What are the three core phases of the Kerberos V5 (RFC 4120) authentication exchange?

Kerberos V5 relies on a trusted third party called the Key Distribution Center (KDC), which contains an Authentication Service (AS) and a Ticket Granting Service (TGS). The exchange occurs in three distinct phases: (1) Authentication Phase (AS-REQ / AS-REP): The client proves identity using a pre-authentication timestamp encrypted with its password hash and receives a Ticket Granting Ticket (TGT) encrypted with the secret key of the krbtgt account. (2) Authorization Phase (TGS-REQ / TGS-REP): The client presents its TGT to the TGS requesting access to a specific Service Principal Name (SPN) and receives a Service Ticket encrypted with the target service's secret key. (3) Service Request Phase (AP-REQ / AP-REP): The client presents the Service Ticket and an authenticator directly to the application server, proving authorization without sending passwords across the wire.

What is a Privilege Attribute Certificate (PAC) and how does Active Directory protect it from tampering?

In Microsoft Active Directory implementations of Kerberos, the user's authorization context (Security Identifiers / SIDs of all global, universal, and nested security groups) is packaged inside an extension called the Privilege Attribute Certificate (PAC), embedded in the authorization-data field of tickets. To prevent a malicious client or service from tampering with group memberships (such as granting themselves Domain Admin SID S-1-5-21-...-512), the PAC is cryptographically protected with two digital signatures: the Server Signature (computed with the target service's key) and the KDC Signature (computed with the krbtgt key). In modern patched AD domains, the KDC also validates PAC ticket signatures during PAC validation calls.

How does a Kerberoasting attack work and how can enterprise infrastructure defend against it?

Any valid domain account can request a TGS service ticket for ANY Service Principal Name (SPN) registered in Active Directory. Because the service ticket is encrypted with the target service account's long-term password hash (often using RC4-HMAC or AES-256), an attacker extracts the encrypted ticket payload from memory and attempts offline dictionary or brute-force cracking using tools like Hashcat. If the service account uses a weak password, the plaintext password is recovered without generating any suspicious network traffic or account lockout events on the domain controller. Defenses include: using Group Managed Service Accounts (gMSA) with automated 120-character random passwords, disabling RC4 encryption across the domain, and alerting on anomalous high-volume TGS requests.

What is a Golden Ticket attack and why is resetting the krbtgt password twice mandatory for recovery?

A Golden Ticket attack occurs when an attacker compromises the secret key (password hash or AES key) of the domain's krbtgt service account. With the krbtgt key, the attacker can forge completely valid TGTs locally without contacting a domain controller, granting themselves arbitrary group memberships (Enterprise Admins), custom validity lifetimes (e.g. 10 years), and any user identity. Because the KDC decrypts TGTs using the krbtgt key, the forged ticket is unconditionally accepted. Active Directory maintains history for the current and immediately previous password of krbtgt; therefore, recovering from a Golden Ticket requires resetting the krbtgt password TWICE, with enough delay between resets for domain replication to complete.

Kerberos V5 & Active Directory Authentication Studio: KDC, TGT, Service Tickets & PAC

Verified Specification | Standardized Formulas | Instant Precision

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Kerberos V5 & Active Directory Authentication Studio

Architect enterprise SSO and identity infrastructure: trace 3-phase ticket exchanges, inspect Privilege Attribute Certificate (PAC) signatures, and audit defenses against Kerberoasting.

AES-256-CTS-HMAC-SHA1-96 KDC Signatures Valid

10.0 Hours

TGT Maximum Validity Window

± 300 sec

Allowed Timestamp Skew Limit (Replay Guard)

14 SIDs

Embedded PAC Security Group Identifiers

Low (gMSA Active)

Kerberoasting Vulnerability Posture

1. Realm & Service Principal Name (SPN) Configuration

Kerberos Realm / Domain

Target Service Principal Name (SPN)

Encryption Type

Service Account Type

2. Complete Kerberos V5 Protocol Ticket Flow

Phase 1: Authentication Service Exchange (AS-REQ → AS-REP)
Client → KDC: AS-REQ { client: alice@CORP.COM, pre-auth: Enc(timestamp, K_alice) }
KDC → Client: AS-REP { TGT: Enc({ alice, session_key_1, PAC }, K_krbtgt), Enc(session_key_1, K_alice) }

Phase 2: Ticket Granting Service Exchange (TGS-REQ → TGS-REP)
Client → KDC: TGS-REQ { TGT, Authenticator: Enc(timestamp, session_key_1), SPN: MSSQLSvc/... }
KDC → Client: TGS-REP { ServiceTicket: Enc({ alice, session_key_2, PAC }, K_service), Enc(session_key_2, session_key_1) }

Phase 3: Application Client-Server Exchange (AP-REQ → AP-REP)
Client → Server: AP-REQ { ServiceTicket, Authenticator: Enc(timestamp, session_key_2) }
Server → Client: AP-REP (Mutual Authentication verified. Authorization granted via validated PAC SIDs).

Click "Run Kerberoasting Audit" to inspect offline cracking resistance for the configured SPN.

3. Production krb5.conf & Active Directory Hardening

// Generated krb5.conf

Kerberos V5 & Active Directory Authentication Studio

1. Realm & Service Principal Name (SPN) Configuration

2. Complete Kerberos V5 Protocol Ticket Flow

3. Production krb5.conf & Active Directory Hardening

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