Kerberos (I): How does Kerberos work? - Theory

Overview

• Aim: Clarify how Kerberos functions, especially for pentesters.
• This is the first post in a series focusing on Kerberos basics, attacks, and complex functionalities like delegation.

What is Kerberos?

• An authentication protocol, not an authorization protocol.
• Identifies users via a secret password but does not determine resource access.
• Used in Active Directory to provide user privilege information.
• Responsibility of individual services to manage access to resources.

Kerberos Items

Transport Layer

• Uses UDP or TCP for transmitting data in cleartext.
• Ports: UDP/88, TCP/88 on Key Distribution Center (KDC).

Agents in Kerberos

• Client or User: Seeks service access.
• Application Server (AP): Provides the required service.
• Key Distribution Center (KDC): Main Kerberos service for issuing tickets, housed in Domain Controller.
• Authentication Service (AS): Issues Ticket Granting Tickets (TGTs).

Encryption Keys

• KDC or krbtgt key: Derived from krbtgt account NTLM hash.
• User Key: Derived from user NTLM hash.
• Service Key: Derived from NTLM hash of service owner.
• Session Key: Negotiated between user and KDC.
• Service Session Key: Used between user and service.

Tickets

• Ticket Granting Service (TGS): Used by users to authenticate against services.
• Ticket Granting Ticket (TGT): Used to request TGSs from KDC.

Privilege Attribute Certificate (PAC)

• Included within tickets to indicate user privileges.
• Signed with KDC key.
• Services can verify PAC with KDC, although rare.

Messages

• KRB_AS_REQ: Requests TGT from KDC.
• KRB_AS_REP: Delivers TGT from KDC.
• KRB_TGS_REQ: Requests TGS using TGT.
• KRB_TGS_REP: Delivers TGS from KDC.
• KRB_AP_REQ: Authenticates user against a service using TGS.
• KRB_AP_REP: Optional, service identifies to user.
• KRB_ERROR: Communicates errors.

Authentication Process

1. KRB_AS_REQ: User requests TGT from KDC.
   • Includes encrypted timestamp, username, service SPN, nonce.
2. KRB_AS_REP: KDC verifies user and responds.
   • Provides TGT, session key, expiration, PAC.
3. KRB_TGS_REQ: User requests TGS from KDC using TGT.
4. KRB_TGS_REP: KDC returns TGS containing service session key.
5. KRB_AP_REQ: User authenticates with service using TGS.
   • If privileges are correct, user gains access.

Attacks on Kerberos

Overpass The Hash/Pass The Key (PTK)

• Uses user hash to impersonate a user.
• Attacker with user hash can access various services.

Pass The Ticket (PTT)

• Involves using a user's ticket to impersonate them.
• Requires both the ticket and session key.

Golden Ticket and Silver Ticket

• Golden Ticket: Builds a TGT using the krbtgt account NTLM hash.
• Silver Ticket: Similar, but for TGS. Requires service key.

Kerberoasting

• Crack user account passwords offline using TGS.
• TGS encrypted with service key, derived from service owner account NTLM hash.

ASREPRoast

• Crack passwords by exploiting accounts with DONT_REQ_PREAUTH flag.
• KDC provides encrypted response that can be cracked.

Conclusion

• This post covered Kerberos authentication and attacks.
• Future posts will detail practical attacks and delegation.

References

• Various resources and articles on Kerberos and related attacks.
• Links to external articles and tools for deeper understanding.