Cisco Security Advisory

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

This section provides details on affected products.

Vulnerable Products

Products Confirmed Not Vulnerable

ILMI (Interim Local Management Interface) is an independent industry standard used for configuration of ATM (Asynchronous Transfer Mode) interfaces. The standard specifies the use of mechanisms and formats previously defined by SNMP (Simple Network Management Protocol). Although it is based on SNMP, ILMI communication actually occurs using a transport other than IP (Internet Protocol) that traverses only the physical ATM link. ILMI is essential to functions such as ATM auto-discovery and LANE (LAN Emulation).

SNMP "objects" are variables that are organized into a MIB (Management Information Base). The MIB has a tree structure and contains both operational (read-only) data as well as configuration (read-write) options. By specifying a community string of "ILMI" in an SNMP request, access can be obtained to read the objects in three specific parts of the overall management tree structure on any device affected by this vulnerability: the MIB-II system group, the LAN-EMULATION-CLIENT MIB, and the PNNI (Private Network-to-Network Interface) MIB. A subset of objects in each part can be modified using the same "ILMI" community string.

The MIB-II system group contains basic information about the device itself. The number of objects that can be modified is limited. Examples include:

• system.sysContact -- The contact information for the person or organization responsible for managing the device.
  
• system.sysLocation -- A description of the physical location where the device is installed or operating.
  
• system.sysName -- The hostname of the device, how it identifies itself at the console prompt. (This might not be the same name by which the device is known to other hosts on the network.)
  

Most of the objects in the system MIB are read-only and cannot be changed via SNMP, such as the time elapsed since the previous restart and textual descriptions of the device's hardware and software.

Numerous objects can be viewed in the LAN-EMULATION-CLIENT MIB and PNNI MIB, and modification of some of the read-write objects can have an affect on ATM operation of the device. The objects in the LAN-EMULATION-CLIENT MIB can only be viewed or modified if LANE has already been configured on the device.

Access to SNMP in Cisco IOS software can be limited by applying access control lists (ACLs), by modifying or removing the SNMP view, by removing the community string from the running configuration, or by disabling the SNMP service. Any SNMP query that does not meet the criteria for access is promptly discarded when such protective measures are in place. If a query does meet the criteria for access, then a response is formulated and sent.

It is possible to configure the device so that the ILMI community string is unavailable in all IOS 11.1 and higher releases. The particular method selected to accomplish this depends on the specific IOS release and configuration.

This defect is documented as CSCdp11863. The vulnerability is repaired by imposing a test such that an SNMP request using the "ILMI" community string will only be recognized if it has been transported by ILMI.

ATM functionality was added in various 10.x releases of Cisco IOS software. However, the function containing the defect was introduced when support for ILMI and other ATM features was added in IOS release 11.0(0.2). Therefore, all prior releases are not vulnerable.

A separate Cisco Security Advisory has been announced regarding additional SNMP vulnerabilities. That advisory, http://www.cisco.com/warp/public/707/ios-snmp-community-vulns-pub.shtml, should be consulted in tandem with this notice.

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 .

If SNMP requests can be received by an affected device, then certain MIB objects can be viewed without proper authorization, causing a violation of confidentiality.

A subset of the readable MIB objects can be modified without authorization to cause a failure of integrity. For example, the hostname can be modified so as to confuse network adminstrators, or the contact and location information could be changed with a goal of disrupting operations or embarrassing whoever is responsible for the device.

Objects in the LAN-EMULATION-CLIENT and PNNI MIBs can be viewed and modified, thus resulting in changes to the operation of ATM functions. If ATM is in use on the device, this may result in a failure of availability.

Any affected device that is not otherwise protected against the receipt of SNMP packets is vulnerable to a denial-of-service (DoS) attack by flooding the SNMP port with read or write requests.

The following table summarizes the known affected Cisco IOS software releases and the earliest estimated dates of availability for fixed releases. All dates are tentative and subject to change.

Each row of the table describes a release train and the platforms or products for which it is intended. If a given release train is vulnerable, then the earliest possible releases that contain the fix and the anticipated date

When selecting a release, keep in mind the following definitions:

• Maintenance - Most heavily tested and highly recommended release of any label in a given row of the table.
  
• Rebuild - Constructed from the previous maintenance or major release in the same train, it contains the fix for a specific defect. Although it receives less testing, it contains only the minimal changes necessary to effect the repair.
  
