Implementing Protected Users and Authentication Policies in AD

In the modern threat landscape, the perimeter has dissolved. As organizations transition to hybrid environments, the focus of identity-based attacks has shifted from breaching the network boundary to exploiting the inherent trust mechanisms within Active Directory (AD). Techniques such as Pass-the-Hash (PtH), Pass-the-Ticket (PtT), and Kerberos Delegation abuse rely on the permissive nature of default AD configurations.

To counter these lateral movement vectors, Microsoft introduced two critical security features: the Protected Users security group and Authentication Policies and Silos. While powerful, these features are not "set and forget" configurations. They require a deep understanding of your environment's authentication flows to avoid widespread service disruption.

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The Mechanics of the "Protected Users" Group

The `Protected Users` group is a specialized security group designed to mitigate the risk of credential theft from the Local Security Authority (LSA) and memory-resident attacks like Mimikatz. When a user is a member of this group, the Domain Controller (DC) enforces a much stricter set of authentication constraints, regardless of the user's other group memberships.

Key Constraints Enforced:

1. Disabling NTLM Authentication: This is the most impactful change. Members of this group cannot authenticate using NTLM. All authentication requests must use Kerberos. This effectively kills the Pass-the-Hash attack vector for these users.
2. Restricting Kerberos Delegation: Members cannot be used in unconstrained, constrained, or resource-based constrained delegation. This prevents an attacker from hijacking a user's identity to impersonate them to other services in the domain.
3. Eliminating Weak Encryption: The group prohibits the use of DES and RC4 encryption types. Authentication is restricted to AES (AES128 and AES256) only. This mitigates attacks targeting the weaker RC4-based Kerberos implementation.
4. Shortened Ticket Lifetimes: The lifetime of the Ticket Granting Ticket (TGT) is significantly reduced. Furthermore, TGTs for protected users cannot be renewed indefinitely; they must be re-authenticated, reducing the window of opportunity for a stolen ticket to be used.
5. No Credential Caching: The system is instructed not to cache the user's password or long-term credentials in the LSA process, making it much harder to scrape credentials from memory.

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Authentication Policies and Silos: Advanced Segmentation

While the Protected Users group protects specific identities, Authentication Policies and Silos allow you to protect the relationships between identities and resources. This is the logical evolution of the "least privilege" principle applied to authentication.

Authentication Policies

An Authentication Policy defines the "rules of engagement" for a user. It specifies which Kerberos encryption types are allowed and, most importantly, which hosts the user is permitted to authenticate to.

Authentication Silos

An Authentication Silo is a container that groups users and computers together. When a user is placed in a silo with specific computers, they are restricted from authenticable to any computer outside that silo. This creates a "logical boundary" within a flat network, preventing an attacker who has compromised a workstation in "Silo A" from using those credentials to move to a server in "Silo B."

By combining these, you can create a highly controlled environment where, for example, a Tier 0 administrator can only authenticate to Domain Controllers and a specific management workstation, with no possibility of their credentials being used on a standard user workstation.

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Implementation Strategy: An Auditing-First Approach

Implementing these features without rigorous auditing is a recipe for a massive outage. The primary failure mode is the breakage of legacy applications that rely on NTLM or unconstrained delegation.

Phase 1: The Audit

Before moving any user into the Protected Users group, you must identify NTLM dependencies.

• Enable NTLM Auditing: Use Group Policy to enable "Audit NTLM authentication" (specifically, `Audit NTLM authentication` and `Audit NTLM Netlogon validation`).
• Monitor Event Logs: Scrutinize Event ID 4624 (Logon) and Event ID 4776 (NTLM validation) on your Domain Controllers. Look for the `Authentication Package: NTLM` field.
• Identify Service Accounts: Service accounts are often the "silent killers" in this implementation. Many legacy services (printers, older web apps, scanners) rely on NTLM or constrained delegation.

Phase 2: The Pilot

Start with a small subset of human users who do not use legacy applications.

1. Create a test OU.
2. Add test users to the `Protected Users` group.
3. Monitor for `Event ID 4769` (Kerberos Service Ticket Request) failures, which might indicate an attempt to use a forbidden encryption type or delegation path.

Phase 3: Enforcement

Once auditing shows zero NTLM usage for your target user group over a full business cycle (including month-end processes), you can move to broader enforcement.

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Risks, Trade-offs, and Common Pitfalls

1. The Service Account Trap

Never add a service account to the Protected Users group unless you have verified it does not require NTLM or delegation. If a service account manages a SQL cluster that uses Kerberos Constrained Delegation (KCD) to impersonate users, adding it to the Protected Users group will immediately break the application's ability to function.

2. Breaking Multi-Tier Applications

In environments using web tier -> app tier -> database tier architectures, the middle tier often uses Kerberos delegation to pass user identity. The Protected Users

Conclusion

As shown across "The Mechanics of the "Protected Users" Group", "Authentication Policies and Silos: Advanced Segmentation", "Implementation Strategy: An Auditing-First Approach", a secure implementation for implementing protected users and authentication policies in ad depends on execution discipline as much as design.

The practical hardening path is to enforce deterministic identity policy evaluation with deny-by-default semantics, continuous control validation against adversarial test cases, and high-fidelity telemetry with low-noise detection logic. This combination reduces both exploitability and attacker dwell time by forcing failures across multiple independent control layers.

Operational confidence should be measured, not assumed: track false-allow rate and time-to-revoke privileged access and reduction in reachable unsafe states under fuzzed malformed input, then use those results to tune preventive policy, detection fidelity, and response runbooks on a fixed review cadence.