Note: please test this in a test environment before applying this in production. This should be considered as Proof of Concept code. Tested on Windows Server 2019 with an 2016 forest functional level.

Due the lack of public examples on how to monitor for ACE changes in AD I created these 2 PoC's. There are probally better was to monitor this that either work or scale better, (for example using a SIEM), but this is mainly to show that if you know where to look you can collect enough data to monitor for ACE changes in Active Directory.

GetKnownGoods.ps1 will collect SDDL's of all the current AD Objects. This example then exports these values to a XML file.
This process could be implemented to run on a schedule to collect this data over time. Another process could be implemented that compares these results to detect malicious activity.

After configuring Audit Directory Services Changes and a SACL whenever a change to a ACE of a AD Object is made a Event with EventID 5136 is logged. MonitorEvents.ps1 uses the data collected by previous example GetKnownGoods.ps1 as known good values. It then compares these values against triggered events to determine what changed from the previously known 'good' state.

With known good states

Without known good states

1. Create a GPO and follow this guide to apply the 'Baseline Recommendation' at minimum.
   https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/plan/security-best-practices/audit-policy-recommendations

2. Create a second GPO to enforce Advanced Auditing.
   https://docs.microsoft.com/en-us/windows-server/identity/ad-ds/plan/security-best-practices/Monitoring-Active-Directory-for-Signs-of-Compromise#enforcing-traditional-auditing-or-advanced-auditing

3. Apply a SACL on the root of AD that monitors changes made to properties and permissions.
   

4. Apply GPO's to the Domain Controllers OU and reboot them.
   

5. Run GetKnownGoods.ps1. Make sure to update the paths for Export-CliXML and Import-CliXML.

MIT

Justin Perdok (@justin-p), Orange Cyberdefense

Feel free to open issues, contribute and submit your Pull Requests. You can also ping me on Twitter (@JustinPerdok)