• Interim - Built at regular intervals between maintenance releases and receive less testing. Interims should be selected only if there is no other suitable release that addresses the vulnerability. Interim releases are usually not available for customer download via CCO without prior arrangement.
  

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 TAC for assistance as shown in the following section.

More information on IOS release names and abbreviations is available at http://www.cisco.com/warp/public/620/1.html.

Several workarounds are available based on customer needs, equipment, and software features. The usefulness and practicality of each workaround depends on the IOS release running on the device and many variables in the customer's environment. Customers are urged to consider each of the following alternatives carefully before deploying. These workarounds are only needed if it is not possible to upgrade to an unaffected release of IOS software.

1. Default workaround for use with releases for which no other workarounds are effective:
   
   1. Applying access lists to all the interfaces of the vulnerable device blocking SNMP from all hosts but those authorized to manage the devices.
      
   2. Blocking SNMP access at the edge of the network to prevent undesirable SNMP traffic from entering the network containing the vulnerable device.
      
   Access lists should be deployed with careful consideration of the possible effects on network operation and performance. Also note that authentication based on an IP source address is weak, so the preceding method will not protect against certain types of attacks in which the IP source address has been spoofed. Further information can be found in the Cisco document "Improving Security on Cisco Routers", available at http://www.cisco.com/warp/public/707/21.html
   
2. For affected releases based on IOS 11.1:
   
   1. Remove the view so that the ILMI community cannot be reached:
      no snmp-server view *ilmi
      
   2. This configuration will not survive a system reload. The command must be re-entered after every restart of the system.
      
3. For affected releases of IOS 11.2 through 11.3(8) NOT REQUIRING ATM:
   In this affected range of releases, the ILMI community string can be modified or deleted. However, the changes will not persist through a reboot of the device. These instructions must be reapplied following every system reload of the affected device:
   
   1. Expose the undocumented ILMI community string so it can be modified:
      snmp-server community ILMI RW
      The preceding command may cause an error that can be safely ignored.
      
   2. Disable read-write capability for the same community:
      no snmp-server community ILMI RW
      If an error is displayed, then this workaround cannot be applied to the device. Use the default workarounds presented in the first item above.
      
   3. Since this configuration will not survive a system reload, the command must be reentered after every restart of the system.
      
   If the command in item 2 above did not generate an error and ATM is not needed on this device, then this workaround is complete.
   
4. For affected releases of IOS 11.2 through 11.3(8) THAT REQUIRE ATM:
   

   Note: This workaround is not valid for ATM switches such as the LS1010 and 8500 series. See section I for those devices.

   This workaround will allow ILMI to continue to function for ATM while constraining who may reconfigure the device by way of the ILMI community string:
   
   1. Create a simple ACL to deny access using the following command. If "66" is already in use, choose a different two-digit number:
      access-list 66 deny any
      
   2. Apply it generally to the ILMI community to restrict its view:
      snmp community ILMI view *ilmi RW 66
      An error will be reported if the *ilmi view doesn't exist. If that occurs, then use the following command to explicitly restrict the ILMI view:
      snmp community ILMI RW 66
      If the preceding command produces persistent errors, then this workaround cannot be applied to this device. Use the default workarounds presented in the first item above.
      
5. For affected releases of IOS 11.3(9) through 12.0(2)T NOT REQUIRING ATM:
   All versions of IOS in this range will accept this workaround. However, the changes will not persist through a reboot of the device. These instructions must be reapplied following every system reload of the affected device:
   
   1. Expose the undocumented ILMI community string so it can be modified:
      snmp-server community ILMI RW
      The preceding command may cause an error that can be safely ignored.
      
   2. Disable read-write capability for the same community:
      no snmp-server community ILMI RW
      If an error is displayed, then this workaround cannot be applied to the device. Stop this procedure and use the default workarounds presented in the first item above.
      
   3. This configuration will not survive a system reload. The command must be reentered after every restart of the system.
      
6. For affected releases of IOS 11.3(9) through 12.0(2)T THAT REQUIRE ATM:
   

   Note: This workaround is not valid for ATM switches such as the LS1010 and 8500 series. This workaround is also not valid for 12.0 releases that are based on 12.0(3)T and higher such as 12.0S. See section I for those devices.

