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Cisco Security Advisory: Wireless ARP Storm Vulnerabilities

Advisory ID: cisco-sa-20070724-arp

http://www.cisco.com/warp/public/707/cisco-sa-20070724-arp.shtml

Revision 1.0

For Public Release 2007 July 24 1600 UTC (GMT)

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Summary
=======

Cisco Wireless LAN Controllers (WLC) contain multiple vulnerabilities
in the handling of Address Resolution Protocol (ARP) packets that could
result in a denial of service (DoS) in certain environments.

Cisco is notifying customers and partners and has made free software
available to address these vulnerabilities for affected customers.
There are workarounds available to mitigate the effects of these
vulnerabilities.

This advisory is posted at 
http://www.cisco.com/warp/public/707/cisco-sa-20070724-arp.shtml.

Affected Products
=================

Unless otherwise specified, the vulnerabilities addressed in this
document affect versions 4.1, 4.0, 3.2, and prior versions of the
Wireless LAN Controller software. To identify the earliest software
releases that include fixes for these vulnerabilities, please consult
the Software Versions and Fixes section of this advisory.

To determine the version of WLC system software running on a particular
device, one of the following methods may be used:

  * In the web interface, choose the Monitor tab, click Summary in the
    left-hand pane, and note the "Software Version."
  * From the command-line interface, type "show sysinfo" and note the
    "Product Version."

Vulnerable Products
+------------------

Vulnerable versions of software may be running on any of the following
hardware platforms:

  * Cisco 4100 Series Wireless LAN Controllers
  * Cisco 4400 Series Wireless LAN Controllers
  * Cisco Airespace 4000 Series Wireless LAN Controller
  * Cisco Catalyst 6500 Series Wireless Services Module (WiSM)
  * Cisco Catalyst 3750 Series Integrated Wireless LAN Controllers

Products Confirmed Not Vulnerable
+--------------------------------

The following hardware platforms are not affected by these
vulnerabilities:

  * Cisco 2000 Series Wireless LAN Controllers
  * Cisco 2100 Series Wireless LAN Controllers
  * Cisco Airespace 3500 Series WLAN Controller
  * Cisco 526 Wireless Express Mobility Controller
  * Cisco Wireless LAN Controller Module
    (NM-AIR-WLC6-K9,NME-AIR-WLC8-K9,NME-AIR-WLC12-K9)
  * Standalone Access Points such as the 1100 Series, 1200 Series and
    AP340/350
  * Cisco 3800 Series Integrated Services Routers
  * Cisco 2800 Series Integrated Services Routers
  * Cisco 1800 Series Integrated Services Routers
  * Cisco 800 Series Routers

Details
=======

Cisco Wireless LAN Controllers provide real-time communication between
lightweight access points and other Wireless LAN controllers for
centralized system wide WLAN configuration and management functions.

The Address Resolution Protocol, or ARP, provides a mapping between a
device's IP address and its hardware address on the local network.

The WLC contains vulnerabilities in the processing of unicast ARP
traffic where a unicast ARP request may be flooded on the LAN links
between Wireless LAN Controllers in a mobility group.

RFC4436 defines a method for IP Version 4 hosts to
detect if they have re-attached to a previously attached network. In
such cases, it may be unnecessary to request a new DHCP address lease
if the current lease is still active. To determine reattachment, the
host may send a unicast ARP request to the address of the default
gateway that it had previously used.

A vulnerable WLC may mishandle unicast ARP requests from a wireless
client leading to an ARP storm. In order for the vulnerability to be
exposed, two WLCs attached to the same set of Layer-2 VLANs must each
have a context for the wireless client. This can occur after a Layer-3
(cross-subnet) roam or when guest WLAN (auto-anchor) is in use.

If the client sends a unicast ARP request with a destination MAC
address that has not been learned by the Layer-2 infrastructure, that
request will be flooded to all ports in the Layer-2 domain after
egressing the WLC. This allows the second WLC to reprocess the ARP
request and incorrectly reforward this packet back into the network.
This vulnerability is documented as CSCsj69233.

If the arpunicast feature has been enabled on the WLC, the WLC will
re-forward broadcast ARP packets targeting the IP address of a known
client context. This creates an ARP storm if more than one WLC is
installed on the corresponding VLAN. This vulnerability is documented
as CSCsj50374 and only affects version 4.1 of the WLC software 
(versions 4.0, 3.2, or previous versions are not affected).

In a Layer-3 (L3) roaming scenario, a wireless client moves from one
controller to another where the wireless LAN interfaces configured on
different controllers are on different IP subnets. In this scenario, a
unicast ARP may not tunneled back to the anchor controller, but may
instead be sent by the foreign controller out to a local VLAN. This
vulnerability is documented as CSCsj70841. 

Note: In versions of software prior to 4.1, a unicast ARP request from
a wireless client that performed a Layer-3 roam was dropped at the
Foreign WLC. This behavior has been corrected as part of CSCsj70841. 

Vulnerability Scoring Details
+----------------------------

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 1.0.

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.

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.

