Detection: O365 Email New Inbox Rule Created

Updated Date: 2026-05-13 ID: 449f525a-7b42-47be-96a7-d9724e336c19 Author: Steven Dick Type: Anomaly Product: Splunk Enterprise Security

Description

The following analytic identifies the creation of new email inbox rules in an Office 365 environment. It detects events logged under New-InboxRule and Set-InboxRule operations within the o365_management_activity data source, focusing on parameters that may indicate mail forwarding, removal, or obfuscation. Inbox rule creation is a typical end-user activity however attackers also leverage this technique for multiple reasons.

Search

 1`o365_management_activity` Workload=Exchange AND (Operation=New-InboxRule OR Operation=Set-InboxRule) Parameters{}.Name IN (SoftDeleteMessage,DeleteMessage,ForwardTo,ForwardAsAttachmentTo,RedirectTo,MoveToFolder,CopyToFolder)
 2
 3| eval file_path = mvappend(MoveToFolder,CopyToFolder), recipient=mvappend(ForwardTo, ForwardAsAttachmentTo, RedirectTo), user = lower(UserId), signature = Operation, src = if(match(ClientIP, "^\["), ltrim(mvindex(split(ClientIP, "]:"), 0), "["), mvindex(split(ClientIP,":"),0)), desc = Name, action = 'Parameters{}.Name'
 4
 5
 6| stats values(action) as action, values(src) as src, values(recipient) as recipient, values(file_path) as file_path, count, min(_time) as firstTime, max(_time) as lastTime by user, signature, desc
 7
 8| `security_content_ctime(firstTime)`
 9
10| `security_content_ctime(lastTime)`
11
12| `o365_email_new_inbox_rule_created_filter`

Data Source

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

Macros Used

Name	Value
o365_management_activity	`sourcetype=o365:management:activity`
o365_email_new_inbox_rule_created_filter	`search *`

o365_email_new_inbox_rule_created_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

Users may create email rules for legitimate purposes. Filter as needed.

Associated Analytic Story

Office 365 Collection Techniques

Intermediate Findings

Message	Entity Field	Entity Type	Risk Score
A new email inbox rule was created for $user$	user	user	20

Threat Objects

Field	Type
desc	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: 6