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Thread: Cisco Security Advisory: Cisco Telnet Denial of Service Vulnerability

  1. #1
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    Oct 2002

    Cisco Security Advisory: Cisco Telnet Denial of Service Vulnerability


    Document ID: 61671
    Revision 1.2
    Last Updated 2004 August 27 1630 UTC
    For Public Release 2004 August 27 1000 UTC
    Please provide your feedback on this document.


    Affected Products
    Software Versions and Fixes
    Obtaining Fixed Software
    Exploitation and Public Announcements
    Status of This Notice: INTERIM
    Revision History
    Cisco Security Procedures


    A specifically crafted Transmission Control Protocol (TCP) connection to a telnet or reverse telnet port of a Cisco device running Internetwork Operating System (IOS)® may block further telnet, reverse telnet, Remote Shell (RSH), Secure Shell (SSH), and in some cases Hypertext Transport Protocol (HTTP) access to the Cisco device. Telnet, reverse telnet, RSH and SSH sessions established prior to exploitation are not affected.

    All other device services will operate normally. Services such as packet forwarding, routing protocols and all other communication to and through the device are not affected.

    Cisco will make free software available to address this vulnerability. Workarounds, identified below, are available that protect against this vulnerability.

    This vulnerability is documented in Cisco bug ID CSCef46191 ( registered customers only) .

    This Advisory is available at

    Affected Products:

    Vulnerable Products

    This vulnerability affects all Cisco devices that permit access via telnet or reverse telnet. Any IOS train without specific fixed releases listed in the Software Versions and Fixes section should be considered vulnerable.
    Products Confirmed Not Vulnerable

    Cisco products that do not run IOS are not affected.


    Telnet, RSH and SSH are used for remote management of Cisco IOS devices. The SSH protocol is also used for Secure Copy (SCP), which allows an encryption-protected transfer of files to and from Cisco devices.

    Services operating over IPv4 and IPv6 are similarly affected.

    HTTP is also used for management of certain Cisco devices. IOS versions prior to12.2(15)T include HTTP server version 1.0, which, if configured, will be unresponsive on a device that is under exploitation. IOS versions after and including 12.2(15)T include HTTP server version 1.1, which is unaffected.

    Reverse telnet is a feature that allows you to telnet to a Cisco device and then connect to a third device through an asynchronous serial connection. For more information on reverse telnet, consult the following documents:

    Cisco devices that are operating as a reverse telnet server may have ports open in the ranges of:
    2001 to 2999

    3001 to 3099

    6001 to 6999

    7001 to 7099

    After a specially crafted TCP connection to an IOS device on TCP port 23 or the reverse telnet ports listed above, all subsequent telnet, reverse telnet, RSH (TCP port 514), SSH, SCP (SSH and SCP use TCP port 22), and in some cases HTTP (TCP port 80) connections to the device experiencing exploitation will be unsuccessful. Any telnet, reverse telnet, RSH, SSH, SCP and HTTP sessions that are already established with the device will continue to function properly.

    In Cisco IOS, telnet, reverse telnet, RSH, SSH, SCP and some HTTP sessions are handled by a virtual terminal (VTY). Each telnet, reverse telnet, RSH, SSH and SCP session consumes a VTY. After successful exploitation, the Cisco device can no longer accept any subsequent VTY connections.

    Though it is not possible to establish new telnet, reverse telnet, RSH, SSH, SCP or HTTP connections to the device after a successful exploitation, the device is only vulnerable on TCP port 23 and the reverse telnet ports listed above.

    A successful exploitation of this vulnerability requires a complete 3-way TCP handshake, which makes it very difficult to spoof the source IP address.

    Only remote access services that use VTYs are affected. This includes telnet, reverse telnet, RSH, SSH, SCP and version 1.0 of the HTTP server. Other device services including, but not limited to, routing protocols, TACACS/RADIUS, Voice over IP (VoIP) and packet forwarding are not affected.

    This vulnerability is addressed by Cisco bug ID:
    CSCef46191 ( registered customers only)

    To determine the software running on a Cisco product, log in to the device and issue the show version command to display the system banner. Cisco IOS software will identify itself as "Internetwork Operating System Software" or simply "IOS®". On the next line of output, the image name will be displayed between parentheses, followed by "Version" and the IOS release name. Other Cisco devices will not have the show version command or will give different output.

    The following example identifies a Cisco product running IOS release 12.0(3) with an installed image name of C2500-IS-L:

    Cisco Internetwork Operating System Software IOS (TM)

    2500 Software (C2500-IS-L), Version 12.0(3), RELEASE SOFTWARE

    The release train label is "12.0".

    The next example shows a product running IOS release 12.0(2a)T1 with an image name of C2600-JS-MZ:

    Cisco Internetwork Operating System Software IOS (tm)

    C2600 Software (C2600-JS-MZ), Version 12.0(2a)T1, RELEASE SOFTWARE (fc1)

    Additional information about Cisco IOS Banners is available at


    Exploitation of this vulnerability may result in the denial of new telnet, reverse telnet, RSH, SSH, SCP and HTTP connections to a device running IOS. Other access to the device via the console or SNMP is not affected. The device will remain in this state until the problematic TCP connection is cleared, or the device is reloaded (which will clear the problematic session). If no other access methods are available, exploitation of this vulnerability could deny remote access to the device.

