Summary

• A vulnerability in the Secure Login Enhancements capability of Cisco Nexus 1000V Switch for VMware vSphere could allow an unauthenticated, remote attacker to cause an affected Nexus 1000V Virtual Supervisor Module (VSM) to become inaccessible to users through the CLI.

  The vulnerability is due to improper resource allocation during failed CLI login attempts when login parameters that are part of the Secure Login Enhancements capability are configured on an affected device. An attacker could exploit this vulnerability by performing a high amount of login attempts against the affected device. A successful exploit could cause the affected device to become inaccessible to other users, resulting in a denial of service (DoS) condition requiring a manual power cycle of the VSM to recover.

  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:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20200226-nexus-1000v-dos

  This advisory is part of the February 2020 Cisco FXOS and NX-OS Software Security Advisory Bundled Publication, which includes six Cisco Security Advisories that describe six vulnerabilities. For a complete list of the advisories and links to them, see Cisco Event Response: February 2020 Cisco FXOS and NX-OS Software Security Advisory Bundled Publication.
  

Affected Products

• Vulnerable Products

  This vulnerability affects the Cisco Nexus 1000V Switch for VMware vSphere Virtual Supervisor Module (VSM) if it is running Cisco NX-OS Software Release 5.2(1)SV3(4.1a) and has been configured with login parameters that are part of the Secure Login Enhancements capability of the device.

  Administrators can determine whether the login parameters of the Secure Login Enhancements capability are configured on a device by using the show running-config | include "login block-for" command in the device CLI. The following example shows the output of the command for a device that is configured with login parameters:

  switch# show running-config | include "login block-for"
  login block-for 120 attempts 10 within 60

  If the command does not return any output, no affected configuration is present, and the device is not vulnerable.

  Products Confirmed Not Vulnerable

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

  Cisco has confirmed that this vulnerability does not affect the following Cisco products:

  • Firepower 1000 Series
  • Firepower 2100 Series
  • Firepower 4100 Series
  • Firepower 9300 Security Appliances
  • MDS 9000 Series Multilayer Switches
  • Nexus 1000 Virtual Edge for VMware vSphere
  • Nexus 1000V Switch for Microsoft Hyper-V
  • Nexus 3000 Series Switches
  • Nexus 5500 Platform Switches
  • Nexus 5600 Platform Switches
  • Nexus 6000 Series Switches
  • Nexus 7000 Series Switches
  • Nexus 9000 Series Fabric Switches in Application Centric Infrastructure (ACI) mode
  • Nexus 9000 Series Switches in standalone NX-OS mode
  • UCS 6200 Series Fabric Interconnects
  • UCS 6300 Series Fabric Interconnects
  • UCS 6400 Series Fabric Interconnects

Details

• The Secure Login Enhancements capability of Cisco Nexus 1000V Switch for VMware vSphere has been added in Cisco NX-OS Software Release 5.2(1)SV3(4.1a) and allows the configuration of login parameters that can temporarily block login attempts when a suspected DoS attack is detected. This capability can slow down dictionary attacks by enforcing a quiet period when multiple failed login attempts are detected.

  All login parameters are disabled by default. If no login parameter is configured, the device is not affected by the vulnerability described in this security advisory.

  Further information about Secure Login Enhancements is available in the Cisco Nexus 1000V for VMware vSphere Security Configuration Guide.

Indicators of Compromise

• Exploitation of this vulnerability requires a high amount of failed CLI login attempts against an affected device. Administrators may identify failed login attempts by using the show login failures CLI command, or by looking for repeated Authentication failed log messages similar to the following:

  2020 Feb 13 17:58:32 switch %AUTHPRIV-3-SYSTEM_MSG: pam_aaa:Authentication failed for user xxx from ...

  For failed login attempts made through SSH, the source IP address of the SSH connection is reported in the Authentication failed log message, as well as in the output of the show login failures CLI command, and can be used to identify eventual attackers. The source IP address is not reported for login attempts made through Telnet. Telnet connections to the device are disabled by default.

  A successful exploitation of this vulnerability would cause the CLI login process to start failing to authenticate any user, generating error messages similar to the following:

  2020 Feb 13 18:18:03 switch %AUTHPRIV-3-SYSTEM_MSG: PAM _pam_init_handlers: no default config /etc/pam.d/other - aaad
  2020 Feb 13 18:18:03 switch %AUTHPRIV-3-SYSTEM_MSG: PAM error reading PAM configuration file - aaad
  2020 Feb 13 18:18:03 switch %AUTHPRIV-3-SYSTEM_MSG: PAM pam_start: failed to initialize handlers - aaad
  2020 Feb 13 18:18:03 switch %AUTHPRIV-3-SYSTEM_MSG: pam_aaa:Authentication failed for user xxx from ...

  Customers are advised to contact their support organization to review the error messages and determine whether the vulnerability has been exploited on the device.

Workarounds

• There are no workarounds that address this vulnerability.

  Customers who cannot immediately apply a software fix may evaluate the possibility of disabling Secure Login Enhancements by removing the login parameters from the configuration until a software fix can be applied, and relying on an access control list (ACL) on the management interface (mgmt0) to restrict access to the device only from explicitly trusted source IP addresses.

  Customers should first evaluate the impact of those configuration changes to the device and their security policy. Customers concerned about undesired or unexpected side effects should consult with their support organization before implementing any configuration change.

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 and Alerts 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 NX-OS Software

  To help customers determine their exposure to vulnerabilities in Cisco NX-OS Software, Cisco provides the Cisco Software Checker to identify any Cisco Security Advisories that impact a specific Cisco NX-OS Software release and the earliest release that fixes the vulnerabilities that are 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, platform, 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 or one or more specific advisories.

  Customers can also use the following form to determine whether a release is affected by any Cisco Security Advisory by choosing the Cisco NX-OS Software and platform and then entering a release-for example, 7.0(3)I7(5) for Cisco Nexus 3000 Series Switches or 14.0(1h) for Cisco NX-OS Software in ACI mode:

  Cisco NX-OS Software Cisco NX-OS Software in ACI Mode MDS 9000 Series Multilayer Switches Nexus 1000V Series Switches Nexus 3000 Series Switches Nexus 5000 Series Switches Nexus 6000 Series Switches Nexus 7000 Series Switches Nexus 9000 Series Switches

  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 and check the Medium check box in the drop-down list under Impact Rating when customizing a search.

  Additional Resources

  For help determining the best Cisco NX-OS Software release for a Cisco Nexus Switch, administrators can refer to the following Recommended Releases documents. If a security advisory recommends a later release, Cisco recommends following the advisory guidance.

  Cisco MDS Series Switches
  Cisco Nexus 1000V for VMware Switch
  Cisco Nexus 3000 Series Switches
  Cisco Nexus 5500 Platform Switches
  Cisco Nexus 5600 Platform Switches
  Cisco Nexus 6000 Series Switches
  Cisco Nexus 7000 Series Switches
  Cisco Nexus 9000 Series Switches
  Cisco Nexus 9000 Series ACI-Mode Switches

  To determine the best release for Cisco UCS, see the Recommended Releases documents in the release notes for the device.

Exploitation and Public Announcements

• 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.

Source

• This vulnerability was found during internal security testing.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20200226-nexus-1000v-dos

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	-	Final	2020-February-26

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