Detection: O365 Email Transport Rule Changed

Updated Date: 2026-05-13 ID: 11ebb7c2-46bd-41c9-81e1-d0b4b34583a2 Author: Steven Dick Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic identifies when a user with sufficient access to Exchange Online alters the mail flow/transport rule configuration of the organization. Transport rules are a set of rules that can be used by attackers to modify or delete emails based on specific conditions, this activity could indicate an attacker hiding or exfiltrated data.

Search

 1`o365_management_activity` Workload=Exchange AND Operation IN ("Set-*","Disable-*","New-*","Remove-*") AND Operation="*TransportRule"
 2
 3| eval object_name = case('Parameters{}.Name'=="Name",mvindex('Parameters{}.Value',mvfind('Parameters{}.Name',"^Name$")),true(),ObjectId), object_id = case('Parameters{}.Name'=="Identity",mvindex('Parameters{}.Value',mvfind('Parameters{}.Name',"^Identity$")),true(),Id)
 4
 5| stats values(object_name) as object_name, min(_time) as firstTime, max(_time) as lastTime, count by object_id, UserId, Operation, signature
 6
 7| rename UserId as user
 8
 9| `security_content_ctime(firstTime)`
10
11| `security_content_ctime(lastTime)`
12
13| `o365_email_transport_rule_changed_filter`

Data Source

Name	Platform	Sourcetype	Source
Office 365 Universal Audit Log	Other	`'o365:management:activity'`	`'o365'`

Macros Used

Name	Value
security_content_ctime	`convert timeformat="%Y-%m-%dT%H:%M:%S" ctime($field$)`
o365_email_transport_rule_changed_filter	`search *`

o365_email_transport_rule_changed_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
T1114.003	Email Forwarding Rule	Collection
T1564.008	Email Hiding Rules	Stealth

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 Finding (Notable)	No
Creates Intermediate Finding (Risk Event)	Yes

Anomaly detections generate Intermediate Findings (Risk Events). They do not generate a Finding (Notable) directly.

Implementation

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

Known False Positives

Legitimate administrative changes for business needs.

Associated Analytic Story

Intermediate Findings

Message	Entity Field	Entity Type	Risk Score
The user [$user$] altered the exchange transport rule id [$object_name$]	user	user	20

Threat Objects

Field	Type
object_name	signature
object_id	signature

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: 7