O365 OAuth App Mailbox Access via Graph API

name: O365 OAuth App Mailbox Access via Graph API
id: 9db0d5b0-4058-4cb7-baaf-77d8143539a2
version: 11
creation_date: '2024-02-14'
modification_date: '2026-05-13'
author: Mauricio Velazco, Splunk
status: production
type: TTP
description: The following analytic detects when emails are accessed in Office 365 Exchange via the Microsoft Graph API using the client ID '00000003-0000-0000-c000-000000000000'. It leverages the 'MailItemsAccessed' operation within the Exchange workload, focusing on OAuth-authenticated applications. This activity is significant as unauthorized access to emails can lead to data breaches and information theft. If confirmed malicious, attackers could exfiltrate sensitive information, compromise user accounts, and further infiltrate the organization's network.
data_source:
    - O365 MailItemsAccessed
search: |-
    `o365_management_activity` Workload=Exchange Operation=MailItemsAccessed AppId=* AppId=00000003-0000-0000-c000-000000000000
      | fillnull
      | stats count earliest(_time) as firstTime latest(_time) as lastTime values(ClientIPAddress) as src
        BY user ClientAppId OperationCount
           AppId vendor_account vendor_product
           dest signature
      | `security_content_ctime(firstTime)`
      | `security_content_ctime(lastTime)`
      | `o365_oauth_app_mailbox_access_via_graph_api_filter`
how_to_implement: You must install the Splunk Microsoft Office 365 Add-on and ingest Office 365 management activity events.
known_false_positives: OAuth applications may access mailboxes for legitimate purposes, you can use the ClientAppId to add trusted applications to an allow list.
references:
    - https://attack.mitre.org/techniques/T1114/002/
    - https://learn.microsoft.com/en-us/troubleshoot/azure/active-directory/verify-first-party-apps-sign-in
    - https://learn.microsoft.com/en-us/graph/permissions-reference
finding:
    title: An OAuth application identified with id $ClientAppId$ accesed mailboxes through the Graph API.
    entity:
        field: user
        type: user
        score: 50
analytic_story:
    - Office 365 Collection Techniques
    - NOBELIUM Group
asset_type: O365 Tenant
mitre_attack_id:
    - T1114.002
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: threat