Detection rules › Splunk
O365 Email Suspicious Behavior Alert
The following analytic identifies when one of O365 the built-in security detections for suspicious email behaviors are triggered. These alerts often indicate that an attacker may have compromised a mailbox within the environment. Any detections from built-in Office 365 capabilities should be monitored and responded to appropriately. Certain premium Office 365 capabilities further enhance these detection and response functions.
MITRE ATT&CK coverage
| Tactic | Techniques |
|---|---|
| Collection | T1114.003 Email Collection: Email Forwarding Rule |
Rules detecting the same action
Other rules on this platform that filter on the same API call or operation.
- O365 Email Reported By User Found Malicious (Splunk)
- O365 ZAP Activity Detection (Splunk)
Rule body splunk
name: O365 Email Suspicious Behavior Alert
id: 85c7555a-05af-4322-81aa-76b4ddf52baa
version: 11
creation_date: '2024-04-07'
modification_date: '2026-05-13'
author: Steven Dick
status: production
type: TTP
description: The following analytic identifies when one of O365 the built-in security detections for suspicious email behaviors are triggered. These alerts often indicate that an attacker may have compromised a mailbox within the environment. Any detections from built-in Office 365 capabilities should be monitored and responded to appropriately. Certain premium Office 365 capabilities further enhance these detection and response functions.
data_source:
- Office 365 Universal Audit Log
search: |-
`o365_management_activity` Workload=SecurityComplianceCenter Operation=AlertEntityGenerated Name IN ("Suspicious email sending patterns detected","User restricted from sending email","Suspicious Email Forwarding Activity","Email sending limit exceeded")
| fromjson Data
| rename Name as signature, AlertId as signature_id, ObjectId as user
| fillnull
| stats count min(_time) as firstTime max(_time) as lastTime
BY dest user src
vendor_account vendor_product signature
signature_id
| `security_content_ctime(firstTime)`
| `security_content_ctime(lastTime)`
| `o365_email_suspicious_behavior_alert_filter`
how_to_implement: You must install the Splunk Microsoft Office 365 Add-on and ingest Office 365 management activity events. The alerts must be enabled in the o365 security portal.
known_false_positives: Users emailing for legitimate business purposes that appear suspicious.
references:
- https://learn.microsoft.com/en-us/purview/alert-policies
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: The user $user$ triggered the O365 security alert [$signature$]
entity:
field: user
type: user
score: 50
analytic_story:
- Suspicious Emails
- Office 365 Collection Techniques
- Office 365 Account Takeover
asset_type: O365 Tenant
mitre_attack_id:
- T1114.003
product:
- Splunk Enterprise
- Splunk Enterprise Security
- Splunk Cloud
category: cloud
security_domain: threat
tests:
- name: True Positive Test
attack_data:
- data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1566/o365_various_alerts/o365_various_alerts.log
sourcetype: o365:management:activity
source: o365
test_type: unit
Stages and Predicates
Stage 1: search
`o365_management_activity` Workload=SecurityComplianceCenter Operation=AlertEntityGenerated Name IN ("Suspicious email sending patterns detected","User restricted from sending email","Suspicious Email Forwarding Activity","Email sending limit exceeded")
Stage 2: search
| fromjson Data
Stage 3: rename
| rename Name as signature, AlertId as signature_id, ObjectId as user
Stage 4: fillnull
| fillnull
Stage 5: stats
| stats count min(_time) as firstTime max(_time) as lastTime
BY dest user src
vendor_account vendor_product signature
signature_id
Stage 6: search
| `security_content_ctime(firstTime)`
Stage 7: search
| `security_content_ctime(lastTime)`
Stage 8: search
| `o365_email_suspicious_behavior_alert_filter`
Indicators
Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.
| Field | Kind | Values |
|---|---|---|
Name | in |
|
Operation | eq |
|
Workload | eq |
|
sourcetype | eq |
|
Search terms
Bare-string tokens in the SPL search body. Splunk matches each token against _raw (the untyped raw event text) anywhere it appears, not against a specific field. These don't surface in the Indicators table because they aren't predicates on a known field.
| Stage | Term |
|---|---|
| 2 | fromjson |
| 2 | Data |