O365 Multiple Mailboxes Accessed via API

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Description

The following analytic is designed to trigger when a high number of Office 365 Exchange mailboxes are accessed via API (Microsoft Graph API or Exchange Web Services) in a short time, hinting at possible unauthorized mass email access. It tracks 'MailItemsAccessed' operations in Exchange, using AppId and regex to identify API interactions. Crucial for SOC teams, this analytic focuses on spotting abnormal access patterns, often signaling data exfiltration or account compromise. Security teams should tailor the threshold — set here to flag over five unique mailboxes accessed within 10 minutes — to align with their environment's norms, ensuring effective detection of potential security incidents while maintaining operational efficiency.

• Type: TTP
• Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
• Datamodel: Web
• Last Updated: 2024-02-01
• Author: Mauricio Velazco, Splunk
• ID: 7cd853e9-d370-412f-965d-a2bcff2a2908

Annotations

ATT&CK

ATT&CK

ID	Technique	Tactic
T1114.002	Remote Email Collection	Collection

Kill Chain Phase

• Exploitation

NIST

• DE.CM

CIS20

• CIS 10

CVE

Search

 `o365_management_activity` Workload=Exchange Operation=MailItemsAccessed AppId=* ClientAppId=* 
| bucket span=10m _time 
| eval matchRegex=if(match(ClientInfoString, "^Client=WebServices;ExchangeWebServices"), 1, 0) 
| search (AppId="00000003-0000-0000-c000-000000000000" OR matchRegex=1) 
| stats values(ClientIPAddress) as src_ip dc(user) as unique_mailboxes values(user) as user by _time ClientAppId ClientInfoString 
| where unique_mailboxes > 5 
| `o365_multiple_mailboxes_accessed_via_api_filter`

Macros

The SPL above uses the following Macros:

• o365_management_activity

o365_multiple_mailboxes_accessed_via_api_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.

Required fields

List of fields required to use this analytic.

• _time
• Workload
• Operation
• AppId
• ClientAppId
• ClientInfoString
• ClientIPAddress
• user

How To Implement

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

Known False Positives

Legitimate applications may access multiple mailboxes via an API. You can filter by the ClientAppId or the CLientIpAddress fields.

Associated Analytic Story

• Office 365 Collection Techniques
• NOBELIUM Group

RBA

Risk Score	Impact	Confidence	Message
42.0	70	60	An Oauth application identified with id $ClientAppId$ accessed multiple mailboxes in a short period of time via an API.

The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.

Reference

• 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
• https://attack.mitre.org/techniques/T1114/002/
• https://www.microsoft.com/en-us/security/blog/2024/01/25/midnight-blizzard-guidance-for-responders-on-nation-state-attack/
• https://learn.microsoft.com/en-us/exchange/client-developer/exchange-web-services/ews-applications-and-the-exchange-architecture

Test Dataset

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

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