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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)
- -----------------------------------------------------------------------
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