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AUSCERT External Security Bulletin Redistribution
Cisco Network Services Orchestrator CLI Secure Shell Server
Privilege Escalation Vulnerability
5 August 2021
AusCERT Security Bulletin Summary
Product: Cisco Network Services Orchestrator (NSO)
Publisher: Cisco Systems
Operating System: Cisco
Impact/Access: Execute Arbitrary Code/Commands -- Existing Account
Increased Privileges -- Existing Account
CVE Names: CVE-2021-1572
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Cisco Network Services Orchestrator CLI Secure Shell Server Privilege
Advisory ID: cisco-sa-nso-priv-esc-XXqRtTfT
First Published: 2021 August 4 16:00 GMT
Version 1.0: Final
Workarounds: No workarounds available
Cisco Bug IDs: CSCvy36721
CVE Names: CVE-2021-1572
o A vulnerability in Cisco Network Services Orchestrator (NSO) could allow an
authenticated, local attacker to execute arbitrary commands at the level of
the account under which Cisco NSO is running, which is root by default. To
exploit this vulnerability, an attacker must have a valid account on an
The vulnerability exists because the affected software incorrectly runs the
SFTP user service at the privilege level of the account that was running
when the NSO built-in Secure Shell (SSH) server for CLI was enabled. If the
NSO built-in SSH server was not enabled, the device is not affected by this
vulnerability. An attacker with low-level privileges could exploit this
vulnerability by authenticating to an affected device and issuing a series
of commands at the SFTP interface. A successful exploit could allow the
attacker to elevate privileges to the level of the account under which
Cisco NSO is running, which is root by default.
Note: Any user who can authenticate to the built-in SSH server may exploit
this vulnerability. By default, all Cisco NSO users have this access if the
server is enabled.
Cisco has released software updates that address this vulnerability. There
are no workarounds that address this vulnerability.
This advisory is available at the following link:
o Vulnerable Products
This vulnerability affects the following releases of Cisco NSO if the NSO
built-in SSH server for CLI is enabled:
Releases 5.4 through 220.127.116.11
Releases 5.5 through 18.104.22.168
The NSO built-in SSH server is disabled by default in an NSO system
installation. To determine whether the built-in SSH server is enabled,
navigate to the ncs.conf file, which is usually located under the /etc/ncs
directory. The built-in SSH server is enabled if the value for SSH enabled
is true , as shown in this example:
<!-- Use the builtin SSH server -->
Products Confirmed Not Vulnerable
Only products listed in the Vulnerable Products section of this advisory
are known to be affected by this vulnerability.
o There are no workarounds that address this vulnerability. However, a
mitigation is available.
Administrators can disable the NSO built-in SSH server and run the ncs_cli
program as a login shell. For guidance on using the ncs_cli program, see
NSO 5.5 User Guide: Starting the CLI .
While this mitigation has been deployed and was proven successful in a test
environment, customers should determine the applicability and effectiveness
in their own environment and under their own use conditions. Customers
should be aware that any workaround or mitigation that is implemented may
negatively impact the functionality or performance of their network based
on intrinsic customer deployment scenarios and limitations. Customers
should not deploy any workarounds or mitigations before first evaluating
the applicability to their own environment and any impact to such
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 fixed this vulnerability in Cisco NSO releases 22.214.171.124 and later and
releases 126.96.36.199 and later.
Exploitation and Public Announcements
o The Cisco Product Security Incident Response Team (PSIRT) is not aware of
any public announcements or malicious use of the vulnerability that is
described in this advisory.
o This vulnerability was found during the resolution of a Cisco TAC support
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 |
| 1.0 | Initial public release. | - | Final | 2021-AUG-04 |
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