xalgord Detecting Email Forwarding Rules Attack
When to Use
- When proactively hunting for indicators of detecting email forwarding rules attack in the environment
- After threat intelligence indicates active campaigns using these techniques
- During incident response to scope compromise related to these techniques
- When EDR or SIEM alerts trigger on related indicators
- During periodic security assessments and purple team exercises
Detection Gaps & Validation
- Forwarding hides in three places. Querying only inbox rules (
New-InboxRule) misses mailbox-level forwarding (Set-Mailbox -ForwardingSmtpAddress/-ForwardingAddress) and Exchange transport rules. Hunt all three operations or you cover only a third of the attack surface. - Log source must be on. M365 detection depends on the Unified Audit Log and mailbox auditing. Verify with
Get-AdminAuditLogConfig/Get-Mailbox | fl AuditEnabled; if UAL is off,New-InboxRule/Set-Mailbox/UpdateInboxRulesevents never land. - Evasion patterns: unnamed rules, rules that forward then mark-as-read and move mail to RSS/Deleted Items/Archive (hiding it from the user), client-only OWA rules, and forwarding to a look-alike internal domain.
- No interactive logon: Graph API and malicious OAuth app consent can create rules without a sign-in event — correlate rule creation with AzureAD audit logs and risky/new app consents.
- Validate the rule fires: create a benign inbox rule forwarding to an internal test mailbox and confirm a
New-InboxRule(orSet-Mailbox) entry appears in the Unified Audit Log (allow ~30-60 min for ingestion). - Tune false positives: delegates, helpdesk/shared mailboxes, and vacation auto-forwards are legitimate. Allowlist those and alert specifically on forwarding to external or newly-seen recipient domains.
Prerequisites
- EDR platform with process and network telemetry (CrowdStrike, MDE, SentinelOne)
- SIEM with relevant log data ingested (Splunk, Elastic, Sentinel)
- Sysmon deployed with comprehensive configuration
- Windows Security Event Log forwarding enabled
- Threat intelligence feeds for IOC correlation
Workflow
- Formulate Hypothesis: Define a testable hypothesis based on threat intelligence or ATT&CK gap analysis.
- Identify Data Sources: Determine which logs and telemetry are needed to validate or refute the hypothesis.
- Execute Queries: Run detection queries against SIEM and EDR platforms to collect relevant events.
- Analyze Results: Examine query results for anomalies, correlating across multiple data sources.
- Validate Findings: Distinguish true positives from false positives through contextual analysis.
- Correlate Activity: Link findings to broader attack chains and threat actor TTPs.
- Document and Report: Record findings, update detection rules, and recommend response actions.
Key Concepts
| Concept | Description |
|---|---|
| T1114.003 | Email Forwarding Rule |
| T1114.002 | Remote Email Collection |
| T1098.002 | Additional Email Delegate Permissions |
Tools & Systems
| Tool | Purpose |
|---|---|
| CrowdStrike Falcon | EDR telemetry and threat detection |
| Microsoft Defender for Endpoint | Advanced hunting with KQL |
| Splunk Enterprise | SIEM log analysis with SPL queries |
| Elastic Security | Detection rules and investigation timeline |
| Sysmon | Detailed Windows event monitoring |
| Velociraptor | Endpoint artifact collection and hunting |
| Sigma Rules | Cross-platform detection rule format |
Common Scenarios
- Scenario 1: BEC actor creating forwarding rule to external email
- Scenario 2: Compromised account with rule deleting security alerts
- Scenario 3: Inbox rule forwarding CEO emails to attacker mailbox
- Scenario 4: OAuth app abuse creating transport rules for data collection
Output Format
Hunt ID: TH-DETECT-[DATE]-[SEQ]
Technique: T1114.003
Host: [Hostname]
User: [Account context]
Evidence: [Log entries, process trees, network data]
Risk Level: [Critical/High/Medium/Low]
Confidence: [High/Medium/Low]
Recommended Action: [Containment, investigation, monitoring]