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 https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20070724-arp.

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 ( registered customers only) .

  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 ( registered customers only) 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 ( registered customers only) .

  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 ( registered customers only) .

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 ( registered customers only) 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.
     

Fixed Software

• 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	3.2.195.13
  4.0	4.0.219.0
  4.1	4.1.181.0

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.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20070724-arp

Revision History

• Revision 1.1	2007-July-31	Updated fixed software version information
  Revision 1.0	2007-July-24	Initial public release

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