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Email remains a fundamental mode of correspondence in enterprise environments. However, as organizations adopt cloud-based messaging platforms and hybrid infrastructures, the threat landscape has become increasingly complex. Among the nuanced dangers is a subtle but impactful vector—rule-based relay attacks. These incursions leverage legitimate mailbox rule capabilities to manipulate, redirect, or exfiltrate sensitive communications under the radar. Often missed by conventional threat detection mechanisms, these attacks demand both awareness and proactive strategy.

What is a Rule-Based Relay Attack?

At its core, a rule-based relay attack involves the abuse of inbox automation features to route data—often confidential—to unintended recipients. After compromising account access, adversaries create rules that perform tasks such as forwarding specific messages, redirecting based on subject lines, or deleting certain types of correspondence. These configurations typically appear benign in interface audits, making their discovery difficult without intentional inspection.

Unlike phishing or ransomware, this tactic thrives in silence. There’s no overt disruption. There’s no alarming payload. It’s a quiet siphoning of valuable content—sometimes spanning weeks or months—used for reconnaissance, financial fraud, or competitive intelligence gathering.

Why These Attacks Are So Effective

1. Abuse of Trust

Most email platforms, including Microsoft 365 and Google Workspace, permit users to configure rules for productivity. Adversaries exploit this trust boundary, understanding that security teams rarely scrutinize personal automation unless something breaks.

2. Persistent Exfiltration

Once a rule is configured, it often continues to operate indefinitely unless deliberately removed. Even after a password reset, rule persistence allows ongoing access to vital communications.

3. Low Visibility

Standard security tools often don’t log rule creation events in detail, and endpoint detection solutions typically ignore email client behaviors unless file execution or command-line activity occurs.

4. Non-Invasive Footprint

Unlike malware, which interacts with system processes or triggers antivirus responses, these rules merely use sanctioned functionality. That makes detection substantially more difficult using signature-based or heuristic scanning.

Common Tactics Observed

• Auto-forwarding to foreign domains — forwarding internal conversations to attacker-controlled inboxes.
• Subject-based sorting — rules filtering keywords like “invoice,” “payment,” or “wire” to isolate financial data.
• Inbox silencing — deleting or archiving responses to prevent detection by the account owner.
• Delayed actions — time-based triggers that only activate rules during specific periods (e.g., weekends or off-hours).
• Spoofed auto-replies — mimicking legitimate autoresponders to intercept follow-up emails.

Mitigation Strategy: Comprehensive Risk Reduction

Effective protection against these covert mechanisms requires both policy enforcement and technical safeguards. Below are layered countermeasures aligned with zero-trust principles.

1. Enforce External Forwarding Restrictions

Limiting the ability to forward messages outside the organization’s domain is essential. In Microsoft environments, this involves modifying remote domain properties or using transport rules to restrict flow paths.

2. Centralized Rule Auditing

Implement regular, automated audits of user-configured rules. This can be performed via scripting or through security monitoring tools with integration to Exchange Online or Gmail APIs. Pay attention to:

• Forwarding rules with external recipients
• Redirects to non-company addresses
• Filtering based on financial or credential-related keywords
• Multiple rules created in quick succession

3. Enable Mailbox Activity Logging

By default, many cloud systems don’t track detailed mailbox operations unless auditing is explicitly enabled. Turn on comprehensive logging for rule modifications, configuration changes, and send/receive behavior.

Use solutions that aggregate and normalize this data, making it easier to detect anomalies and trends.

4. Leverage Behavioral Analytics

Security information and event management (SIEM) systems that incorporate behavioral models can flag users acting outside their normal parameters. A sudden burst of rule creation or multiple outbound messages to unusual recipients should trigger investigation.

5. Restrict Rule Creation by Policy

In higher-risk departments (e.g., finance, legal), consider limiting the ability to create inbox rules through administrative control or by policy. This may be implemented using Conditional Access or advanced role-based access schemes.

6. Deploy Data Loss Prevention (DLP) Rules

Create policies that intercept outbound messages containing sensitive terms (SSNs, account numbers, trade secrets) if forwarding is involved. DLP solutions can scan both subject lines and message content before transmission.

