Stop Authentication Coercion Before It Spreads

A single Windows workstation can quietly “order” your most trusted servers to authenticate to an attacker-controlled host. No phishing email. No fake login page. No user typing anything in. Just normal Windows behavior—nudged in the wrong direction.

That’s why authentication coercion has become one of the most reliable on-ramps to credential theft, NTLM relay, and eventually domain compromise. And it’s getting harder to spot because attackers are moving away from famous tricks (like PrintNightmare) and toward rare RPC interfaces and obscure operation numbers (opnums) that many environments never baseline, alert on, or even log properly.

If you’re responsible for Active Directory, Windows servers, or identity security, here’s the stance I’ll take: signature-only defenses for authentication coercion are already behind. The best path forward is behavior-based detection—ideally with AI—paired with targeted hardening that removes easy relay paths.

Authentication coercion: the “hands-free” credential grab

Authentication coercion is an attack pattern where a target machine is forced to initiate authentication to an attacker-controlled system. The attacker doesn’t steal a password directly; they manipulate Windows services so the victim system offers up authentication material (often NTLM hashes) during an outbound connection.

This matters because coerced authentication often targets high-value machines—Domain Controllers, certificate services, Citrix servers, RADIUS servers, file servers—anything that can open doors across the enterprise.

Why Windows makes this possible

Windows environments are built to “just work.” When a system reaches out to a remote resource (a printer, a share, a service endpoint), it tries to authenticate automatically. Attackers exploit that convenience.

A common mechanism behind coercion is Remote Procedure Call (RPC). RPC is everywhere in Windows. It enables one machine to ask another machine to execute a function remotely. Many RPC methods accept parameters like a UNC path (for example, \\server\share\file). If an attacker can cause a server to process a UNC path that points to attacker infrastructure, the server may attempt to authenticate outbound.

Here’s the key line defenders should remember:

Coercion succeeds by blending into legitimate admin plumbing—then weaponizing the auto-authentication that plumbing triggers.

Why coercion keeps evolving (and why defenders keep missing it)

Attackers are shifting from well-known coercion vectors to rarely used RPC functions to avoid detection. When security teams build detections around the “famous” techniques, adversaries simply swap in a different interface, a different opnum, and keep the same outcome: outbound authentication they can capture and relay.

Several coercion tools and techniques are already widely documented and easy to run. That’s a big deal operationally because it lowers the barrier for intrusion crews and affiliates. In practice, it also creates a defensive trap: teams overfit detections to the tools they’ve seen on GitHub, not to the behavior the tools produce.

Common coercion families (what most teams monitor)

You’ll often see defenders focused on a handful of known RPC protocols and tools, such as:

• Print-related coercion (often associated with PrintNightmare-style activity)
• EFSRPC coercion (commonly associated with PetitPotam-style activity)
• DFS namespace coercion
• File server VSS agent coercion
• Remote event log coercion

Those are worth monitoring. But attackers are increasingly comfortable using less popular interfaces because:

• They’re rarely used in day-to-day operations (so the attacker stands out less if you aren’t baselining)
• Many orgs don’t have detections at the opnum level
• Controls may block one coercion method while leaving others untouched

If you only alert on “the top five coercion tools,” you’re basically playing whack-a-mole with Windows internals.

A real attack pattern: rare RPC + fast targeting of Tier 0

Real intrusions show coercion happening as a rapid, automated sweep—often from one compromised internal host toward many critical servers. A compelling pattern observed in the wild is abuse of the EventLog Remoting RPC interface (a protocol most orgs barely think about).

In one case, an attacker used a rare remote event logging method to coerce a server into reaching out to an external IP. That combination—rare interface, unusual destination, and outbound auth attempt—was the signal.

Then the attacker behavior escalated quickly:

• A single internal machine initiated coercion attempts against 10+ remote resources
• Targets included Domain Controllers and other Tier 0-adjacent systems
• Authentication attempts occurred in a tight time window
• The attacker captured machine account NTLM hashes from coerced authentications
• The attacker moved into NTLM relay behavior, aiming at certificate authority services

Here’s why this sequence is so dangerous: machine accounts are powerful, and if attackers can relay those authentications to the right service (especially identity infrastructure like certificate services), they can mint privileges that don’t look like a typical “stolen password” story.

What defenders should extract from the case study

The most useful lesson isn’t “watch for protocol X.” It’s this:

1. Rarity is a detection feature. If a protocol/opnum hasn’t been used in 30 days, treat first-time usage as suspicious until proven otherwise.
2. Coercion is usually noisy across targets. One host hits many servers rapidly.
3. The destination often gives it away. UNC parameters pointing to external IPs or unusual internal hosts are high-signal.
4. Coercion is a setup move. The real objective is credential theft, relay, and privilege escalation.

What AI detection does better than rule stacks

AI-based detection is well-suited to authentication coercion because coercion is fundamentally an anomaly problem, not a malware problem. Most coercion traffic looks like legitimate Windows management traffic—until you add context.

