Microsoft 365 inbox forwarding rule
A Microsoft 365 mailbox inbox rule (New-InboxRule / Set-InboxRule) that forwards, redirects, or auto-deletes mail.
What it detects
A Microsoft 365 mailbox inbox rule (New-InboxRule / Set-InboxRule) that forwards, redirects, or auto-deletes mail. Attacker-created forwarding rules silently exfiltrate a compromised mailbox's mail and hide replies — a classic post-BEC persistence and email-collection move that survives a password reset.
MITRE ATT&CK mapping
- T1114.003Email Collection: Email Forwarding Rule· Collection
False positives and triage
Users and admins do create legitimate forwarding/redirect rules. Confirm the destination is external / unexpected and correlate with the mailbox owner — a rule forwarding to an outside address the user doesn't control is the signal.
The rule
Declarative YAML read by the detection engine — data, never executed. Confidence 80%. Source: rules/cloud/m365_inbox_forwarding_rule.yaml, licensed under the Apache License 2.0.
id: cloud.identity.m365_inbox_forwarding_rule
version: 1.0.0
status: stable
severity: high
confidence: 0.8
platforms: [application]
mitre_techniques: [T1114.003]
description: |
A Microsoft 365 mailbox inbox rule (New-InboxRule / Set-InboxRule) that forwards,
redirects, or auto-deletes mail. Attacker-created forwarding rules silently
exfiltrate a compromised mailbox's mail and hide replies — a classic post-BEC
persistence and email-collection move that survives a password reset.
match:
provider: Office365
event_action_any:
- New-InboxRule
- Set-InboxRule
raw_message_contains_any:
- "ForwardTo="
- "ForwardAsAttachmentTo="
- "RedirectTo="
- "DeleteMessage=True"
correlation_keys: [user_name]
false_positive_guidance: |
Users and admins do create legitimate forwarding/redirect rules. Confirm the
destination is external / unexpected and correlate with the mailbox owner —
a rule forwarding to an outside address the user doesn't control 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.