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

These vulnerabilities are the result of defects in the functions responsible for Simple Network Management Protocol (SNMP), an Internet standard for the remote administration of network devices. SNMP makes use of one or more labels called "community strings" to delimit groups of "objects" (variables) that can be viewed or modified on a device. The SNMP data in such a group is organized in a tree structure called a Management Information Base (MIB). A single device may have multiple MIBs connected together into one large structure, and various community strings may provide read-only or read-write access to different, possibly overlapping portions of the larger data structure. An example of a read-only variable might be a counter showing the total number of octets sent or received through an interface. An example of a read-write variable might be the speed of an interface, or the hostname of a device.

Community strings also provide a weak form of access control in earlier versions of SNMP, v1 and v2c. (SNMPv3 provides much improved access control using strong authentication and should be preferred over SNMPv1 and SNMPv2c wherever it is supported.) If a community string is defined, then it must be provided in any basic SNMP query if the requested operation is to be permitted by the device. Community strings usually allow read-only or read-write access to the entire device. In some cases, a given community string will be limited to one group of read-only or read-write objects described in an individual MIB.

In the absence of additional configuration options to constrain access, knowledge of the single community string for the device is all that is required to gain access to all objects, both read-only and read-write, and to modify any read-write objects. The defects responsible for these vulnerabilities are grouped here by function:

1. The defect arises from implementation of the SNMPv2 "informs" functionality, which involves the exchange of read-only community strings for the sharing of status information. When an affected device processes a command defining a host to receive SNMP "traps" (logging messages) such as the "snmp-server host" command, then the community specified in the trap statement is also configured for general use if it is not already defined in the saved configuration. This occurs even if the community was previously removed and the configuration was saved to memory prior to a system reload.
   
2. The read-write community string is exposed when the device is examined via a "walk", or traversal, of the View-based Access Control MIB (VACM) using the device's read-only community string. View-based Access Control is a feature of SNMPv3 added to IOS in version 12.0(3)T. CSCds32217 describes the defect in IOS, CSCds16384 applies to IOS running on 2900XL and 3500XL switches, and CSCds19674 documents the defect in CatOS on Catalyst switches. Most IOS releases in 12.0 (after 12.0(3)T) as well as most 12.1 releases contain this vulnerability, as well as 12.0(5.2)XU and 12.0(5)XW for the 2900XL and 3500XL switches, and CatOS releases 5.4(1) - 5.5(2) and 6.1(1) for the Catalyst switches.
   
3. Implementation of new cable-industry standards for management of cable modems introduced an undocumented read-write community string, "cable-docsis", which was intended only for DOCSIS-compliant cable-capable devices. It was inadvertently enabled by default for all devices except DOCSIS-compatible cable modems and head end units in a limited range of IOS releases. This defect is documented as CSCdr59314. This vulnerability is confined to a very narrow set of IOS releases based on 12.1(3) and 12.1(3)T, and it is fixed in 12.1(4) and 12.1(5)T releases and following.
   

Full details are provided in the software section below regarding the status of each vulnerability in specific releases.

A separate Cisco Security Advisory has recently been announced regarding an SNMP vulnerability due to an undocumented default "ILMI" read-write community string in IOS. That advisory, http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20010227-ios-snmp-ilmi, 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 .

Knowledge of read-only community strings allows read access to information stored on an affected device, leading to a failure of confidentiality. Knowledge of read-write community strings allows remote configuration of affected devices without authorization, possibly without the awareness of the administrators of the device and resulting in a failure of integrity and a possible failure of availability.

These vulnerabilities could be exploited separately or in combination to gain access to or modify the configuration and operation of any affected devices without authorization. Customers are urged to upgrade affected systems to fixed releases of software, or to apply measures to protect such systems against unauthorized use by restricting access to SNMP services until such time as the devices can be upgraded.

This security advisory represents a combination of multiple related product security vulnerabilities. The affected trains and releases are not identical for all of the defects, but there are significant groups of releases where affected versions intersect with others. Unless otherwise noted, each label displayed under "Availability of Fixed Releases" identifies the release that resolves all of these defects for that specific train. Please note the following exceptions:

• IOS software Major Release version 12.0 and IOS releases based on 11.x or earlier are not affected by the vulnerabilities described in this notice. All other releases of 12.0, such as 12.0DA, 12.0S or 12.0T, may be affected.
  
• CSCdr59314 is only present in certain 12.1(3) releases and does not affect any other IOS releases.
  
• Fixes for all six defects have been integrated into 12.2 prior to its initial availability, and therefore all releases based on 12.2 and all later versions are not vulnerable to the defects described in this advisory.
  

The following table summarizes the IOS software releases that are known to be affected, and the earliest estimated dates of availability for the recommended fixed versions. Dates are always 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 of availability for each are listed in the "Rebuild", "Interim", and "Maintenance" columns. A device running any release in the given train that is earlier the release in a specific column (less than the earliest fixed release) is known to be vulnerable, and it should be upgraded at least to the indicated release or a later version (greater than the earliest fixed release label).

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, and interim images should be upgraded to the next available maintenance release as soon as possible. Interim releases are not available via manufacturing, and usually they are not available for customer download from CCO without prior arrangement with the Cisco TAC.
  

