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

Software packages are available from various commercial and free sites that perform automated remote tests for computer security vulnerabilities by scanning computers on a network for known security flaws. Two security vulnerabilities associated with several UNIX-based platforms are the subject of two specific tests that have the same effect on vulnerable Cisco routers. The scanning program is asserting the Telnet ENVIRON option, #36, before the router indicates that it is willing to accept it, and this causes the router to reload unexpectedly.

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 .

The described defect can be used to mount a consistent and repeatable denial of service (DoS) attack on any vulnerable Cisco product, which may result in violations of the availability aspects of a customer's security policy. This defect by itself does not cause the disclosure of confidential information nor allow unauthorized access.

For the affected Cisco IOS software Major Release version shown in the first column of the table below, customers should upgrade to the known invulnerable releases listed to the right in the same row. In general, customers should upgrade to the release in the column furthest to the right within the same row. For example, any customer running 12.0 "mainline" (Major Release) should upgrade at least to 12.0(7.1), but preferably to 12.0(8).

Any release not specifically listed in the left-most column below is unaffected by the vulnerability.

The projected release date is shown with the software release version number for those releases that are not yet complete or available on CCO.*

An "interim release" is scheduled and contains numerous fixes and occasional enhancements that carry forward into all later versions.** A "maintenance release" is a regularly scheduled event that incorporates significant enhancements and cumulative fixes; it may be the entry point for support of noteworthy new technology in Cisco IOS software.

* All dates are tentative and subject to change

** Interim releases are subjected to less internal testing and verification than are regular releases, may have serious bugs, and should be installed with great care.

*** 12.0(8)SC is not vulnerable to this defect, but due to other issues it is no longer available on CCO as of the date of this notice. Upgrade instead to 12.0(9)SC.

The vulnerability described in this notice can only be exploited if the Telnet service is configured on the affected system and reachable from the attacker's computer. The following recommendations provide an interactive login capability without using the Telnet service, thus mitigating the threat in lieu of a software upgrade while preserving remote access to the router for administrative purposes:

• Prevent access using the Telnet service by defining an appropriate access control list and applying it to the vty line or the router's interfaces using the "access-group" keyword. Security can be increased further by restricting both the virtual terminal lines and the router's physical interfaces with two access-groups, one to control who can connect to the vtys, and the other on the interfaces to control from where those connections can be attempted.
  
• Disable Telnet and use SSH (if it is available to you) to connect to the router for administrative purposes.. After "line vty 0 4" in the router's configuration, add "transport input ssh". This stipulates that only the SSH protocol may be used for interactive logins to the router. As of the date of this notice, SSH is only available on certain products: 7200, 7500, and 12000 series running Cisco IOS software releases such as 12.0S, 12.1S, and 12.1T.
  
• Disable interactive network logins to the router completely by removing the "line" command such that virtual consoles are never enabled. Use an out-of-band method to login to and administer the router such as a hard-wired console. Consider connecting the console to a terminal server which itself is only reachable via a separate parallel network that in turn is restricted by site policy exclusively for administrative purposes.
  

The wide variety of customer configurations make it impossible to judge the effectiveness and relative merits of these workarounds in lieu of a software upgrade. Customers are cautioned to evaluate these recommendations carefully with regard to their specific network configurations.

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

As of the date of this notice, Cisco knows of no publicity, discussion, nor reports of malicious exploitation of this specific vulnerability applied directly against a Cisco product.

The denial of service (DoS) aspect of this vulnerability was reported to Cisco by several different customers who found it while conducting security scans of their networks. The defect that causes this vulnerability, documented in CSCdm70743, was discovered internally by a Cisco development engineer.

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.

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 notice is posted at http://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20000420-ios-telnet.

In addition to posting on the World-Wide Web, a text version of this notice is clear-signed with the Cisco PSIRT PGP key and is posted to the following e-mail lists and Usenet newsgroups:

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

Future updates of this notice, if any, will be placed on Cisco's World-Wide 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.

The web page athttp://www.cisco.com/en/US/products/products_security_vulnerability_policy.html describes how to report security vulnerabilities in Cisco products, obtain assistance with security incidents, and register to receive product security information from Cisco. This includes instructions for press inquiries regarding Cisco Security Advisories and notices. This advisory is Cisco's complete public statement regarding this vulnerability.