Detection: O365 Multiple Mailboxes Accessed via API

Updated Date: 2024-09-30 ID: 7cd853e9-d370-412f-965d-a2bcff2a2908 Author: Mauricio Velazco, Splunk Type: TTP Product: Splunk Enterprise Security

Description

The following analytic detects when a high number of Office 365 Exchange mailboxes are accessed via API (Microsoft Graph API or Exchange Web Services) within a short timeframe. It leverages 'MailItemsAccessed' operations in Exchange, using AppId and regex to identify API interactions. This activity is significant as it may indicate unauthorized mass email access, potentially signaling data exfiltration or account compromise. If confirmed malicious, attackers could gain access to sensitive information, leading to data breaches and further exploitation of compromised accounts. The threshold is set to flag over five unique mailboxes accessed within 10 minutes, but should be tailored to your environment.

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

Data Source

Name Platform Sourcetype Source
O365 MailItemsAccessed N/A 'o365:management:activity' 'o365'

Macros Used

Name Value
o365_management_activity sourcetype=o365:management:activity
o365_multiple_mailboxes_accessed_via_api_filter search *
o365_multiple_mailboxes_accessed_via_api_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
T1114.002 Remote Email Collection Collection
KillChainPhase.EXPLOITAITON
NistCategory.DE_CM
Cis18Value.CIS_10
APT1
APT28
APT29
Chimera
Dragonfly
FIN4
HAFNIUM
Ke3chang
Kimsuky
Leafminer
Magic Hound
Star Blizzard

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

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

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message Risk Score Impact Confidence
An Oauth application identified with id $ClientAppId$ accessed multiple mailboxes in a short period of time via an API. 42 70 60
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: 4