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.

All of the following workarounds must be configured while in enable mode on the affected router or switch. Be sure to save the changes with the "write memory" command after each configuration change.

The workaround for the vulnerability introduced by CSCdr61016 and CSCds49183 is to configure community strings for the snmp-server hosts prior to configuring the snmp-server hosts. This command should include the desired access restrictions on this community string. In the following example, "1.2.3.4" is the IP address of the host intended to receive SNMP traps:

router#config term
   ! create access list
router(config)#access-list 66 deny any
   ! configure community string with access restrictions
router(config)#snmp-server community public ro 66
   ! configure snmp-server host
router(config)#snmp-server host 1.2.3.4 public
router(config)#exit
router#write memory
router#

If the "snmp-server community" command is entered after one or more "snmp-server host" commands have been entered using the same community string, then all of the "snmp-server host" commands must be re-entered due to the otherwise unrelated defect CSCdr21997. This latter defect prevents traps or informs from leaving the router using the community string. The defect is present in some but not all of the same IOS releases as CSCdr61016.

To permanently remove communities after definition of the "snmp-server host" command, the associated "snmp-server host" commands that correspond to those communities must also be removed.

The vulnerability described in CSCds32217 and CSCds16384 can be remedied by using the "snmp-server view" command to block the ability to poll the SNMP-VIEW-BASED-ACM-MIB. The result is a view that restricts the ability to browse the SNMP-VIEW-BASED-ACM-MIB, and it must be applied to all read-only community strings. For example:

router#config term
   ! create view
router(config)#snmp-server view novacm internet included
   ! block vacmSecurityToGroupEntry table
router(config)#snmp-server view novacm internet.6.3.16 excluded
   ! apply view to read-only security string
router(config)#snmp-server community public view novacm RO
router(config)#exit
router#write memory
router#

If the affected router or switch already contains more than one read-write community string, then all read-write community strings must be prevented from reading the SNMP-VIEW-BASED-ACM-MIB. For read-write community strings that do not have a view applied, create a new view and apply it to the community string. If a read-write community string already has a view applied to it, then modify the view to prevent access to the SNMP-VIEW-BASED-ACM-MIB. Both situations are shown below.

If the following example is part of a pre-existing configuration:

router#show running-config
...
snmp-server view oldview internet included
snmp-server view oldview ipRouteTable excluded
snmp-server view oldview ipNetToMediaTable excluded
snmp-server view oldview at excluded
snmp-server community tech view oldview RW
snmp-server community private RW
...

then the following modifications will exclude the SNMP-VIEW-BASED-ACM-MIB:

router#config term
   ! block vacmSecurityToGroupEntry table in existing view
router(config)#snmp-server view oldview internet.6.3.16 excluded
   ! create new view
router(config)#snmp-server view novacm internet included
router(config)#snmp-server view novacm internet.6.3.16 excluded
   ! apply new view
router(config)#snmp-server community private view novacm RW
router(config)#exit
router#write memory
router#

Note: For the fullest protection provided by this workaround, every existing view on the affected switch or router must be modified in a similar manner.

The vulnerability described in CSCds19674 for CatOS can be remedied by using the "set snmp view" command to prevent access to the SNMP-VIEW-BASED-ACM-MIB. For example:

switch#set snmp view defaultUserView 1.3.6.1.6.3.16.1.2 excluded nonvolatile

If the "cable-docsis" community string is deleted from the configuration, then CSCdr59314 causes it to automatically reappear after the system is reloaded. The following workaround prohibits the use of the "cable-docsis" community string by defining an access list statement that completely denies any requests for it:

router#config term
   ! create access list
router(config)#access-list 66 deny any
   ! apply access restrictions to cable-docsis community string
router(config)#snmp-server community cable-docsis ro 66
router(config)#exit
router#write memory
router#

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

CSCdr59314 was discovered internally and repaired. Cisco is aware of one incident in which a customer's routers were modified without authorization by using the "cable-docsis" community string. The vulnerability was brought to the attention of the Cisco Product Security Incident Response Team when the customer reported the incident. The other vulnerabilities were initially reported by customers on one product or confirmed internally on other products during repair.

Although Cisco has no knowledge of a specific program or script designed to make use of these vulnerabilities, there are numerous off-the-shelf programs and scripts available which could be used as-is or modified to exploit any of the vulnerabilities described in this notice.

Cisco is not aware of any general discussion of these vulnerabilities in public forums.

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.

This advisory will be posted on Cisco's worldwide website at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20010228-ios-snmp-community.

In addition to Worldwide Web posting, a text version of this notice will be clear-signed with the Cisco PSIRT PGP key and will be posted to the following e-mail and Usenet news recipients:

• cust-security-announce@cisco.com
  
• bugtraq@securityfocus.com
  
• firewalls@lists.gnac.com
  
• first-teams@first.org (includes CERT/CC)
  
• cisco@spot.colorado.edu
  
• cisco-nsp@puck.nether.net
  
• comp.dcom.sys.cisco
  
• Various internal Cisco mailing lists
  

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.

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 Web site at http://www.cisco.com/en/US/products/products_security_vulnerability_policy.html. This includes instructions for press inquiries regarding Cisco security notices.