Detection: O365 Email Suspicious Search Behavior

Updated Date: 2025-02-27 ID: 3b6e1d36-6916-4eec-a7d5-bc98953ba595 Author: Steven Dick Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic identifies when Office 365 users search for suspicious keywords or have an excessive number of queries to a mailbox within a limited timeframe. This behavior may indicate that a malicious actor has gained control of a mailbox and is conducting discovery or enumeration activities.

Search

 1`o365_management_activity` Operation=SearchQueryInitiatedExchange
 2
 3| eval command = case(Operation=="SearchQueryPerformed",SearchQueryText,true(),QueryText), UserId = lower(UserId), signature_id = CorrelationId, signature=Operation, src = ClientIP, user = lower(UserId), object_name=case(Operation=="SearchQueryPerformed",'EventData',true(),QuerySource), -time = _time, suspect_terms = case(match(command, `o365_suspect_search_terms_regex`),command,true(),null())
 4
 5| where command != "*" AND command != "(*)" 
 6
 7| bin _time span=1hr
 8
 9| `o365_email_suspicious_search_behavior_filter`
10
11| stats values(ScenarioName) as app, values(object_name) as object_name values(command) as command, values(suspect_terms) as suspect_terms, values(src) as src, dc(suspect_terms) as suspect_terms_count, dc(command) as count, min(-time) as firstTime, max(-time) as lastTime by user,signature,_time
12
13| where count > 20 OR suspect_terms_count >= 2
14
15| `security_content_ctime(firstTime)` 
16
17| `security_content_ctime(lastTime)`

Data Source

Name	Platform	Sourcetype	Source
Office 365 Universal Audit Log	N/A	`'o365:management:activity'`	`'o365'`

Macros Used

Name	Value
o365_management_activity	`sourcetype=o365:management:activity`
o365_email_suspicious_search_behavior_filter	`search *`

o365_email_suspicious_search_behavior_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
T1552	Unsecured Credentials	Credential Access

Exploitation

DE.AE

CIS 10

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 Risk Event	True

This configuration file applies to all detections of type anomaly. These detections will use Risk Based Alerting.

Implementation

You must install the Splunk Microsoft Office 365 Add-on and ingest Office 365 management activity events. You must also enable SearchQueryInitiated category as part of your organizations mailbox audit logging policy. The thresholds and match terms set within the analytic are initial guidelines and should be customized based on the organization's user behavior and risk profile. Security teams are encouraged to adjust these thresholds to optimize the balance between detecting genuine threats and minimizing false positives, ensuring the detection is tailored to their specific environment.

Known False Positives

Users searching excessively or possible false positives related to matching conditions.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message:

The user $user$ searched email suspiciously, $count$ unique terms and $suspect_terms_count$ suspect terms were searched within a limited timeframe.

Risk Object	Risk Object Type	Risk Score	Threat Objects
user	user	35	src

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