Detection: O365 New Forwarding Mailflow Rule Created

Updated Date: 2024-05-29 ID: 289ed0a1-4c78-4a43-9321-44ea2e089c14 Author: Mauricio Velazco, Splunk Type: TTP Product: Splunk Enterprise Security

Description

The following analytic detects the creation of new mail flow rules in Office 365 that may redirect or copy emails to unauthorized or external addresses. It leverages Office 365 Management Activity logs, specifically querying for the "New-TransportRule" operation and parameters like "BlindCopyTo", "CopyTo", and "RedirectMessageTo". This activity is significant as it can indicate potential data exfiltration or unauthorized access to sensitive information. If confirmed malicious, attackers could intercept or redirect email communications, leading to data breaches or information leakage.

 1`o365_management_activity` Workload=Exchange Operation="New-TransportRule" 
 2| eval match1=mvfind('Parameters{}.Name', "BlindCopyTo") 
 3| eval match2=mvfind('Parameters{}.Name', "CopyTo") 
 4| eval match3=mvfind('Parameters{}.Name', "RedirectMessageTo") 
 5| where match1>= 0 OR match2>= 0 OR match3>=0 
 6| eval ForwardTo=coalesce(BlindCopyTo, CopyTo, RedirectMessageTo) 
 7| search ForwardTo!=""  
 8| rename UserId as user  
 9| stats count earliest(_time) as firstTime latest(_time) as lastTime by Operation, user, Name, ForwardTo  
10| `security_content_ctime(firstTime)` 
11| `security_content_ctime(lastTime)`   
12| `o365_new_forwarding_mailflow_rule_created_filter`

Data Source

Name Platform Sourcetype Source Supported App
N/A N/A N/A N/A N/A

Macros Used

Name Value
o365_management_activity sourcetype=o365:management:activity
o365_new_forwarding_mailflow_rule_created_filter search *
o365_new_forwarding_mailflow_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 Email Collection Collection
KillChainPhase.EXPLOITAITON
NistCategory.DE_CM
Cis18Value.CIS_10
Magic Hound
Silent Librarian

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

Forwarding mail flow rules may be created for legitimate reasons, filter as needed.

Associated Analytic Story

Risk Based Analytics (RBA)

Risk Message Risk Score Impact Confidence
A new forwarding mailflow rule was created by $user$ 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: 2