Rules can catch known bad combinations. AI can catch weird-but-new combinations.

The specific AI advantages for coercion

1) Baselines per host and per role

A Domain Controller behaves differently than a Citrix server. A print server behaves differently than a database server. AI models can build baselines like:

• Which RPC interfaces are used on this host (and how often)
• Which peers this host normally talks to
• Typical authentication protocols used (Kerberos vs NTLM)
• Normal time-of-day patterns for administrative RPC

2) “Rarity + sequence” detection

Coercion often appears as a sequence:

• unusual RPC call → unusual outbound auth attempt → repeated across multiple targets → relay attempt

AI can score the sequence rather than a single event. That reduces false positives and catches variants that don’t match a signature.

3) Entity-focused correlation across telemetry

Coercion isn’t just network traffic. It touches:

• Endpoint behavior (processes initiating RPC calls)
• Identity logs (NTLM authentications that shouldn’t happen)
• Network destinations (odd UNC/IP targets)
• Server role context (Tier 0 vs noncritical)

AI-driven UEBA-style approaches can correlate those signals into one alert that an analyst can actually act on.

A practical one-liner you can use internally:

Coercion detection works when you treat Windows RPC like identity traffic, not just network noise.

Detection you can implement this quarter (without boiling the ocean)

Start with three detection pillars: UNC inspection, opnum/interface rarity, and Tier 0 targeting. You don’t need perfect visibility into every RPC packet on day one. You need high-signal coverage that catches the common path to compromise.

1) UNC path and parameter monitoring

Most coercion techniques need a remote path. Focus on:

• RPC calls that include UNC-like parameters
• UNC parameters pointing to:
  • external IP addresses
  • newly seen internal hosts
  • systems outside normal admin subnets

Operational tip: filter local-only RPC first. Local RPC is rarely the coercion path.

2) Interface GUID and opnum anomaly detection

Coercion lives at the method-call level. Prioritize monitoring for:

• Known high-risk RPC interfaces
• Rare opnums within those interfaces
• Any “first-time seen” interface/opnum pair per asset role

If you can only do one thing: alert when a workstation calls rare RPC methods against Domain Controllers. That’s not normal in healthy environments.

3) Behavioral sweep detection

Create detections for:

• One source host initiating RPC connections to many servers in <15 minutes
• Coercion-like RPC calls followed by failed NTLM auth attempts across critical servers
• Sudden spikes in NTLM usage from hosts that usually use Kerberos

These are the patterns automated coercion tools generate.

Prevention and hardening: make relays painful and coercion less useful

The goal isn’t to “patch coercion” (you can’t). The goal is to reduce where coerced authentications can be relayed and reduce which services can be abused remotely.

High-impact hardening moves

1. Enforce SMB signing where feasible

   • This blocks or degrades common relay paths involving SMB.

2. Use Extended Protection for Authentication (EPA) on applicable services

   • EPA helps bind authentication to the channel, making credential forwarding and relaying harder.

3. Disable or restrict unused RPC services on critical systems

   • If a Domain Controller doesn’t need remote event log access, shut it off.
   • If print spooler isn’t required on Tier 0 assets, disable it.

4. Constrain NTLM aggressively (especially around identity infrastructure)

   • Many coercion chains culminate in NTLM relay. Reducing NTLM acceptance in the right places changes the economics of the attack.

5. Network segmentation with Tier 0 protection rules

   • Workstations should not have broad RPC reach into Domain Controllers and certificate services.

If you’re choosing where to start, I’d pick: Tier 0 RPC restrictions + SMB signing + EPA where possible. It doesn’t fix everything, but it stops a lot of real intrusion chains.

A 10-step quick plan for security teams (practical and opinionated)

1. Inventory Tier 0 assets: Domain Controllers, CAs, RADIUS, identity brokers, privileged access servers.
2. Block or tightly restrict workstation-to-Tier-0 RPC at the network layer.
3. Disable Print Spooler on Domain Controllers.
4. Disable remote event log access on Domain Controllers unless there’s a proven operational need.
5. Enforce SMB signing on servers (phased rollout if necessary).
6. Identify where NTLM is still required—and where it’s just legacy inertia.
7. Alert on external IPs appearing inside UNC parameters.
8. Alert on rare RPC interface/opnum usage, especially targeting Tier 0.
9. Correlate coercion alerts with follow-on behavior: relay attempts, certificate enrollment anomalies, DCSync-like patterns.
10. Add AI-driven baselining for RPC + auth behavior so variants don’t slip through just because the opnum changed.

What to do next

Authentication coercion isn’t “back.” It never left. It’s simply shifted into the parts of Windows RPC that most teams don’t watch.

If your detection strategy relies on a list of known coercion tools, you’re betting your domain on attackers being predictable. They won’t be. AI-based behavioral detection—paired with Tier 0 hardening and relay-resistant authentication—gives you a realistic way to catch coercion early, before it becomes credential theft and privilege escalation.

What would change in your environment if you treated every rare RPC call into Tier 0 as a potential breach attempt until proven otherwise?