Summary

• A vulnerability in the Kerberos authentication feature of Cisco Adaptive Security Appliance (ASA) Software could allow an unauthenticated, remote attacker to impersonate the Kerberos key distribution center (KDC) and bypass authentication on an affected device that is configured to perform Kerberos authentication for VPN or local device access.

  The vulnerability is due to insufficient identity verification of the KDC when a successful authentication response is received. An attacker could exploit this vulnerability by spoofing the KDC server response to the ASA device. This malicious response would not have been authenticated by the KDC. A successful attack could allow an attacker to bypass Kerberos authentication.

  Cisco has released software updates that address this vulnerability. There are no workarounds that address this vulnerability.

  Note: Configuration changes after the software upgrade are necessary to address this vulnerability. See the Details section of this advisory for additional information.

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-asa-kerberos-bypass-96Gghe2sS

  This advisory is part of the May 2020 Cisco ASA, FMC, and FTD Software Security Advisory Bundled Publication, which includes 12 Cisco Security Advisories that describe 12 vulnerabilities. For a complete list of the advisories and links to them, see Cisco Event Response: May 2020 Cisco ASA, FMC, and FTD Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco products if they are running Cisco ASA Software with Kerberos authentication configured for VPN or local device access.

  Determine Whether Kerberos Authentication Is Configured

  Administrators can use the show running-config aaa-server | include kerberos command to determine whether a Kerberos server is configured. The following example shows the output of the command for a device that has one Kerberos server configured that is named asaKerberosTestServer:

  device(config)# show running-config aaa-server | include kerberos
  aaa-server asaKerberosTestServer protocol kerberos
  kerberos-realm DEV.ASA.TEST

  If the Kerberos server name that is returned in the output is referenced elsewhere in the configuration¹, that Kerberos server is being used for authentication. Administrators can use the show running-config all | include <kerberos server name> command to verify whether Kerberos authentication is configured. In the following example, the Kerberos server name asaKerberosTestServer is configured for Secure Shell (SSH) console authentication (Kerberos authentication can also be configured for VPN access):

  device(config)# show running-config all | include asaKerberosTestServer
  aaa-server asaKerberosTestServer protocol kerberos
  aaa-server asaKerberosTestServer (inside) host DEV.ASA.TEST
  aaa authentication ssh console asaKerberosTestServer

  1. The kcd-server <kerberos server name> CLI command is an exception. If the only instance of the Kerberos server name is this command, 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 Cisco Firepower Management Center (FMC) Software or Cisco Firepower Threat Defense (FTD) Software.

Details

• Configuration changes after the software upgrade are necessary to address this vulnerability. Cisco ASA devices are vulnerable and can still be exploited unless the CLI commands validate-kdc and aaa kerberos import-keytab are configured. These new configuration commands ensure that the ASA validates the KDC during every user authentication transaction, which prevents the vulnerability that is described in this security advisory.

  Administrators can configure the new commands by entering the validate-kdc command and then the aaa kerberos import-keytab command in the device CLI.

  When the validate-kdc command is enabled, the ASA will validate the Kerberos server (KDC) during every user authentication transaction with that server by requesting a service ticket for the user and verifying the response against a previously stored key table (keytab). The aaa kerberos import-keytab command imports a Kerberos keytab file to the ASA.

  The following output shows the configuration of the validate-kdc and aaa kerberos import-keytab commands:
  

  device(config)# validate-kdc
  device(config)# aaa kerberos import-keytab disk0:mykeytab
  
  device# show aaa kerberos keytab
  
  Principal:   host/testing@DEV.ASA.TEST
  
  Key version: 10
  
  Key type:    arcfour (23)
  

  For more information about the new commands, see the Cisco ASA Series Command Reference, A - H Commands (aaa kerberos import-keytab command) and the Cisco ASA Series Command Reference, T - Z Commands (validate-kdc command).

Workarounds

• There are no workarounds that address this vulnerability.

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.

  Fixed Releases

  In the following table(s), the left column lists Cisco software releases. The center column indicates whether a release is affected by the vulnerability described in this advisory and the first release that includes the fix for this vulnerability. The right column indicates whether a release is affected by any of the vulnerabilities described in this bundle and which release includes fixes for those vulnerabilities.

  Cisco ASA Software

  Cisco ASA Software Release	First Fixed Release for This Vulnerability	First Fixed Release for All Vulnerabilities Described in the Bundle of Advisories
  Earlier than 9.61	Migrate to a fixed release.	Migrate to a fixed release.
  9.6	Migrate to a fixed release.	Migrate to a fixed release.
  9.71	Migrate to a fixed release.	Migrate to a fixed release.
  9.8	9.8.4.15	9.8.4.20
  9.9	9.9.2.66	9.9.2.67
  9.10	9.10.1.37	9.10.1.40
  9.12	9.12.3.2	9.12.3.9
  9.13	9.13.1.7	9.13.1.10
  9.14	Not vulnerable.	Not vulnerable.

  1. Cisco ASA Software releases 9.5 and earlier, as well as Release 9.7, have reached end of software maintenance. Customers are advised to migrate to a supported release that includes the fix for this vulnerability.

  Note 1: This vulnerability is fixed in Cisco ASA Software releases 9.8 and later through a new set of configuration commands. Cisco ASA devices are vulnerable and can still be exploited unless the CLI commands validate-kdc and aaa kerberos import-keytab are configured. For more information, see the Details section of this advisory.

  Note 2: Cisco does not recommend that customers use Kerberos authentication if the Kerberos authentication server is outside of the known, trusted network for any Cisco ASA Software release unless the validate-kdc and aaa kerberos import-keytab commands have been configured.

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

• Cisco would like to thank Yoav Iellin, Yaron Kassner, Dor Segal, and Rotem Zach of Silverfort for reporting this vulnerability.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-asa-kerberos-bypass-96Gghe2sS

Revision History

• Version	Description	Section	Status	Date
  1.1	Updated ASA Fixed Release table to indicate 9.10.1.40 as the correct fixed release instead of 9.10.1.39.	Fixed Releases	Final	2020-MAY-06
  1.0	Initial public release.	—	Final	2020-MAY-06

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