Summary

• On February 28, 2020, APPGATE published a blog post regarding CVE-ID CVE-2020-10188, which is a vulnerability in Telnet servers (telnetd).

  For more information about this vulnerability, see the Details section.

  Cisco will release software updates that address this vulnerability. There are workarounds that address this vulnerability.

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-telnetd-EFJrEzPx

Affected Products

• Cisco investigated its product line to determine which products may be affected by this vulnerability. The Vulnerable Products section includes Cisco bug IDs for each affected product. The bugs are accessible through the Cisco Bug Search Tool and contain additional platform-specific information, including workarounds (if available) and fixed software releases.

  Vulnerable Products

  The following table lists Cisco products that are affected by the vulnerabilities that are described in this advisory.

  Product	Cisco Bug ID	Fixed Release Availability
  Cisco IOS XE Software when persistent Telnet is configured	CSCvu66723	Consult the Cisco Software Checker for details.

  Products Confirmed Not Vulnerable

  Only products listed in the Vulnerable Products section of this advisory are known to be affected by this vulnerability.

Details

• Cisco IOS XE Software is affected only if the device is configured with the persistent Telnet feature. The Telnet service that is used for VTY lines in Cisco IOS Software and Cisco IOS XE Software is not affected.

  Determine the Status of Persistent Telnet

  To determine whether the persistent Telnet feature is enabled for a device, log in to the device and use the show running-config | include transport type persistent telnet command in the CLI to check for the presence of the transport type persistent telnet input command in the global configuration. If the command returns no output or an error message, then persistent Telnet is not configured and the device is not vulnerable.

  Proof of concept code that is currently available results in high CPU usage on the affected device. To recover the device, a power cycle needs to occur.

Indicators of Compromise

• The Cisco Security Indicators of Compromise Reference Guide lists commonly observed indicators of compromise (IoCs), which can help identify devices that may have been impacted by the vulnerability that is disclosed in this advisory.

  Router#show processes cpu platform sorted | include telnetd
    5827    5714     99%     14%     14%  R        193015808  in.telnetd
    5713    5600     99%     14%     14%  R        196431872  in.telnetd
    5599    5486     99%     15%     15%  R        194543616  in.telnetd
    5485    5372     99%     15%     15%  R        207822848  in.telnetd
    5371    5258     99%     15%     15%  R        209567744  in.telnetd
    5257    5144     99%     15%     15%  R        209879040  in.telnetd

Workarounds

• Disable the persistent Telnet feature and use persistent Secure Shell (SSH) instead. For more information, see the Console Port, Telnet, and SSH Handling configuration guide.

Fixed Software

• Cisco has released free software updates that address the vulnerability described in this advisory. Customers may only install and expect support for software versions and feature sets for which they have purchased a license. By installing, downloading, accessing, or otherwise using such software upgrades, customers agree to follow the terms of the Cisco software license:
  https://www.cisco.com/c/en/us/products/end-user-license-agreement.html

  Additionally, customers may only download software for which they have a valid license, procured from Cisco directly, or through a Cisco authorized reseller or partner. In most cases this will be a maintenance upgrade to software that was previously purchased. Free security software updates do not entitle customers to a new software license, additional software feature sets, or major revision upgrades.

  When considering software upgrades, customers are advised to regularly consult the advisories for Cisco products, which are available from the Cisco Security Advisories page, to determine exposure and a complete upgrade solution.

  In all cases, customers should ensure that the devices to be upgraded contain sufficient memory and confirm that current hardware and software configurations will continue to be supported properly by the new release. If the information is not clear, customers are advised to contact the Cisco Technical Assistance Center (TAC) or their contracted maintenance providers.

  Customers Without Service Contracts

  Customers who purchase directly from Cisco but do not hold a Cisco service contract and customers who make purchases through third-party vendors but are unsuccessful in obtaining fixed software through their point of sale should obtain upgrades by contacting the Cisco TAC: https://www.cisco.com/c/en/us/support/web/tsd-cisco-worldwide-contacts.html

  Customers should have the product serial number available and be prepared to provide the URL of this advisory as evidence of entitlement to a free upgrade.

  Cisco IOS and IOS XE Software

  To help customers determine their exposure to vulnerabilities in Cisco IOS and IOS XE Software, Cisco provides the Cisco Software Checker to identify any Cisco Security Advisories that impact a specific software release and the earliest release that fixes the vulnerabilities described in each advisory (“First Fixed”). If applicable, the tool also returns the earliest release that fixes all the vulnerabilities described in all the advisories identified (“Combined First Fixed”).

  Customers can use the Cisco Software Checker to search advisories in the following ways:

  • Choose the software and one or more releases
  • Upload a .txt file that includes a list of specific releases
  • Enter the output of the show version command

  After initiating a search, customers can customize the search to include all Cisco Security Advisories, a specific advisory, or all advisories in the most recent bundled publication.

  Customers can also use the following form to determine whether a release is affected by any Cisco Security Advisory by entering a Cisco IOS or IOS XE Software release-for example, 15.1(4)M2 or 3.13.8S:

  By default, the Cisco Software Checker includes results only for vulnerabilities that have a Critical or High Security Impact Rating (SIR). To include results for Medium SIR vulnerabilities, customers can use the Cisco Software Checker on Cisco.com and check the Medium check box in the drop-down list under Impact Rating when customizing a search.

Recommendations

• Telnet is an insecure protocol. Customers are advised to instead use SSH or HTTPS on Cisco products that support them.

Exploitation and Public Announcements

• The Cisco Product Security Incident Response Team (PSIRT) is aware that proof-of-concept exploit code is available for the vulnerability that is described in this advisory.

  The Cisco PSIRT is not aware of any malicious use of the vulnerability that is described in this advisory.

Source

• This vulnerability was discovered while investigating the impact of CVE-ID CVE-2020-10188 on Cisco products.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-telnetd-EFJrEzPx

Revision History

• Version	Description	Section	Status	Date
  1.1	Corrected broken form in software checker text.	Fixed Software	Interim	2020-JUL-08
  1.0	Initial public release.	-	Interim	2020-JUN-24

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