    Depending on your network architecture, workarounds may be available to mitigate this vulnerability. Software will be available to repair this vulnerability.

    Software Versions and Fixes:

    Cisco is working to release fixes for this vulnerability in all currently maintained IOS releases. No software upgrade is required in order to mitigate this vulnerability. See the information below regarding the available configuration workarounds. The software fixes will appear in regularly scheduled maintenance releases of IOS software.

    As fixed software becomes available for public release, Cisco will update this section of the advisory.
    Obtaining Fixed Software
    Customers with Service Contracts

    As fixed software becomes available, customers with contracts should obtain the fixed software through their regular update channels. For most customers, this means that such software 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 assistance with the upgrade or fixed software, which should be free of charge.
    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 at obtaining fixed software through their point of sale should get their fixed software 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)


    Please have your product serial number available and give the URL of this notice as evidence of your entitlement to a free upgrade. Free upgrades for non-contract customers must be requested through the TAC.

    Please do not contact either "" or "" for software upgrades.

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

    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


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

    Mitigation Strategies

    Not all of the mitigation strategies listed will work for all customers. Some of the workarounds listed are dependent on which versions and feature-sets of IOS you have in your network.

    Enabling SSH and disabling telnet

    Note: SSH support is only available in certain IOS feature sets and platforms

    Cisco devices that support SSH can enable it by following the steps listed here:

    To disable telnet access to the device, configure the following on all your VTY lines:

    Router(config)# line vty 0 4
    Router(config-line)# transport input ssh

    Note: Even if SSH is enabled, the IOS device is not protected until telnet access is disabled.

    Configuring a VTY Access Class

    It is possible to limit the exposure of the Cisco device by applying a VTY access class to permit only known, trusted devices to connect to the device via telnet, reverse telnet and SSH.

    For more information on restricting traffic to VTYs, please consult:

    The following example permits access to VTYs from the netblock and the single IP address while denying access from everywhere else:

    Router(config)# access-list 1 permit
    Router(config)# access-list 1 permit host
    Router(config)# line vty 0 4
    Router(config-line)# access-class 1 in

    For devices acting as a terminal server, to apply the access class to reverse telnet ports, the access-list must be configured for the aux port and terminal lines as well:

    Router(config)# line 1 <x>
    Router(config-line)# access-class 1 in

    Different Cisco platforms support different numbers of terminal lines. Check your device's configuration to determine the correct number of terminal lines for your platform.

    Configuring Access Lists (ACLs)

    In addition to configuring a VTY Access Class, it may be desirable to block all telnet and reverse telnet traffic destined to your network infrastructure.

    Telnet and reverse telnet should be blocked as part of a Transit ACL controlling all access to the trusted network. Transit ACLs are considered a network security best practice and should be considered as a long-term addition to good network security, as well as a workaround for this specific vulnerability. The white paper entitled "Transit Access Control Lists: Filtering at Your Edge" presents guidelines and recommended deployment techniques for transit ACLs:

    Configuring Infrastructure Access Lists (iACLs)

    Although it is often difficult to block traffic transiting your network, it is possible to identify traffic which should never be allowed to target your infrastructure devices and block that traffic at the border of your network. Infrastructure ACLs are considered a network security best practice and should be considered as a long-term addition to good network security as well as a workaround for this specific vulnerability. The white paper entitled "Protecting Your Core: Infrastructure Protection Access Control Lists" presents guidelines and recommended deployment techniques for infrastructure protection ACLs:

    Configuring Receive Access Lists (rACLs)

    For distributed platforms, rACLs may be an option starting in Cisco IOS Software Versions 12.0(21)S2 for the 12000 series GSR and 12.0(24)S for the 7500 series. The receive access lists protect the device from harmful traffic before the traffic can impact the route processor. Receive path ACLs are considered a network security best practice, and should be considered as a long-term addition to good network security, as well as a workaround for this specific vulnerability. The CPU load is distributed to the line card processors and helps mitigate load on the main route processor. The white paper entitled "GSR: Receive Access Control Lists" will help identify and allow legitimate traffic to your device and deny all unwanted packets:

    Exploitation and Public Announcements:

    The Cisco PSIRT is aware of exploitation of this vulnerability and is recommending customers take action to protect themselves.

    Status of This Notice: INTERIM


    This advisory will be posted on Cisco's worldwide website at

    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. (includes CERT/CC)

    Future updates of this advisory, if any, will be placed on Cisco's worldwide website, but may or may not be actively announced on mailing lists or newsgroups. Users concerned about this problem are encouraged to check the above URL for any updates.
    Revision History

    Revision 1.2


    Updated the Vulnerable Products section.

    Updated the Configuring a VTY Access Class description in the Workarounds section.

    Revision 1.1


    Added the second paragraph to the Details section.

    Changed the Configuring a VTY Access Class and the Configuring Access Lists (ACLs) descriptions in the Workarounds section.

    Revision 1.0


    Initial public release.

    Cisco Security Procedures:

    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 website at This includes instructions for press inquiries regarding Cisco security notices. All Cisco security advisories are available at
    For more information, and the official article from Cisco, visit here.
    Space For Rent.. =]

  2. #2
    Member n00bius's Avatar
    Join Date
    Mar 2005
    really...interesting, and the obvious question is...but alas, a lot of business still use the 2600 series which is EOL and will remain unfixed and vulnerable (aside from the aforementioned external securing).
    ...:::Pure Kn0wledge:::...

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