O365 External Guest User Invited

name: O365 External Guest User Invited
id: 8c6d52ec-d5f2-4b2f-8ba1-f32c047a71fa
version: 9
creation_date: '2024-04-13'
modification_date: '2026-05-13'
author: Steven Dick
status: production
type: TTP
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.
data_source:
    - Office 365 Universal Audit Log
search: "`o365_management_activity` Workload=AzureActiveDirectory AND Operation=\"Add user*\" AND ModifiedProperties{}.NewValue=\"[*Guest*]\" AND ModifiedProperties{}.NewValue=\"[*Invitation*]\" | 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)) | rex field=user \"(?<user> [ \\w\\.-]+@ [ \\w-]+\\. [ \\w-]{2,4})\" | rename Operation as signature, Id as signature_id | fillnull | stats count min(_time) as firstTime max(_time) as lastTime by dest user src vendor_account vendor_product signature signature_id src_user | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `o365_external_guest_user_invited_filter`"
how_to_implement: 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.
references:
    - https://dirkjanm.io/assets/raw/US-22-Mollema-Backdooring-and-hijacking-Azure-AD-accounts_final.pdf
    - https://www.blackhat.com/us-22/briefings/schedule/#backdooring-and-hijacking-azure-ad-accounts-by-abusing-external-identities-26999
    - https://attack.mitre.org/techniques/T1136/003/
    - https://docs.microsoft.com/en-us/azure/active-directory/external-identities/b2b-quickstart-add-guest-users-portal
finding:
    title: Azure Guest User $user$ invited by $src_user$
    entity:
        field: src_user
        type: user
        score: 50
intermediate_findings:
    entities:
        - field: user
          type: user
          score: 50
          message: Azure Guest User $user$ invited by $src_user$
analytic_story:
    - Azure Active Directory Persistence
asset_type: O365 Tenant
mitre_attack_id:
    - T1136.003
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: identity