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Securing Communication: Best Practices for Email Masking in On-Premises Exchange

Jan 18, 2024
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Users should not share or transmit confidential information, such as personal data, medical information, financial records, proprietary source code, or credit card details, via email without proper authorization and oversight. Notably, this holds particular significance in light of various data protection regulations, such as CCPA and GDPR. Nonetheless, there are instances where employees may accidentally send such information due to inattentiveness, and some may do so intentionally. Therefore, every organization needs to establish comprehensive, system-wide data protection measures that extend to email communication.

One such measure is email masking. It is a long-standing feature, commonly referred to as email aliases, which enhances the privacy and security of your email communication. In particular, this feature generates a randomized email address to obscure your actual email address when setting up an online account. Subsequently, any messages sent to the concealed email address are automatically forwarded to your primary inbox. Notably, the sender remains unaware of the ultimate destination of the email, and they can only determine it if you accidentally respond to a message using your primary account.

Email masking offers dual advantages. First and foremost, it enhances your privacy. Specifically, the more you employ distinct masked email addresses, ideally one for each online account, the more you reduce the potential risks associated with information leakage in the event of a data breach. These masked email addresses cannot be used as login IDs on other websites or for password resets. Furthermore, malicious actors cannot hijack the masked email address as they would with a legitimate account. Essentially, they function solely as forwarding addresses, and they are disposable.


Why Email Masking for your on-premise Exchange email server?

The Federal Bureau of Investigation (FBI) and the Cybersecurity and Infrastructure Security Agency (CISA) have jointly issued reports to caution on-premise Exchange users regarding how malicious cyber actors have been actively employing zero-day exploits to infiltrate on-premises Microsoft Exchange servers. Importantly, these threat actors possess the capability to pilfer credentials and mailbox data, which contain and transmit sensitive data, including financial data, proprietary information, and personally identifiable information (PII).

In addition, an entirely different set of vulnerabilities in Exchange, which were recently discovered and actively exploited, remain unpatched. Researchers have demonstrated that Microsoft's initial efforts to address these flaws were unsuccessful. These vulnerabilities represent the latest in a series of security issues that have afflicted Exchange's code over the years.

Furthermore, even when Microsoft does release patches for Exchange, they often go unimplemented due to the labor-intensive technical process required for installation. Consequently, many who have observed the growing security challenges associated with running an Exchange server are left with a clear message: An Exchange server poses a security risk requiring robust security measures to maintain data privacy and security.

Moreover, the challenge faced by email administrators in maintaining fully patched On-premise Exchange servers can be traced back to the difficulties in installing Exchange updates. Essentially, this complexity emerges from the age of its underlying code and the potential risks linked to modifying interdependent mechanisms within the software, which can disrupt its functionality.

To illustrate, security researcher Kevin Beaumont shared his personal experience on Twitter while updating an On-premise Exchange email server. He documented numerous issues, crashes, and hiccups during the process. Despite the server having received an update just a few months prior, the update process took nearly three hours to complete.

In light of these challenges, email masking for on-premise Exchange servers is necessary to protect sensitive data from unauthorized access. Email masking provides a critical layer of security for businesses. It serves as a safeguard against ever-evolving cyber threats. Besides, in an era where cyber actors frequently exploit vulnerabilities in these servers, the need for enhanced protection cannot be underscored.

Email masking plays a pivotal role by obfuscating email addresses, introducing substantial challenges for malicious actors seeking to launch phishing attacks or compromise sensitive data. This additional security measure serves as a bulwark, ensuring data privacy and defending against unauthorized access to valuable information.


Step-by-step instructions on how to mask emails on your on-premise Exchange 

Step 1: Plan Your Email Masking Strategy

  • Define the purpose of email masking (e.g., privacy, security, spam prevention).
  • Decide on the email alias format and structure (e.g., firstname.lastname@yourdomain.com -> alias@yourdomain.com).
  • Determine which users or groups should have alias email addresses and what their corresponding real email addresses will be.

Step 2: Create Alias Email Addresses

  • Open the Exchange Management Console (EMC) or Exchange Admin Center (EAC), depending on your Exchange Server version.
  • Create alias email addresses for users:
    • For EAC:
      • Navigate to "Recipients" and select "Mailboxes."
      • Locate the user for whom you want to create an alias, then click "Edit."
      • Under the "Email Address" section, add a new alias address, and if necessary, set it as the primary address.

Step 3: Update Email Address Policies

  • Configure email address policies to ensure that new users receive alias addresses as their primary email addresses. Adjust existing policies or create new ones as needed.

Step 4: Configure Outbound Email Address Rewriting

  • Ensure that emails sent from the Exchange Server use alias addresses as the "Reply-To" or "From" addresses to maintain user anonymity.
  • In Exchange Management Shell, use the Set-SenderIDConfig cmdlet to enable alias address rewriting: Set-SenderIDConfig -AliasEnabled $true.

