O365 New Forwarding Mailflow Rule Created

SPLUNK detection rule by Mauricio Velazco, Splunk · Apache-2.0

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.

name: O365 New Forwarding Mailflow Rule Created
id: 289ed0a1-4c78-4a43-9321-44ea2e089c14
version: 9
creation_date: '2024-04-17'
modification_date: '2026-05-13'
author: Mauricio Velazco, Splunk
status: production
type: TTP
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.
data_source: []
search: "`o365_management_activity` Workload=Exchange Operation=\"New-TransportRule\" | eval match1=mvfind('Parameters{}.Name',\"BlindCopyTo\") | eval match2=mvfind('Parameters{}.Name',\"CopyTo\") | eval match3=mvfind('Parameters{}.Name', \"RedirectMessageTo\") | where match1>= 0 OR match2>= 0 OR match3>=0 | eval ForwardTo=coalesce(BlindCopyTo, CopyTo, RedirectMessageTo) | search ForwardTo!=\"\" | rename UserId as user | fillnull | stats count earliest(_time) as firstTime latest(_time) as lastTime by user, Name, ForwardTo, vendor_account, vendor_product, dest, signature | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `o365_new_forwarding_mailflow_rule_created_filter`"
how_to_implement: 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.
references:
    - https://attack.mitre.org/techniques/T1114/
    - https://learn.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/mail-flow-rules
    - https://learn.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/mail-flow-rule-actions
drilldown_searches:
    - name: View the detection results for - "$user$"
      search: '%original_detection_search% | search  user = "$user$"'
      earliest_offset: $info_min_time$
      latest_offset: $info_max_time$
    - name: View risk events for the last 7 days for - "$user$"
      search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
      earliest_offset: 7d
      latest_offset: "0"
finding:
    title: A new forwarding mailflow rule was created by $user$
    entity:
        field: user
        type: user
        score: 50
analytic_story:
    - Office 365 Collection Techniques
asset_type: O365 Tenant
mitre_attack_id:
    - T1114
product:
    - Splunk Enterprise
    - Splunk Enterprise Security
    - Splunk Cloud
category: cloud
security_domain: audit
tests:
    - name: True Positive Test
      attack_data:
        - data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1114/o365_new_forwarding_mailflow_rule_created/o365_new_forwarding_mailflow_rule_created.log
          sourcetype: o365:management:activity
          source: o365
      test_type: unit

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