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Mail Rules: Security Risks and How to Identify Threats in Your Organization

Email is one of the most critical attack vectors in cybersecurity, and mail rules (also known as email filters, inbox rules, or transport rules) can be exploited to bypass security controls. While mail rules help automate email management, misconfigured, overly permissive, or malicious rules can create serious security risks—including data exfiltration, undetected phishing attacks, and compromised accounts.

In this article, we will explore potential security issues in mail rules, how they can be abused by attackers, and how security teams can review and detect suspicious rules in an organization.

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1. Security Risks in Mail Rules

Mail rules can be exploited in various ways, either by malicious insiders, external attackers, or compromised accounts. Here are some common security risks:

1.1 Automatic External Forwarding

• Risk: A rule that forwards emails to an external email address (e.g., Gmail, Yahoo, or another domain) can be used to exfiltrate sensitive data.
• Attack Scenario: An attacker who gains access to an employee’s account can set a forwarding rule to receive all internal emails without raising suspicion.

1.2 Hiding Security Alerts

• Risk: A rule that automatically deletes, archives, or moves security alerts to another folder can prevent users from seeing warnings about suspicious logins or phishing attempts.
• Attack Scenario: If an account is compromised, an attacker can configure a rule that hides emails from IT security, preventing the victim from receiving password reset warnings.

1.3 Auto-Deletion of Specific Emails

• Risk: A rule that automatically deletes emails matching certain criteria (e.g., emails containing the words "security alert" or "unauthorized login") can be used to suppress warning messages.
• Attack Scenario: After compromising an account, an attacker adds a rule that deletes emails from the security team, preventing the user from noticing unusual login activity.

1.4 Mailbox Monitoring (Silent Copies to Another User)

• Risk: Rules that silently copy or redirect emails to another internal or external mailbox allow attackers to spy on sensitive communications.
• Attack Scenario: An insider threat or compromised administrator account could create a rule that copies emails from executives or HR personnel to another user without their knowledge.

1.5 Phishing or Malicious Redirects

• Risk: A rule that modifies incoming emails (e.g., changing subject lines or replacing links) can be used for phishing or impersonation attacks.
• Attack Scenario: A malicious rule might replace all incoming emails from "CEO@example.com" with "FakeCEO@malicious.com," making phishing emails appear legitimate.

1.6 Bypassing Security Controls

• Risk: If mail rules automatically mark phishing emails as safe, attackers can bypass spam and security filters.
• Attack Scenario: A rule that moves phishing emails to a trusted folder (instead of spam) increases the likelihood of employees falling for scams.

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2. How to Search for Security Issues in Mail Rules

Security teams must actively review mail rules to detect possible threats. Below are key steps to audit and investigate risky configurations:

2.1 Reviewing Mail Rules in Microsoft 365 (Exchange Online)

Admins can use PowerShell to list all mailbox rules and detect suspicious behavior:

powershell
CopyEdit
Get-InboxRule -Mailbox user@company.com | Select Name, Description, Enabled, ForwardTo, DeleteMessage, MoveToFolder

• Look for external forwarding rules (ForwardTo values outside the organization).
• Identify rules that delete messages automatically.
• Detect rules that move security alerts or IT communications to another folder.

2.2 Reviewing Mail Rules in Google Workspace

Google Admins can check for forwarding rules using:

1. Google Admin Console → Apps → Google Workspace → Gmail → Routing.
2. Security Investigation Tool → Search for message forwarding rules in Gmail logs.

2.3 Searching for Suspicious Rule Patterns

When reviewing rules, look for the following red flags:

✅ Forwarding rules to external domains (e.g., john.doe@gmail.com).

✅ Rules that delete or move emails from security teams (security@example.com).

✅ Rules that apply to all incoming messages (too broad).

✅ Rules that modify email content or redirect emails to another user.

2.4 Automating Mail Rule Audits

Organizations can automate rule reviews with SIEM tools like:

• Microsoft Sentinel
• Google Security Command Center
• Splunk

Set alerts for:

🚨 Users enabling automatic forwarding to external addresses.

🚨 Rules deleting security alerts or login notifications.

🚨 Mail rules created after a suspicious sign-in event.

