DCSync is one of those attacks that feels “too quiet for how catastrophic it is.” It doesn’t need malware on a domain controller, and it can look like normal replication unless you know what “normal” actually looks like. This guide focuses on patterns: the repeatable signals in logs, identity context, and network behavior that separate legitimate directory replication from credential-theft replication.
What DCSync is (and why it blends in)
At a high level, a DCSync-style intrusion is an abuse of directory replication privileges. Instead of “dumping passwords from LSASS on a DC,” the attacker convinces a domain controller to replicate sensitive directory data to them—especially credential material (hashes and related secrets).
DCSync is fundamentally a permissions problem and a replication-traffic anomaly. If you can (1) confidently identify who is allowed to replicate, and (2) baseline where replication traffic comes from, you can usually detect it quickly.
If your replication fundamentals are rusty, start with: Active Directory Replication: What it is and how it works. Understanding the “normal engine” makes it far easier to spot replication being used as a weapon.
Baseline first: what “normal replication” looks like
Most failed DCSync detections happen for one reason: teams try to write “the DCSync rule” before they can answer: what does legitimate replication look like in this forest?
Normal replication tends to be…
- DC-to-DC (source is another domain controller)
- Machine-account driven (e.g.,
DC01$) - Predictable cadence (site topology + schedules)
- Consistent ports & RPC patterns (varies, but stable over time)
- Consistent naming contexts (domain/config/schema, as expected)
Suspicious replication tends to be…
- Non-DC source host (workstation/app server/jump box)
- User or service account not known to be replication-capable
- Bursty or “one-off” at unusual times
- Targeted and intense (many objects/secrets in a short window)
- Preceded by ACL/permission change granting replication rights
Baseline doesn’t need to be perfect. You just need enough to confidently say: “Replication from these hosts/accounts is expected; everything else is suspicious until proven otherwise.”
Permission baselining is often where the real work is. If you need a refresher on how AD permissions behave (inheritance, scope, delegated rights), see Active Directory Permissions Explained.
High-signal indicators and common patterns
Below are the most useful patterns for analysis. You’ll notice they aren’t “single event = compromise.” They’re clusters that, when correlated, reliably identify abuse.
Pattern 1: Replication rights used from a non-DC host
This is the classic DCSync shape: a host that is not a domain controller initiates replication-like operations against a DC. In most environments, this is rare enough to treat as high severity.
| Signal | Where to see it | Why it matters | Common false positives |
|---|---|---|---|
| Replication-like directory access from non-DC IP/hostname | DC Security log (directory service access auditing), SIEM correlation, network telemetry | Replication is typically DC-to-DC; non-DC sources are unusual | AD assessment tools, backup/DR products, identity governance tooling (verify vendor behavior) |
| Account performing replication does not match expected replication principals | DC Security log + identity inventory | Highlights credential abuse or over-permissioned service accounts | Legacy service accounts with broad rights (often the real problem) |
Pattern 2: “Grant then replicate” (ACL change precedes replication use)
Another frequent pattern: an attacker first modifies permissions (or group membership) to gain replication rights, then immediately performs the replication abuse.
If you’re trying to catch DCSync early, don’t start with “replication events.” Start with “who just gained replication rights?” and track forward from there.
Pattern 3: The “quiet account” suddenly acts like Tier 0
Many environments have dormant or low-activity accounts that should never behave like Tier 0. If a rarely-used service account suddenly performs high-impact directory operations, that’s a strong anomaly.
This is why auditing group nesting and effective privilege paths matters: Auditing Nested Group Memberships: An Expert Guide. Attackers love hidden privilege inheritance because it lets them “look legitimate” on paper.
Pattern 4: Replication permissions exist where they shouldn’t
In many DCSync incidents, the attacker didn’t need to “add” rights—someone had already granted them years ago. Excess permissions are not just messy; they’re a standing invitation to domain compromise.
If you suspect “legacy sprawl” is the real culprit, this is a strong companion read: Excess Permissions: Lessons from Legacy Setups.
Telemetry setup: the minimum you need for reliable detection
You can’t analyze patterns without consistent telemetry. The goal is not “turn on every audit setting.” The goal is: capture identity + host + operation for replication-sensitive directory access.
1) Enable the right auditing on domain controllers
- Directory Service Access / Object Access (so DCs log the relevant directory operations).
- Directory Service Changes (so ACL changes and sensitive attribute changes are visible).
- Ensure events include source network address where possible (critical for “non-DC host” detection).
The exact audit policy and SACL configuration varies by Windows Server version and your logging strategy. The key outcome is consistent: you need events that show who performed the replication-sensitive operation and from where.
2) Watch for the replication rights GUIDs (defender signal)
When Directory Service Access auditing is configured, defenders often key on replication-control access rights. The commonly monitored control access rights are:
| Right (concept) | Common name | What to do with it |
|---|---|---|
| Replicate directory changes | DS-Replication-Get-Changes | Alert if used by non-DC hosts or unexpected accounts |
| Replicate all directory changes | DS-Replication-Get-Changes-All | High sensitivity; few accounts should ever need this |
| Replicate changes in filtered set | DS-Replication-Get-Changes-In-Filtered-Set | High sensitivity; investigate unusual usage |
3) Collect complementary telemetry (identity + network)
- Logons to DCs (successful/failed), especially remote logons and unusual authentication patterns.
