Fixes

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.

Cisco Nexus 3000 NX-OS Software

This vulnerability has been corrected in Cisco Nexus 3000 NX-OS Software Release 5.0(3)U1(2a) or 5.0(3)U2(1) and later.

Cisco Nexus 3000 NX-OS Software can be downloaded from the following link:http://www.cisco.com/cisco/software/find.html?q=nx-os

Cisco Nexus 5000 NX-OS Software

This vulnerability has been corrected in Cisco Nexus 5000 NX-OS Software Releases 5.0(3)N2(1) and later.

Cisco Nexus 5000 NX-OS Software can be downloaded from the following link:http://www.cisco.com/cisco/software/find.html?q=nx-os


Workarounds

The effects of this vulnerability are experienced when an ACL remark is configured prior to any deny statement on the ACL. As a workaround, remarks can be removed from the configuration to mitigate this vulnerability. ACL remarks can be removed using the no remark command under each configured ACL.

Obtaining Fixed Software

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 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 Using Third-Party Support Organizations

Customers whose Cisco products are provided or maintained through prior or existing agreements 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 Without Service Contracts

Customers who purchase direct from Cisco but do not hold a Cisco service contract, and customers who purchase through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should acquire 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

Customers should have their product serial number available and be prepared to give the URL of this notice as evidence of entitlement to a free upgrade. Free upgrades for non-contract customers must be requested through the TAC.

Refer to http://www.cisco.com/en/US/support/tsd_cisco_worldwide_contacts.html for additional TAC contact information, including localized telephone numbers, and instructions and e-mail addresses for use in various languages.

Exploitation and Public Announcements

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

This vulnerability was found during the troubleshooting of a customer service request.

Status of This Notice: Final

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.


Distribution

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

http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20110907-nexus

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.


Revision History

Revision 1.0

2011-September-07

Initial public release.

Cisco Security Procedures

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/web/about/security/psirt/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.


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