Detection: O365 External Guest User Invited

Updated Date: 2024-04-11 ID: 8c6d52ec-d5f2-4b2f-8ba1-f32c047a71fa Author: Steven Dick Type: TTP Product: Splunk Enterprise Security

Description

The following analytic identifies the invitation of an external guest user within Azure AD. With Azure AD B2B collaboration, users and administrators can invite external users to collaborate with internal users. 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. This detection leverages the Universal Audit Log (UAL)/o365:management:activity sourcetype as a detection data source.

1`o365_management_activity` Workload=AzureActiveDirectory AND Operation="Add user*" AND ModifiedProperties{}.NewValue="[*Guest*]" AND ModifiedProperties{}.NewValue="[*Invitation*]" 
2| eval user = (mvindex('ModifiedProperties{}.NewValue',5)), src_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| rex field=user "(?<user>[\\w\\.-]+@[\\w-]+\\.[\\w-]{2,4})" 
4| stats values(user) as user, min(_time) as firstTime, max(_time) as lastTime, count by Operation,Id,src_user 
5| rename Operation as signature, Id as signature_id 
6| `security_content_ctime(firstTime)`  
7| `security_content_ctime(lastTime)` 
8| `o365_external_guest_user_invited_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_guest_user_invited_filter search *
o365_external_guest_user_invited_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

Administrator may legitimately invite external guest users. Filter as needed.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message Risk Score Impact Confidence
Azure Guest User $user$ invited by $src_user$ 25 50 50
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: 1