Step 5: DNS Configuration

  • Update your DNS records to properly handle incoming and outgoing email for both real and alias addresses. This typically involves:
    • Setting up MX records to direct incoming emails to your Exchange Server.
    • Configuring SPF (Sender Policy Framework) records to authenticate outbound emails sent from your domain.
    • Implementing DKIM (DomainKeys Identified Mail) records for email authentication and security.

Step 6: Testing and Verification

  • Thoroughly test the setup by sending and receiving emails to and from alias addresses. Verify that emails are correctly routed and the alias addresses appear as intended.

Step 7: Security and Access Control

  • Implement robust security measures to protect alias addresses from unauthorized access or misuse. Consider using strong passwords, multi-factor authentication, and access control policies.

Step 8: Monitoring and Logging

  • Set up monitoring and logging solutions to track email activities, ensuring that you can identify any issues or security breaches quickly.

Step 9: User Training and Education

  • Provide training and education to your users on how to use alias email addresses effectively. Ensure they understand the purpose and limitations of email masking.

Step 10: Backup and Recovery

Establish a robust backup and recovery strategy to safeguard your Exchange Server configuration. Regularly back up configurations and email data to ensure you can recover from unexpected issues.

Step 11: Documentation

Create comprehensive documentation of your email masking setup. Include configurations, policies, and any custom scripts or settings applied. This documentation will be valuable for reference and future maintenance.

Step 12: Regular Maintenance

Continuously monitor and update your email masking solution as needed. Stay current with Exchange Server updates and security patches to maintain a secure and functional system.

Common challenges that may arise

Configuration Errors: Comprises a range of potential mistakes in the setup process, including the following:
  • Incorrect email alias addresses: Creating email alias addresses with errors, such as misspelled or improperly formatted aliases, can result in email routing problems.
  • Email policy misconfigurations: Incorrectly configuring email policies can lead to the unintended assignment of alias addresses or cause issues with email distribution.
  • DNS record issues: Mistakes in setting up MX, SPF, or DKIM records may result in misrouted emails, email deliverability problems, or emails being flagged as spam.

    Compatibility issues: Compatibility challenges may arise when your Exchange Server version does not fully align with specific features or scripts required for email masking. These issues include:
  • Lack of native support: Some Exchange Server versions may lack built-in features for email masking, necessitating the use of custom scripts or third-party tools.
  • Script or plugin compatibility: Custom scripts or plugins used for alias management may not seamlessly integrate with your Exchange Server version, leading to functionality limitations or errors.

    Mail Deliverability: Challenges related to email deliverability can carry significant consequences:
  • SPF misconfigurations: Incorrect Sender Policy Framework (SPF) configurations can result in emails sent from alias addresses being flagged as suspicious or rejected by recipient servers.
  • DKIM problems: Misconfigured DomainKeys Identified Mail (DKIM) records may lead to email authentication failures, resulting in deliverability issues or triggering spam flags.


Overcome challenges with expert assistance

Pulsar Security's experts assume a crucial role in enabling the successful implementation of email masking on on-premises Exchange servers. Firstly, they conduct comprehensive assessments of the existing email infrastructure to pinpoint vulnerabilities and potential points of exposure. This involves gaining an understanding of the organization's specific needs and potential risks, enabling them to recommend customized email masking solutions that align with the company's distinct security requirements.

Secondly, our security professionals take charge of deploying and configuring email masking tools, ensuring their seamless integration with the current Exchange server environment. They establish appropriate policies and guidelines for creating and managing masked email aliases, thereby minimizing the risk of configuration errors. Furthermore, these experts provide training to both IT teams and end-users, ensuring that they comprehend the significance of email masking and possess the necessary skills to use it effectively. Continuous monitoring and maintenance of email masking solutions, along with proactive threat assessments, contribute to an organization's overall security posture. Luckily, experts constantly adapt and refine these measures to address evolving threats and maintain the efficacy of email masking to safeguard sensitive data and thwart cyber attacks.



Corey Belanger

Corey Belanger

Corey is a Security Consultant and leads QA of product development, using his expertise in these dual roles to more effectively test and secure applications, whether while building enterprise applications or while performing penetration tests and vulnerability assessments for customers. An Army veteran with a tour of duty in Afghanistan, Corey has built a post-military career in security while earning Network+, Security+, GIAC Certified Incident Handler, GIAC Python Coder, GIAC Web App Penetration Testing, and GIAC Penetration Tester certifications. Corey is also a BsidesNH organizer and founding member of TechRamp, avenues which he uses to help others build their skills for careers in security and technology. Fun Fact: When not manning a terminal or watching the Bruins, Corey can often be found snowboarding or riding his motorcycle.