- Network flows to DCs (RPC endpoint mapper and dynamic RPC behavior can be a useful corroborator).
- Privileged group changes and ACL changes on domain root/config/schema objects.
Understanding authentication mechanics can help explain why certain logons appear the way they do: NTLM authentication and Kerberos Authentication Protocols Explained.
Hunting playbook: triage questions and correlation strategy
When an alert fires (or you suspect DCSync), don’t start by asking “is it DCSync?” Start by asking a sequence of questions that quickly narrows the search space.
Triage questions that move fast
- Which host initiated the replication-sensitive activity? Is it a DC? If not, treat as high risk.
- Which account was used? Is it a DC machine account or an approved replication-capable service?
- Is this a first-seen behavior for that account/host pair? First-time + Tier 0 operations is a strong anomaly.
- Was there a recent permission or group membership change? “Grant then replicate” is a classic chain.
- How “wide” was the operation? One object vs many; bursty activity suggests extraction rather than normal ops.
Correlation strategy (what to tie together)
| Data type | What to correlate | Why it helps |
|---|---|---|
| Directory access events | Replication-right usage + source IP/host + subject account | Core “who did what from where” signal |
| Directory changes | ACL edits on domain root/config/schema + timing vs replication use | Detects “grant then replicate” chain |
| Logon events | New logon to DCs or privileged systems by the subject account | Shows how the attacker established access |
| Network telemetry | Unusual RPC/SMB flows to DCs from non-DC segments | Confirms the host-level origin and scope |
If the suspect account is able to replicate, assume credential exposure may already have occurred. Your investigation should run in parallel with containment steps.
Hardening: reducing the blast radius
“Detect DCSync” is important. But “make DCSync hard to do” is better. The most effective prevention is removing replication rights from anything that isn’t a domain controller (or a tightly controlled, justified exception).
Practical hardening moves
- Inventory who has replication rights (explicitly and via group nesting) and justify each one.
- Remove legacy broad delegations that creep onto domain root/configuration objects over time.
- Tier 0 isolation: restrict where privileged credentials can log on; avoid using admin creds on user workstations.
- Protect high-value accounts (admin hygiene, MFA where applicable, strong operational controls).
- Harden and monitor delegation and permissions to stop “grant then replicate” at the source.
Many DCSync incidents are simply the end result of “permission sprawl.” Treat excess permissions as a security debt you actively pay down, not a cleanup task you postpone forever.
Response: what to do when you suspect DCSync
DCSync is a “credential material exposure” event. If replication rights were abused, assume an attacker may have obtained hashes for sensitive accounts (potentially including highly privileged secrets).
Immediate containment (minutes to hours)
- Identify and isolate the source host (remove from network or restrict its access to DCs).
- Disable or contain the suspect account (or remove replication rights immediately if disabling is risky operationally).
- Block suspicious paths to DCs at network boundaries where feasible (especially from user segments).
- Preserve evidence: export relevant DC logs, capture network telemetry, and record time windows.
Credential reset strategy (hours to days)
- Reset passwords for exposed privileged accounts (prioritize Tier 0, service accounts, and any account used in the event).
- If you suspect the domain’s ticket-signing secret was exposed, follow best practice for KRBTGT password resets (typically involves a carefully planned double reset with validation).
- Review and remove unauthorized delegations and group memberships added during the intrusion window.
Eradication and recovery (days to weeks)
- Root-cause the permissions path: how did replication rights exist or get granted?
- Audit other DCs and privileged endpoints for lateral movement evidence.
- Implement sustained monitoring for recurrence: replication-right usage from non-DC hosts should become a durable alert.
A strong DCSync response isn’t just “close the one hole.” It’s “prove we removed the class of holes”: replication rights inventory, permission governance, and tiering controls.
Quick checklist
- Baseline expected replication sources (DC-to-DC hosts, machine accounts, cadence).
- Alert on replication-right usage from non-DC hosts.
- Detect chains: ACL/group change ➜ replication-right usage ➜ unusual logons/network flows.
- Inventory & justify all principals with replication permissions; remove legacy sprawl.
- Hunt anomalies: first-seen Tier 0 behavior, bursty replication-like activity, rare accounts acting privileged.
- Respond fast: isolate host, contain account, preserve evidence, execute credential reset plan.
Related reading on windows-active-directory.com: AD Replication fundamentals · AD permissions and inheritance · Nested group audit (hidden privilege paths) · Kerberos vs NTLM (auth context) · Excess permissions in legacy AD
Key takeaways
- DCSync is replication privilege abuse. Treat it as a permissions + anomaly problem, not just “malware.”
- The fastest wins come from baselining normal replication and alerting on non-DC replication sources.
- Look for chains: permission change ➜ replication use ➜ credential exposure response.
- Long-term prevention is permission governance: eliminate replication rights sprawl and hidden privilege inheritance.
