Security News > 2021 > April > Vulnerability Exposes F5 BIG-IP to Kerberos KDC Hijacking Attacks

F5 Networks this week released patches to address an authentication bypass vulnerability affecting BIG-IP Access Policy Manager, but fixes are not available for all impacted versions.

Tracked as CVE-2021-23008, the high-severity vulnerability allows for the bypass of BIG-IP APM AD authentication if the attacker can hijack a Kerberos KDC connection using a spoofed AS-REP. Authentication bypass is also possible from an AD server that the attacker has already compromised, F5 explains.

The Kerberos protocol leverages a client-server model for on-premise authentication, with KDC being an intermediary that holds shared secret keys and user access information.

If the Kerberos KDC authentication to the server is compromised, the attacker can hijack the connection between the client and the domain controller, thus bypassing authentication.

Such attacks typically exploit misconfigurations in Kerberos protocol implementations, and the same applies to F5 BIG-IP as well.

"For an APM access policy configured with AD authentication and SSO agent, if a spoofed credential related to this vulnerability is used, depending 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," F5 notes.

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