Windows 2000 Kerberos Ticket Renewal Enforcement

In Microsoft Windows 2000, Kerberos user ticket renewal policy limits are enforced directly by the Key Distribution Center (KDC) running on Active Directory domain controllers. By embedding strict expiration and renewal timestamps inside the encrypted Ticket-Granting Ticket (TGT) during initial authentication, the KDC ensures that clients cannot renew a session beyond the maximum renewal lifetime configured in the domain’s Group Policy without requiring the user to re-authenticate with primary credentials.

Domain Policy Configuration

The enforcement mechanism begins with the domain-wide Kerberos policy defined in Active Directory. In Windows 2000, these rules are configured via the Default Domain Policy Group Policy Object (GPO) under:

Computer Configuration -> Windows Settings -> Security Settings -> Account Policies -> Kerberos Policy

The relevant setting is Maximum lifetime for user ticket renewal, which has a default setting of 7 days (10,080 minutes). This policy sets the absolute ceiling on how long a TGT can be refreshed before full re-authentication is mandatory. A companion setting, Maximum lifetime for user ticket, defines the initial validity period of the TGT (defaulting to 10 hours).

Timestamp Generation During Initial Authentication

When a user logs on, the Kerberos client sends an Authentication Service request (KRB_AS_REQ) to the KDC. When generating the response (KRB_AS_REP), the KDC constructs the TGT and includes three critical timestamp fields inside the encrypted ticket payload:

  1. authtime: The timestamp indicating when the initial authentication took place.
  2. endtime: The expiration time of the current ticket, determined by the maximum ticket lifetime (e.g., current time + 10 hours).
  3. renew-till: The hard cutoff for renewals, calculated by adding the domain's renewal policy limit to the initial authentication time (e.g., authtime + 7 days).

Because these values reside inside the encrypted portion of the TGT—secured using the KDC's own secret key (the krbtgt account key)—the client cannot alter them.

Renewal Request Processing

Before a TGT reaches its endtime, the Windows 2000 client requests a renewal by sending a Ticket-Granting Service request (KRB_TGS_REQ) with the renewable flag set. The KDC processes this request through the following steps:

  1. Decryption and Validation: The KDC decrypts the presented TGT using its krbtgt key and verifies its cryptographic integrity.
  2. Flag Verification: The KDC checks that the original TGT was marked as renewable.
  3. Time Comparison: The KDC compares its current system time against the renew-till timestamp contained within the decrypted ticket:
    • If the current time is earlier than the renew-till time, the request is valid.
    • If the current time has exceeded the renew-till time, the KDC rejects the request with the error code KDC_ERR_TGT_REVOKED or KDC_ERR_ETYPE_NOSUPP.

Issuance of the Renewed Ticket

If the renewal request is approved, the KDC issues a new TGT. In the new ticket:

  • The endtime is advanced by another standard ticket lifetime (e.g., up to 10 hours from the renewal moment), provided this does not exceed the renew-till limit. If the remaining time until renew-till is less than the standard ticket lifetime, the endtime is truncated to match renew-till.
  • The original renew-till and authtime timestamps remain unchanged from the original ticket.

Once the renew-till deadline is reached, the KDC refuses any further renewals for that ticket lineage. The client is forced to initiate a brand-new KRB_AS_REQ exchange, requiring the user to supply valid primary credentials (such as entering their password or smart card PIN) to obtain a new TGT.