   This workaround will allow ILMI to continue to function for ATM while constraining who may reconfigure the device by way of the ILMI community string:
   
   1. Create a simple ACL to deny access using the following command. If "66" is already in use, choose a different two-digit number:
      access-list 66 deny any
      
   2. Apply it generally to the ILMI community to restrict its view:
      snmp community ILMI view *ilmi RW 66
      An error will be reported if the *ilmi view doesn't exist. If that occurs, then use the following command to explicitly restrict the ILMI view:
      snmp community ILMI RW 66
      If the preceding command produces persistent errors, then this workaround cannot be applied to this device. Use the default workaround presented in the first item above.
      
7. For affected releases of IOS 12.0(3)T and later:
   These releases of IOS include support for Simple Network Management Protocol version 3 (SNMPv3), which is required for this workaround.
   Confirm the presence of SNMPv3 support by asking the console CLI (command-line interpreter) for assistance with options to complete the snmp-server command. Enter config mode, enter the command shown below, and note the expected response:
   snmp-server user test test ?
   

   remote  Specify a remote SNMP entity to which the user belongs
   v1      user using the v1 security model
   v2c     user using the v2c security model
   v3      user using the v3 security model
   
   

   If the preceding command did not produce the expected results, then SNMPv3 is not supported in the release and this workaround cannot be applied. Stop this procedure and consider applying the default workaround presented above in the first item.
   Otherwise, if the device responded as expected, continue with the following explanation and instructions.
   In these IOS releases (12.0(3)T and later), ILMI packets are processed by the SNMP engine in the same manner as ordinary IP SNMP packets. An access control list or a view applied to the ILMI community string will be processed whether the transport is ILMI or IP. However, the only types of access control lists that can be applied to a community string are via IP access-list statements, which when applied, block ALL non-IP packets, including ILMI packets. Modifying or deleting the *ilmi view will also affect the packets transported by ILMI, so workarounds that change the view are equally ineffective at permitting ILMI while denying SNMP. In this range of releases, it is not possible to apply a workaround that denies IP SNMP packets that does not also deny ILMI SNMP packets.
   
8. For affected releases of IOS 12.0(3)T and later NOT REQUIRING ATM:
   
   1. Expose the undocumented ILMI community string so it can be modified:
      snmp-server community ILMI RW
      The preceding command may cause an error that can be safely ignored.
      
   2. Disable read-write capability for the same community:
      no snmp-server community ILMI RW
      If an error is displayed, then this workaround cannot be applied to the device. Stop this procedure and consider using the default workaround.
      
9. For affected releases of IOS 12.0(3)T and later THAT REQUIRE ATM:
   

   Note: This section also applies to ATM switch software such as for the LS1010 and the 8500 series. This section also applies to other 12.0 releases that are based on 12.0(3)T and higher such as 12.0S.

   The only effective workaround for systems in this category is the default workaround:
   
   1. Applying access lists to all the interfaces of the vulnerable device blocking SNMP from all hosts but those authorized to manage the devices.
      
   2. Blocking SNMP access at the edge of the network to prevent undesirable SNMP traffic from entering the network containing the vulnerable device.
      
   Access lists should be deployed with careful consideration of the possible effects on network operation and performance. Also note that authentication based on an IP source address is weak, so the preceding method will not protect against certain types of attacks in which the IP source address has been spoofed.
   In this range of releases it is not possible to block IP SNMP packets while permitting ILMI SNMP packets. The alternative workarounds presented previously will almost certainly cause a failure of ATM ILMI communications resulting in a loss of ATM connectivity, either immediately upon configuration, or unexpectedly at some later time. Either use the default workaround or upgrade to fixed software.
   

Cisco has made free software available to address this vulnerability for affected customers. 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 Using Third-Party Support Organizations

Customers Without Service Contracts

This vulnerability is known to the engineering staff of several Cisco customers. Cisco considers it known to the public prior to the publication of this notice.

Cisco is aware of one recent incident involving the unauthorized modification of a router that appears to have resulted from this vulnerability. However, it may have been the unintended side-effect of a test of the vulnerability.

Cisco is not aware of any available tools specifically designed to make use of this vulnerability. However, various off-the-shelf network management programs could easily be used to test for this vulnerability and to exploit it. Certain widely-available programs known to the cracker community could be modified by any reasonably competent programmer to automate the abuse of this vulnerability.

Cisco is not aware of any general public discussion of this vulnerability other than the exceptions previously noted.

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This notice will be posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20010227-ios-snmp-ilmi.

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:

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Future updates of this notice, if any, will be placed on Cisco's Worldwide Web server, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the URL given above for any updates.

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