7. Simulated Attack Campaigns

Regularly run controlled red team exercises or simulated phishing campaigns to observe how users respond to compromise attempts. Use results to refine education and response procedures.

Monitoring: Indicators of Abuse

Timely detection depends on vigilance across multiple telemetry sources. Below are critical signs to monitor:

• Sudden or unexplained forwarding rules
• Unexpected logins from international IPs followed by rule configuration
• Messages sent during off-peak hours with external CC/BCC recipients
• Mailbox delegations to unknown entities
• Login failures followed by password resets and forwarding changes

Incident Response Workflow

Step 1: Isolate the Account

Immediately disable sign-in or require a password reset. Avoid deleting mailbox data before forensic preservation.

Step 2: Review All Rules

Enumerate and document existing inbox rules. Remove anything unfamiliar or forwarding externally.

Step 3: Audit Recent Logins

Determine when the intrusion began and what systems were accessed. Use IP tracing and device fingerprints.

Step 4: Preserve Logs

Export mailbox audit logs and any relevant system activity for evidence retention. Avoid overwriting timestamps.

Step 5: Notify Affected Stakeholders

If messages were rerouted to competitors or malicious third parties, alert impacted users and leadership immediately.

Step 6: Harden Mailbox Configuration

Reinforce account settings: enable MFA, disable legacy authentication, and set forward-only restrictions.

Step 7: Conduct Root Cause Analysis

Determine how credentials were acquired: phishing, password reuse, or brute force. Implement remediation based on findings.

Tools to Assist Detection and Prevention

Tool / Platform	Utility
Microsoft Defender for Office 365	Rule creation alerts, anomaly detection
Azure Sentinel	Log ingestion, SIEM correlation
PowerShell	Manual rule enumeration and audit
Wazuh / OSSEC	Log analysis and SIEM-style alerting for cloud logs
Google Admin Console	Audit forwarding rules and apply restrictions
Okta / Duo	Enforce strong multifactor authentication
Splunk	Behavioral analysis and mail rule tracking

Strengthening Security Culture

Technology alone is insufficient. Educating users about subtle threats like rule abuse empowers them to recognize when something feels off. Train users to:

• Periodically check their own inbox rules
• Recognize unauthorized changes to mail behavior
• Report delays or missing emails immediately
• Use strong, unique passwords and MFA

Regular newsletters, lunch-and-learn sessions, and policy reminders help reinforce this awareness.

Case Study: Quiet Espionage via Forwarding Rule

In a real-world example, a mid-sized engineering firm discovered that their CFO’s inbox had been compromised for several weeks. A rule forwarded every message with the word “quote,” “invoice,” or “RFP” to a ProtonMail address. The attacker harvested project pricing, then underbid them in a competitive government contract. There were no signs of malware, no user complaints—just a quiet relay operating within authorized features.

Post-mortem analysis revealed that initial access came via credential reuse. A previous account breach in an unrelated service provided the attacker with valid login credentials. MFA had not yet been rolled out. Rule creation occurred less than 10 minutes after initial login from a foreign IP.

Long-Term Recommendations

• Zero-Trust Architecture: Assume no internal system is immune to misuse. Verify every access attempt based on user, location, device, and behavior.
• Continuous Policy Review: Periodically reevaluate what is allowed by default and whether those defaults align with risk posture.
• Integration of Cloud Access Security Broker (CASB): Use CASB tools to monitor cloud-based mail traffic, apply inline policies, and alert on violations.
• Board-Level Engagement: Ensure executive leadership understands the implications of stealth attacks and funds the appropriate defense efforts.

Final Thoughts

Rule-based relay attacks exemplify how legitimate features can be repurposed for malicious gain. These quiet operations bypass traditional malware signatures and blend in with ordinary user behavior. That makes them exceptionally dangerous in the modern workplace, where information velocity is high and oversight often lags behind usage.

By aligning technical controls with behavioral analysis and user education, organizations can minimize exposure and maintain the integrity of their most vital communication channel—email.