CSCsj69233 - "WLCs loop unicast ARPs to wrong subnet between each other"
  CVSS Base Score - 3.3
    Access Vector -            Remote
    Access Complexity -        Low
    Authentication -           Not Required
    Confidentiality Impact -   None
    Integrity Impact -         None
    Availability Impact -      Complete
    Impact Bias -              Normal
  CVSS Temporal Score - 2.7
    Exploitability -           Functional
    Remediation Level -        Official-Fix
    Report Confidence -        Confirmed

CSCsj50374 - "ARP requests could cause broadcast loop on WLCs"
  CVSS Base Score - 3.3
    Access Vector -            Remote
    Access Complexity -        Low
    Authentication -           Not Required
    Confidentiality Impact -   None
    Integrity Impact -         None
    Availability Impact -      Complete
    Impact Bias -              Normal
  CVSS Temporal Score - 2.7
    Exploitability -           Functional
    Remediation Level -        Official-Fix
    Report Confidence -        Confirmed

CSCsj70841 - "Unicast ARP forwarding broken after l3 roam"
  CVSS Base Score - 4.7
    Access Vector -            Remote
    Access Complexity -        Low
    Authentication -           Not Required
    Confidentiality Impact -   None
    Integrity Impact -         Partial
    Availability Impact -      Partial
    Impact Bias -              Normal
  CVSS Temporal Score - 3.9
    Exploitability -           Functional
    Remediation Level -        Official-Fix
    Report Confidence -        Confirmed

Impact
======

Successful exploitation of these vulnerabilities may result in a DoS
condition.

Software Version 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.

+-------------------------------------------------------------+
|      Major Release       |  Availability of Fixed Releases  |
|--------------------------+----------------------------------|
| 3.2                      | available 27-July-2007           |
|--------------------------+----------------------------------|
| 4.0                      | available 27-July-2007           |
|--------------------------+----------------------------------|
| 4.1                      | 4.1.181.0                        |
+-------------------------------------------------------------+

Workarounds
===========

For enhanced security, Cisco recommends that operators require all
clients to obtain their IP addresses from a DHCP server. To enforce
this requirement, all WLANs can be configured with a DHCP Required
setting, which disallows client static IP addresses. If DHCP Required
is selected, clients must obtain an IP address via DHCP. Any client
with a static IP address will not be allowed on the network. The
controller monitors DHCP traffic because it acts as a DHCP proxy for
the clients.

This workaround is generally effective for wireless clients employing
the mechanisms defined in RFC4436 when joining a network. It is not 
effective against deliberate attempts to craft packets that create an 
ARP storm.

Customers experiencing exploitation from the vulnerability associated
with CSCsj50374 may configure the WLC to disable arpunicast processing 
via the CLI:

    config network arpunicast disable

This section provides both GUI and CLI instructions for configuring
your WLAN to use a DHCP server.

Using the GUI to Configure DHCP
+------------------------------

 1. In the web user interface, navigate to the WLANs page.
 2. Locate the WLAN you wish to configure for a DHCP server, and click
    the associated Edit link to display the WLANs > Edit page.
 3. Under General Policies, check the DHCP Relay/DHCP Server IP Addr 
    check box to verify whether you have a valid DHCP server assigned
    to the WLAN. If you do not have a DHCP server assigned to the WLAN,
    continue with Step 4. Otherwise, continue with Step 9.
 4. Under General Policies, uncheck the Admin Status check box.
 5. Click Apply to disable the WLAN.
 6. In the DHCP Relay/DHCP Server IP Addr edit box, enter a valid DHCP
    server IP address for this WLAN.
 7. Under General Policies, check the Admin Status check box.
 8. Click Apply to assign the DHCP server to the WLAN and to enable the
    WLAN. You are then returned to the WLANs page.
 9. In the upper-right corner of the WLANs page, click Ping and enter
    the DHCP server IP address to verify that the WLAN can communicate
    with the DHCP server.

Using the CLI to Configure DHCP
+------------------------------

 1. In the CLI, enter "show wlan" to verify whether you have a valid DHCP
    server assigned to the WLAN. If you do not have a DHCP server
    assigned to the WLAN, continue with Step 2. Otherwise, continue
    with Step 4.
 2. If necessary, use the following commands:

        config wlan disable <wlan-id>
        config wlan dhcp_server <wlan-id> <dhcp-server-ip-address>
        config wlan enable <wlan-id>


    In these commands, wlan-id = 1 through 16, and
    dhcp-server-ip-address = DHCP server IP address.
 3. Enter "show wlan" to verify that you have a DHCP server assigned to
    the WLAN.
 4. Enter "ping dhcp-ip-address" to verify that the WLAN can communicate
    with the DHCP server.

Obtaining Fixed Software
========================

Cisco will make free software available to address these
vulnerabilities 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 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 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 without Service Contracts
+----------------------------------

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.

Exploitation and Public Announcements
=====================================

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

These vulnerabilities were reported to Cisco by customers.

Status of this Notice: INTERIM
==============================

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. CISCO EXPECTS TO UPDATE THIS DOCUMENT AS NEW
INFORMATION BECOMES AVAILABLE.

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://www.cisco.com/warp/public/707/cisco-sa-20070724-arp.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.

Revision History
================

+-------------------------------------------------------------+
| Revision 1.0   | 2007-July-24   | 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/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.

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All contents are Copyright 2006-2007 Cisco Systems, Inc. All rights
reserved. 
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Updated: Jul 24, 2007                                Document ID: 97823

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