Active Directory Replication User Backdoor

Metadata


id	WIN-190101151110
author	Roberto Rodriguez @Cyb3rWard0g
creation date	2019/01/01
platform	Windows
playbook link	WIN-180815210510

Technical Description

Active Directory replication is the process by which the changes that originate on one domain controller are automatically transferred to other domain controllers that store the same data. Active Directory data takes the form of objects that have properties, or attributes. Each object is an instance of an object class, and object classes and their respective attributes are defined in the Active Directory schema. The values of the attributes define the object, and a change to a value of an attribute must be transferred from the domain controller on which it occurs to every other domain controller that stores a replica of that object. An adversary with enough permissions (domain admin) can add an ACL to the Root Domain for any user, despite being in no privileged groups, having no malicious sidHistory, and not having local admin rights on the domain controller. This is done to bypass detection rules looking for Domain Admins or the DC machine accounts performing active directory replication requests against a domain controller.

The following access rights / permissions are needed for the replication request according to the domain functional level

Control access right symbol	Identifying GUID used in ACE
DS-Replication-Get-Changes	1131f6aa-9c07-11d1-f79f-00c04fc2dcd2
DS-Replication-Get-Changes-All	1131f6ad-9c07-11d1-f79f-00c04fc2dcd2
DS-Replication-Get-Changes-In-Filtered-Set	89e95b76-444d-4c62-991a-0facbeda640c

Additional reading

https://github.com/hunters-forge/ThreatHunter-Playbook/tree/master/docs/content/library/active_directory_replication.md

Hypothesis

Adversaries with enough permissions (domain admin) might be adding an ACL to the Root Domain for any user to abuse active directory replication services.

Analytics

Initialize Analytics Engine



In [ ]:

    
from openhunt.mordorutils import *
spark = get_spark()

Download & Process Mordor File



In [ ]:

    
mordor_file = "https://raw.githubusercontent.com/hunters-forge/mordor/master/small_datasets/windows/defense_evasion/file_permissions_modifications_T1222/ad_object_modification/empire_dcsync_acl.tar.gz"
registerMordorSQLTable(spark, mordor_file, "mordorTable")

Analytic I

FP Rate	Log Channel	Description
Low	['Security']	Look for any user accessing directory service objects with replication permissions GUIDs



In [ ]:

    
df = spark.sql(
    '''
SELECT `@timestamp`, computer_name, SubjectUserName, ObjectName, OperationType
FROM mordorTable
WHERE channel = "Security"
    AND event_id = 4662
    AND ObjectServer = "DS"
    AND AccessMask = "0x40000"
    AND ObjectType LIKE "%19195a5b_6da0_11d0_afd3_00c04fd930c9%"
    '''
)
df.show(1,False)

Analytic II

FP Rate	Log Channel	Description
Low	['Security']	Look for any user modifying directory service objects with replication permissions GUIDs



In [ ]:

    
df = spark.sql(
    '''
SELECT `@timestamp`, computer_name, SubjectUserName, ObjectDN, AttributeLDAPDisplayName
FROM mordorTable
WHERE channel = "Security"
    AND event_id = 5136
    AND lower(AttributeLDAPDisplayName) = "ntsecuritydescriptor"
    AND (AttributeValue LIKE "%1131f6aa_9c07_11d1_f79f_00c04fc2dcd2%"
        OR AttributeValue LIKE "%1131f6ad_9c07_11d1_f79f_00c04fc2dcd2%"
        OR AttributeValue LIKE "%89e95b76_444d_4c62_991a_0facbeda640c%")
    '''
)
df.show(1,False)

Detection Blindspots

Hunter Notes

Hunt Output

Category	Type	Name
signature	SIGMA	win_ad_object_writedac_access
signature	SIGMA	win_ad_replication_user_backdoor