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3. Preventing Malicious Mail Rules

To mitigate risks, organizations should enforce security policies and restrict rule creation:

3.1 Disable External Forwarding (Recommended Best Practice)

• In Microsoft 365, admins can block external email forwarding:

  powershell
  CopyEdit
  Set-TransportConfig -ExternalMailEnabled $false

• In Google Workspace, disable forwarding under Gmail Routing Settings.

3.2 Enable Multi-Factor Authentication (MFA)

• If an attacker gains access to an account, they can create rules without detection. MFA prevents unauthorized logins.

3.3 Regularly Audit Mail Rules

• Schedule reviews for mail rules in high-risk accounts (executives, finance, HR).
• Set up alerts for new rules in Microsoft Defender or Google Security Center.

3.4 Train Users to Recognize Malicious Rules

Educate employees to check their mail rules periodically and report unusual changes.

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Final Thoughts

Mail rules are essential for productivity but can introduce security risks if misused. Attackers often exploit mail rules to exfiltrate data, hide alerts, and manipulate messages. By regularly auditing rules, blocking external forwarding, and setting up automated alerts, organizations can detect and mitigate threats before they cause damage.

🚀 Pro Tip:

If you're an admin, run a scheduled script every week to identify newly created forwarding rules and report them for review.

By staying proactive, organizations can ensure email security remains a strength, not a weakness.

Frequently Asked Questions

Sign-ins

What is an "Impossible Travel" sign-in event?

This occurs when a user logs in from two distant locations within a time period that would be physically impossible. For example:

• 8:00 AM: Sign-in from New York.
• 8:15 AM: Sign-in from London. This indicates possible account compromise or session hijacking.

Why is sign-in monitoring important for security?

Sign-in logs help detect unauthorized access attempts, brute-force attacks, and compromised accounts. Reviewing sign-ins can reveal impossible travel scenarios, MFA bypass attempts, or login anomalies that indicate potential breaches.

How can creating policies help enhance security?

Creating policies allows you to:

• Proactively Manage Threats: Implement rules that automatically respond to unusual or unauthorized activities, helping to mitigate potential risks before they escalate.
• Customize Security Measures: Tailor security settings to fit the unique needs of your organization, ensuring that protective measures align with your specific security goals.
• Ensure Compliance: Maintain compliance with regulatory requirements by enforcing consistent policies that govern data access and user behavior.

Remediations

How to enforce MFA on a user

How to enforce MFA on a user

To enforce Multi-Factor Authentication (MFA) on a user in our platform, follow these steps:

1. Go to Identity Hub.
2. Click on the selected user.
3. Navigate to the Remediations tab.
4. Scroll to the Enforce MFA section.
5. Select the application where you wish to enforce MFA for the user.
6. Click Enforce.

MFA will then be enforced for the user on the selected application.

How to reset a user password

How to reset a user password

To reset a user’s password on our platform, follow these steps:

1. Go to Identity Hub.
2. Click on the selected user.
3. Navigate to the Remediations tab.
4. Scroll to the Reset Password section.
5. Select the integration where you wish to reset the user's password.
6. Click Reset Password.
7. Follow any additional instructions if prompted.

The user’s password will then be reset according to the selected integration’s requirements.

How to Suspend a User

How to suspend a user

To suspend a user on our platform, follow these steps:

1. Go to Identity Hub.
2. Click on the selected user.
3. Navigate to the Remediations tab.
4. Scroll to the Suspend Account section.
5. Select the integrations from which you'd like to suspend the user.
6. Click Suspend.

The user will then be suspended from the chosen integrations.

General

How Rotate Enhances Identity Security

Rotate's Identity Hub offers a comprehensive suite of features designed to strengthen your organization's identity security:

• User and Device Management: Gain visibility into user details, monitor actions, and manage connected devices and applications.
• Security Enforcement: Enforce Multi-Factor Authentication (MFA) and strong password policies, and enroll employees in security awareness training programs.
• Remediation Actions: Quickly respond to threats by suspending accounts, resetting passwords, disconnecting users from all apps and devices, and enforcing MFA.
• Access Monitoring and Anomaly Detection: Track login behaviors with geo-location heatmaps and AI-driven insights to detect risky activities and anomalies, such as impossible travel or suspicious login times and locations.
• Policy Management: Create and enforce security policies with specific actions like alerts or restrictions to address potential threats.

By integrating these capabilities, Rotate's Identity Hub provides a robust framework to protect your organization against identity-related threats.

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