Suspicious Machine Account Replication - DcSync Indicator

Status: test
Severity: medium
Log source: product windows, service security
Author: Benjamin Delpy, Florian Roth (Nextron Systems), Scott Dermett, Sorina Ionescu
Source: github.com/SigmaHQ/sigma

Detects suspicious Active Directory Replication Service (ADRS) requests originating from a machine account (SubjectUserName ending in '$') rather than a legitimate Domain Controller. Under normal operation, only Domain Controllers initiate replication requests carrying the DS-Replication-Get-Changes-All right. If a threat actor obtains valid machine account credentials — for example by abusing certificate-based authentication (PKINIT) to impersonate a DC after exploiting a CA vulnerability such as CVE-2026-54121 (Certighost), where a temporary machine account is created to request a DC certificate and then used to perform DCSync — they can dump all domain credential material including the krbtgt hash.

Known false positives

Valid DC Sync that is not covered by the filters; please report

MITRE ATT&CK coverage

Tactic	Techniques
Credential Access	`T1003.006` OS Credential Dumping: DCSync

Telemetry coverage

Provider	Record / event type
Security-Auditing	Event ID 4662: An operation was performed on an object.

Rule body

title: Suspicious Machine Account Replication - DcSync Indicator
id: 611eab06-a145-4dfa-a295-3ccc5c20f59a
related:
    - id: 17d619c1-e020-4347-957e-1d1207455c93 # Active Directory Replication from Non Machine Account
      type: similar
status: test
description: |
    Detects suspicious Active Directory Replication Service (ADRS) requests originating from
    a machine account (SubjectUserName ending in '$') rather than a legitimate Domain Controller.

    Under normal operation, only Domain Controllers initiate replication requests carrying the
    DS-Replication-Get-Changes-All right. If a threat actor obtains valid machine account
    credentials — for example by abusing certificate-based authentication (PKINIT) to
    impersonate a DC after exploiting a CA vulnerability such as CVE-2026-54121 (Certighost),
    where a temporary machine account is created to request a DC certificate and then used to
    perform DCSync — they can dump all domain credential material including the krbtgt hash.
references:
    - https://twitter.com/gentilkiwi/status/1003236624925413376
    - https://gist.github.com/gentilkiwi/dcc132457408cf11ad2061340dcb53c2
    - https://blog.blacklanternsecurity.com/p/detecting-dcsync?s=r
    - https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-10/security/threat-protection/auditing/event-4662
    - https://github.com/aniqfakhrul/CVE-2026-54121
author: Benjamin Delpy, Florian Roth (Nextron Systems), Scott Dermett, Sorina Ionescu
date: 2018-06-03
modified: 2026-07-30
tags:
    - attack.credential-access
    - attack.s0002
    - attack.t1003.006
    - cve.2026-54121
logsource:
    product: windows
    service: security
    definition: |
        Requirements: A SACL must be configured on the domain NC root object (e.g. DC=domain,DC=com)
        to generate Event 4662. Add via ADSI Edit: navigate to the domain NC root → Properties →
        Security → Advanced → Auditing → add an ACE for Everyone, Type: Success, Applies to:
        This object only, rights: DS-Replication-Get-Changes and DS-Replication-Get-Changes-All.
        The OS audit subcategory must also be enabled:
        auditpol /set /subcategory:"Directory Service Access" /success:enable
detection:
    selection:
        EventID: 4662
        Properties|contains:
            - '1131f6ad-9c07-11d1-f79f-00c04fc2dcd2' # DS-Replication-Get-Changes-All
            - '1131f6aa-9c07-11d1-f79f-00c04fc2dcd2' # DS-Replication-Get-Changes
            - '9923a32a-3607-11d2-b9be-0000f87a36b2' # DS-Replication-Synchronize
            - '89e95b76-444d-4c62-991a-0facbeda640c' # DS-Replication-Get-Changes-In-Filtered-Set
        SubjectUserName|endswith: '$'
    filter_main_subject_usersid:
        SubjectUserSid|startswith: 'S-1-5-18'
    condition: selection and not 1 of filter_main_*
falsepositives:
    - Valid DC Sync that is not covered by the filters; please report
level: medium

Stages and Predicates

Stage 0: `condition`

selection and not 1 of filter_main_*

Stage 1: `selection`

selection:
    EventID: 4662
    Properties|contains:
        - '1131f6ad-9c07-11d1-f79f-00c04fc2dcd2'
        - '1131f6aa-9c07-11d1-f79f-00c04fc2dcd2'
        - '9923a32a-3607-11d2-b9be-0000f87a36b2'
        - '89e95b76-444d-4c62-991a-0facbeda640c'
    SubjectUserName|endswith: '$'

Stage 2: `not filter_main_subject_usersid`

filter_main_subject_usersid:
    SubjectUserSid|startswith: 'S-1-5-18'

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`SubjectUserSid`	starts_with	`S-1-5-18`	`excludes:SubjectUserSid` `field:"SubjectUserSid" value:"S-1-5-18"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`Properties`	match	`1131f6aa-9c07-11d1-f79f-00c04fc2dcd2` corpus 6 (sigma 3, elastic 2, kusto 1) `1131f6ad-9c07-11d1-f79f-00c04fc2dcd2` corpus 6 (sigma 3, elastic 2, kusto 1) `89e95b76-444d-4c62-991a-0facbeda640c` corpus 6 (sigma 3, elastic 2, kusto 1) `9923a32a-3607-11d2-b9be-0000f87a36b2` corpus 3 (sigma 3)	`field:"Properties" kind:match`
`SubjectUserName`	ends_with	`$` corpus 5 (sigma 4, splunk 1)	`field:"SubjectUserName" kind:ends_with value:"$"`