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AUSCERT External Security Bulletin Redistribution
Telnet Vulnerability Affecting Cisco Products: June 2020
9 July 2020
AusCERT Security Bulletin Summary
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Execute Arbitrary Code/Commands -- Remote/Unauthenticated
Denial of Service -- Remote/Unauthenticated
Access Confidential Data -- Remote/Unauthenticated
CVE Names: CVE-2020-10188
Revision History: July 9 2020: Fixed software section has been updated to correct
broken form in software checker text
June 25 2020: Initial Release
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Telnet Vulnerability Affecting Cisco Products: June 2020
Advisory ID: cisco-sa-telnetd-EFJrEzPx
First Published: 2020 June 24 16:00 GMT
Last Updated: 2020 July 8 16:58 GMT
Version 1.1: Interim
Cisco Bug IDs: CSCvu66723
o 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:
o 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
The following table lists Cisco products that are affected by the
vulnerabilities that are described in this advisory.
Product Cisco Bug Fixed Release Availability
Cisco IOS XE Software when CSCvu66723 Consult the Cisco Software
persistent Telnet is configured 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.
o 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 <name> 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
Indicators of Compromise
o 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
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
o 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.
o 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
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
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
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
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
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
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 :
[ ] [Check]
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.
o Telnet is an insecure protocol. Customers are advised to instead use SSH or
HTTPS on Cisco products that support them.
Exploitation and Public Announcements
o 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.
o This vulnerability was discovered while investigating the impact of CVE-ID
CVE-2020-10188 on Cisco products.
Cisco Security Vulnerability Policy
o To learn about Cisco security vulnerability disclosure policies and
publications, see the Security Vulnerability Policy . This document also
contains instructions for obtaining fixed software and receiving security
vulnerability information from Cisco.
| Version | Description | Section | Status | Date |
| 1.1 | Corrected broken form in | Fixed | Interim | 2020-JUL-08 |
| | software checker text. | Software | | |
| 1.0 | Initial public release. | - | Interim | 2020-JUN-24 |
o 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 standalone copy or paraphrase of the text of this document that omits the
distribution URL is an uncontrolled copy and may lack important information
or contain factual errors. The information in this document is intended for
end users of Cisco products.
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