analyzing-office365-audit-logs-for-compromise

name	analyzing-office365-audit-logs-for-compromise
description	Parse Office 365 Unified Audit Logs via Microsoft Graph API to detect email forwarding rule creation, inbox delegation, suspicious OAuth app grants, and other indicators of account compromise.
domain	cybersecurity
subdomain	cloud-security
tags	["Office365","Microsoft-Graph","audit-logs","email-compromise","inbox-rules","OAuth","BEC"]
version	1.0
author	mahipal
license	Apache-2.0
nist_csf	["PR.IR-01","ID.AM-08","GV.SC-06","DE.CM-01"]

Analyzing Office 365 Audit Logs for Compromise

Overview

Business Email Compromise (BEC) attacks often leave traces in Office 365 audit logs: suspicious inbox rule creation, email forwarding to external addresses, mailbox delegation changes, and unauthorized OAuth application consent grants. This skill uses the Microsoft Graph API to query the Unified Audit Log, enumerate inbox rules across mailboxes, detect forwarding configurations, and identify compromised account indicators.

When to Use

• When investigating security incidents that require analyzing office365 audit logs for compromise
• When building detection rules or threat hunting queries for this domain
• When SOC analysts need structured procedures for this analysis type
• When validating security monitoring coverage for related attack techniques

Detection Gaps & Validation

• Forwarding hides in more than inbox rules: check New-InboxRule/Set-InboxRule/UpdateInboxRules and mailbox-level Set-Mailbox -ForwardingSmtpAddress/-ForwardingAddress, plus transport rules (New-TransportRule). A clean inbox-rule list does not mean no forwarding.
• Operations attackers use: Add-MailboxPermission/Add-RecipientPermission (delegation), Consent to application / Add app role assignment grant to user (illicit OAuth consent), Add delegated permission grant, and Set-MailboxAuditBypassAssociation (silences mailbox auditing for an identity — a strong tamper signal).
• MailItemsAccessed (access-scoping for BEC blast radius) only logs with E5/Advanced Audit; on E3 you cannot prove which messages were read.
• UAL realities: ingestion latency (minutes to hours) means ago(1h) windows miss late events — pivot on the event's own timestamp; default retention is 90/180 days; verify auditing is even on with Get-AdminAuditLogConfig/Get-Mailbox | fl AuditEnabled before trusting an empty result.
• Validate the query fires: create a benign external-forwarding inbox rule on a test mailbox, then confirm it surfaces via Search-UnifiedAuditLog -Operations New-InboxRule / the Graph auditLogs query; tune FPs by allow-listing approved enterprise apps (AppId) and known delegation service accounts.

Prerequisites