F5 BIG-IP APM AD (Active Directory) Authentication Flaw Bypassed using a Spoofed AS-REP

Cybersecurity researchers on Wednesday disclosed a new bypass vulnerability (CVE-2021-23008) in the Kerberos Key Distribution Center (KDC) security feature impacting F5 Big-IP application delivery services.

“BIG-IP APM AD (Active Directory) authentication can be bypassed using a spoofed AS-REP (Kerberos Authentication Service Response) response sent over a hijacked KDC (Kerberos Key Distribution Center) connection, or from an AD server compromised by an attacker”, according to F5 Networks.

Kerberos is an authentication protocol that relies on a client-server model for mutual authentication and requires a trusted intermediary called Key Distribution Center (KDC), a Kerberos Authentication Server (AS) or a Ticket Granting Server in this case, that acts as a repository of shared secret keys of all users as well as information about which users have access privileges to which services on which network servers.

“A remote attacker can hijack a KDC connection using a spoofed AS-REP response”, F5 Networks noted in the alert.

For an APM access policy configured with AD authentication and SSO (single sign-on) agent, if a spoofed credential related to this vulnerability is used, depending on how the back-end system validates the authentication token it receives, access will most likely fail.

An APM access policy can also be configured for BIG-IP system authentication. A spoofed credential related to this vulnerability for an administrative user through the APM access policy results in local administrative access.

“The KDC Spoofing vulnerability allows an attacker to bypass the Kerberos authentication to Big-IP Access Policy Manager (APM), bypass security policies and gain unfettered access to sensitive workloads,” Silverfort researchers Yaron Kassner and Rotem Zach said in a report.

The spoofing attack, therefore, hinges on the possibility that there exist insecure Kerberos configurations to hijack the communication between the client and the domain controller, leveraging it to create a fraudulent KDC that diverts the traffic intended for the controller to the fake KDC, and subsequently authenticate itself to the client.

Patches Available

F5 Networks has released patches to address the weakness (CVE-2021-23008, CVSS score 8.1), with fixes introduced in BIG-IP APM versions 12.1.6, 13.1.4, 14.1.4, and 15.1.3.

A similar patch for version 16.x is expected at a future date.

F5 recommend customers running 16.x check the security advisory to assess their exposure and get details on mitigations for the vulnerability.

Mitigation

APM access policy:

The company recommends configuring multi-factor authentication (MFA) or deploying an IPSec tunnel between the affected BIG-IP APM system and the Active Directory servers.

BIG-IP System Authentication

If BIG-IP system authentication uses AD authentication from an APM access policy, the company advises an alternative remote authentication option from the User Directory options that have the SSL-based authentication feature.

The key configuration enables the ‘SSL’ option and configures it as needed for the listed remote authentication alternative configurations:

• Active Directory
• LDAP
• ClientCert LDAP

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