Detection: O365 External Identity Policy Changed

Description

The following analytic identifies when changes are made to the external guest policies within Azure AD. With Azure AD B2B collaboration, users and administrators can invite external users to collaborate with internal users. This detection also attempts to highlight what may have changed. External guest account invitations should be monitored by security teams as they could potentially lead to unauthorized access. An example of this attack vector was described at BlackHat 2022 by security researcher Dirk-Jan during his tall Backdooring and Hijacking Azure AD Accounts by Abusing External Identities.

 1`o365_management_activity` Workload=AzureActiveDirectory Operation="Update policy." Target{}.ID="B2BManagementPolicy" 
 2| eval object_attrs = mvindex('ModifiedProperties{}.NewValue',0), object_attrs_old = mvindex('ModifiedProperties{}.OldValue',0), object_name = mvindex('Target{}.ID',3), signature=Operation, user = case(match(mvindex('Actor{}.ID',-1),"User"),mvindex('Actor{}.ID',0),match(mvindex('Actor{}.ID',-1),"ServicePrincipal"),mvindex('Actor{}.ID',3),true(),mvindex('Actor{}.ID',0)) 
 3| spath input=object_attrs_old output=B2BOld path={} 
 4| spath input=B2BOld 
 5| rename B2BManagementPolicy.* as B2BManagementPolicyOld.* 
 6| spath input=object_attrs output=B2BNew path={} 
 7| spath input=B2BNew 
 8| eval object_attrs = 'B2BManagementPolicy.InvitationsAllowedAndBlockedDomainsPolicy.AllowedDomains{}' , object_attrs_old = 'B2BManagementPolicyOld.InvitationsAllowedAndBlockedDomainsPolicy.AllowedDomains{}' 
 9| eval diff_add=mvmap(object_attrs,if(isnull(mvfind(object_attrs_old,object_attrs)),object_attrs,null)) 
10| eval diff_remove=mvmap(object_attrs_old,if(isnull(mvfind(object_attrs,object_attrs_old)),object_attrs_old,null)) 
11| eval result = case(isnotnull(diff_add),"Added ".mvjoin(diff_add,","),isnotnull(diff_remove),"Removed ".mvjoin(diff_remove,",")), action = case(isnotnull(diff_add),"created",isnotnull(diff_remove),"deleted") 
12| stats values(object_attrs) as object_attrs, values(action) as action, values(result) as result, values(B2BManagementPolicy*) as B2BManagementPolicy*, count, min(_time) as firstTime, max(_time) as lastTime by user,signature,object_name 
13| `security_content_ctime(firstTime)`  
14| `security_content_ctime(lastTime)` 
15| `o365_external_identity_policy_changed_filter`

Data Source

Name Platform Sourcetype Source Supported App
N/A N/A N/A N/A N/A

Macros Used

Name Value
o365_management_activity sourcetype=o365:management:activity
o365_external_identity_policy_changed_filter search *
o365_external_identity_policy_changed_filter is an empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Annotations

- MITRE ATT&CK
+ Kill Chain Phases
+ NIST
+ CIS
- Threat Actors
ID Technique Tactic
T1136.003 Cloud Account Persistence
KillChainPhase.INSTALLATION
NistCategory.DE_CM
Cis18Value.CIS_10
APT29
LAPSUS$

Default Configuration

This detection is configured by default in Splunk Enterprise Security to run with the following settings:

Setting Value
Disabled true
Cron Schedule 0 * * * *
Earliest Time -70m@m
Latest Time -10m@m
Schedule Window auto
Creates Notable Yes
Rule Title %name%
Rule Description %description%
Notable Event Fields user, dest
Creates Risk Event True
This configuration file applies to all detections of type TTP. These detections will use Risk Based Alerting and generate Notable Events.

Implementation

You must install the Splunk Microsoft Office 365 Add-on and ingest Office 365 management activity events.

Known False Positives

Business approved changes by known administrators.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message Risk Score Impact Confidence
User $user$ changed the external identity [$object_name$] policy - $result$ 75 75 100
The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

References

Detection Testing

Test Type Status Dataset Source Sourcetype
Validation Passing N/A N/A N/A
Unit Passing Dataset o365 o365:management:activity
Integration ✅ Passing Dataset o365 o365:management:activity

Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI. Alternatively you can replay a dataset into a Splunk Attack Range


Source: GitHub | Version: 2