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Cisco Security Advisory

Cisco IOS Software TELNET Option Handling Vulnerability

Advisory ID: cisco-sa-20000420-ios-telnet

Revision 1.0

For Public Release 2000 April 20 13:00  UTC (GMT)


A defect in multiple Cisco IOS software versions will cause a Cisco router to reload unexpectedly when the router is tested for security vulnerabilities by security scanning software programs. The defect can be exploited repeatedly to produce a consistent denial of service (DoS) attack.

Customers using the affected Cisco IOS software releases are urged to upgrade as soon as possible to later versions that are not vulnerable to this defect. Vulnerable products and releases are listed in detail below.

The security scanner is testing for the presence of two specific vulnerabilities that affect certain UNIX-based systems. The vulnerabilities are unrelated to Cisco IOS software and Cisco IOS software is not directly at risk from them. However, a side-effect of the tests exposes the defect described in this security advisory, and the router will reload unexpectedly as soon as it receives any subsequent traffic.

This defect is documented as Cisco Bug ID CSCdm70743.

This advisory is posted at

Affected Products

This section provides details on affected products.

Vulnerable Products

The following Major Releases of Cisco IOS software are vulnerable to this defect:

  • 11.3AA
  • 12.0 releases: 12.0(2) up to and including 12.0(6)
  • 12.0(7), except that 12.0(7)S, 12.0(7)T, and 12.0(7)XE are not vulnerable

This vulnerability affects the following Cisco hardware products if they are running affected software:

  • AS5200, AS5300, and AS5800 series access servers
  • 7200 and 7500 series routers
  • ubr7200 series cable routers
  • 7100 series routers
  • 3660 series routers
  • SC3640 System Controllers (see the explanation below)
  • AS5800 series Voice Gateway products
  • AccessPath LS-3, TS-3, and VS-3 Access Solutions products

The SC3640 System Controller is a Cisco 3640 router customized to provide local management of multiple access servers. The Cisco SC3640 binary image contains the defect and thus is vulnerable if it is possible for the attacker to telnet to the device. However, the original Cisco 3640 router does not contain the defect and is not vulnerable to the denial of service attack described in this notice.

Products Confirmed Not Vulnerable

Cisco customers running Cisco IOS software versions 11.3, 11.3T, 11.2 or lower, and 12.0(8) or 12.1 or higher are not affected. Details regarding specific releases of Cisco IOS software and suggested upgrade paths are provided below in the section "Software Versions and Fixes".

No other Cisco products are currently known to be affected by these vulnerabilities.


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.

Vulnerability Scoring Details

Cisco has provided 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 2.0.

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 .

Cisco has also provided a CVSS calculator to help compute the environmental impact for individual networks at .


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.

Software Versions and Fixes

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.

Major Release


Projected Fixed Regular or Interim** Releases

Projected Fixed Regular Maintenance Releases

Unaffected Earlier Releases

11.2 and earlier, all variants

Multiple releases



11.3-based Releases


AS5800 support and other dial platforms



12.0-based Releases


12.0 mainline




ISP support: 7200, RSP, GSR12000






Cable ISP support: ubr7200


12.0(8)SC*** or 12.0(9)SC



12.0 new technology early deployment release





12.0 for Catalyst 8500 and LS1010


12.0(6.5)W5(17), 2000/04/18*


Short-life release for selected enterprise features, 7200 & 7500




Short-life release for Dial/Voice, 5200, 5300, 5800, 2600, & 3600



12.1-based Releases

12.1 and later, all variants

Multiple releases



* 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.

Obtaining 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, or as otherwise set forth at Downloads at

Do not contact either "" or "" for software upgrades.

Customers with Service Contracts

Customers with contracts should obtain upgraded software through their regular update channels. For most customers, this means that upgrades should be obtained through the Software Center on Cisco's worldwide website at

Customers Using Third-Party Support Organizations

Customers whose Cisco products are provided or maintained through prior or existing agreement with third-party support organizations such as Cisco Partners, authorized resellers, or service providers should contact that support organization for guidance and assistance with the appropriate course of action in regards to this advisory.

The effectiveness of any workaround or fix is dependent on specific customer situations such as product mix, network topology, traffic behavior, and organizational mission. Due to the variety of affected products and releases, customers should consult with their service provider or support organization to ensure any applied workaround or fix is the most appropriate for use in the intended network before it is deployed.

Customers Without Service Contracts

Customers who purchase direct from Cisco but who do not hold a Cisco service contract and customers who purchase through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain software patches and bug fixes by contacting the Cisco Technical Assistance Center (TAC). TAC contacts are as follows.

  • +1 800 553 2447 (toll free from within North America)
  • +1 408 526 7209 (toll call from anywhere in the world)
  • e-mail:

Have your product serial number available and provide the URL of this notice as evidence of entitlement to a software patch or bug fix. Customers without service contracts should request a software patch or bug fix through the TAC.

Refer to for additional TAC contact information, including special localized telephone numbers and instructions and e-mail addresses for use in various languages.

Exploitation and Public Announcements

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.

Status of This Notice: Final


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

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:

  • (includes CERT/CC)
  • 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.

Revision History



Initial public release.

Cisco Security Procedures

The web page at 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.

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