O365 New Email Forwarding Rule Created

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

The following analytic identifies the creation of new email forwarding 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 like ForwardTo, ForwardAsAttachmentTo, and RedirectTo. This activity is significant as unauthorized email forwarding can lead to data exfiltration and unauthorized access to sensitive information. If confirmed malicious, attackers could intercept and redirect emails, potentially compromising confidential communications and leading to data breaches.

name: O365 New Email Forwarding Rule Created
id: 68469fd0-1315-44ba-b7e4-e92847bb76d6
version: 10
creation_date: '2024-04-17'
modification_date: '2026-05-13'
author: Mauricio Velazco, Splunk
status: production
type: TTP
description: The following analytic identifies the creation of new email forwarding 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 like ForwardTo, ForwardAsAttachmentTo, and RedirectTo. This activity is significant as unauthorized email forwarding can lead to data exfiltration and unauthorized access to sensitive information. If confirmed malicious, attackers could intercept and redirect emails, potentially compromising confidential communications and leading to data breaches.
data_source: []
search: "`o365_management_activity` (Operation=New-InboxRule OR Operation=set-InboxRule) | eval match1=mvfind('Parameters{}.Name', \"ForwardTo\") | eval match2=mvfind('Parameters{}.Name', \"ForwardAsAttachmentTo\") | eval match3=mvfind('Parameters{}.Name', \"RedirectTo\") | where match1>= 0 OR match2>= 0 OR match3>= 0 | eval ForwardTo=coalesce(ForwardTo, ForwardAsAttachmentTo, RedirectTo) | fillnull | stats count min(_time) as firstTime max(_time) as lastTime values(Name) as Name by signature dest user src vendor_account vendor_product ForwardTo | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)` | `o365_new_email_forwarding_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: Users may create email forwarding rules for legitimate purposes. Filter as needed.
references:
    - https://attack.mitre.org/techniques/T1114/003/
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 forwarding email inbox rule was created for $user$
    entity:
        field: user
        type: user
        score: 50
analytic_story:
    - Office 365 Collection Techniques
asset_type: O365 Tenant
mitre_attack_id:
    - T1114.003
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/T1114.003/o365_email_forwarding_rule_created/o365_email_forwarding_rule_created.log
          sourcetype: o365:management:activity
          source: o365
      test_type: unit

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

07c3892f-894b-4f3a-b4b0-e54c6af918e7

Propose edits, view related rules, and engage with the community through comments.

Executable code, not a declarative rule

This submission is arbitrary [[ currentRule.format ? currentRule.format.toUpperCase() : '' ]] code, not a declarative detection rule — review it as you would any script before running it, and note that it cannot be safely aggregated or auto-executed alongside YARA/Sigma/Suricata/CRS rules.

Rejected pattern present

[[ reason ]]

What does this mean?

Format & documentation suggestions

Not a security issue — this rule is valid and was accepted as-is. These are optional fields/conventions its own format recommends, mainly affecting its quality score and how well it documents itself for other reviewers.

[[ reason ]]

What do these messages mean?

[[ currentRule.title && currentRule.title.length > 60 ? currentRule.title.slice(0, 60) + '…' : currentRule.title ]]

[[ currentRule.title ]]

Community Discussion