Microsoft 365 transport rule
A Microsoft 365 org-wide Exchange transport (mail-flow) rule (New-TransportRule / Set-TransportRule) that blind-copies, forwards, or redirects mail.
What it detects
A Microsoft 365 org-wide Exchange transport (mail-flow) rule (New-TransportRule / Set-TransportRule) that blind-copies, forwards, or redirects mail. Unlike a single inbox rule this affects the whole tenant — attackers use one to silently exfiltrate mail organization-wide or to bypass anti-phishing / safe-links controls.
MITRE ATT&CK mapping
- T1114.003Email Collection: Email Forwarding Rule· Collection
False positives and triage
Journaling, archiving, DLP, and encryption legitimately use transport rules. Confirm the destination is external / unexpected — a rule that blind-copies, redirects, or forwards mail to an outside address is the signal.
The rule
Declarative YAML read by the detection engine — data, never executed. Confidence 70%. Source: rules/cloud/m365_transport_rule.yaml, licensed under the Apache License 2.0.
id: cloud.identity.m365_transport_rule
version: 1.0.0
status: stable
severity: high
confidence: 0.7
platforms: [application]
mitre_techniques: [T1114.003]
description: |
A Microsoft 365 org-wide Exchange transport (mail-flow) rule (New-TransportRule /
Set-TransportRule) that blind-copies, forwards, or redirects mail. Unlike a single
inbox rule this affects the whole tenant — attackers use one to silently exfiltrate
mail organization-wide or to bypass anti-phishing / safe-links controls.
match:
provider: Office365
event_action_any:
- New-TransportRule
- Set-TransportRule
raw_message_contains_any:
- "BlindCopyTo"
- "RedirectMessageTo"
- "AddToRecipients"
- "ForwardTo"
correlation_keys: [user_name]
false_positive_guidance: |
Journaling, archiving, DLP, and encryption legitimately use transport rules.
Confirm the destination is external / unexpected — a rule that blind-copies,
redirects, or forwards mail to an outside address is the signal.
Related detections
Run this rule on your own logs — offline
KIROSEC runs this rule and the rest of the library over Windows, Linux, macOS and cloud audit logs on the analyst's own machine. Every alert links to the events that raised it, and nothing leaves the device.