Back in the early days of the internet, people looked forward to hearing that deep, robotic voice announcing “you’ve got mail!” Today, whether you like it or not, email is fundamental to personal and business communications. In 2022, people sent and received an estimated 333 billion emails daily, with the number expected to increase to 392.5 billion by 2026.
Experiencing a security incident on your email server can interrupt business operations leading to lost revenue. Equally concerning, attackers can use the server to send spam and phishing emails with your domain. To mitigate risk, you should consider these email server security vulnerabilities and best practices.
How does an email server work?
When a person uses an email application, the email server sends and receives the messages using protocols that decide which server processes the request, where the message goes, and how the intended mail client wants it delivered.
Many people use the term mail server for both outgoing and incoming servers. However, the two programs use different protocols:
- Mail transfer agents (MTA): software transmits messages between sender and recipient devices using the outgoing Simple Mail Transfer Protocol (SMTP)
- Mail delivery agents (MDA): software that receives the email from the MTA then delivers it locally to the recipient’s inbox using an incoming mail protocol like Internet Message Access Protocol IMAP) or Post Office Protocol Version 3 (POP3)
When someone sends an email, the message typically goes through the following process:
- MTA uses SMTP to check the Domain Name System (DNS) for the recipient and translate the domain to an IP address.
- MTA queries the mail exchange (MX) records to identify the delivery agent.
- MX records tell the MTA where the message should go.
- MDA retrieves the message from the server.
- MDA delivers the message to the recipient’s email application so it lands in the inbox.
Why is a secure email server important?
Since an email server manages the delivery of email across the network, email server security mitigates risk by protecting the sensitive information that these communications contain. Email remains a common attack vector, with the 2024 Data Breach Investigation Report (DBIR) noting email as the second most prevalent action in breaches.
A secure email server enhances data protection by:
- Encrypting data to make it unusable for anyone with unauthorized access
- Identifying and blocking suspicious emails with filters to reduce phishing attack risk
- Using advanced authentication methods to prevent malicious actors from sending emails using the organization’s domain
- Scanning for malware to quarantine or delete suspicious attachments
- Detecting and blocking the transmission of sensitive information to prevent accidental or intentional sensitive data leakage
What are some common email server vulnerabilities?
Email servers, like any other technology, are prone to vulnerabilities that attackers can exploit to gain unauthorized access to systems, networks, and data.
Software vulnerabilities
Common vulnerabilities and exposures (CVEs) are security weaknesses in the software running on the server. Some examples of vulnerabilities include:
- SMTP injection: improper input validation that enables malicious actors to modify messages content, recipients, or create false sender identity
- Open mail relays: lack of authentication to the server allows malicious actors to send messages from it, often as part of a phishing attack
- Improper validation: failure to sanitize commands that allows malicious actors to forge email headers or manipulate messages
Some recent examples of CVEs that impact emails servers include:
- CVE-2023-42115: Vulnerability on Exim MTA within the SMTP service that remote attackers could use to execute arbitrary code in the context of the service account.
- CVE-2024-21410: Vulnerability on Microsoft Exchange Server that attackers could use to escalate privileges and authenticate as a user.
- CVE-2023-38181: Vulnerability on Microsoft Exchange Server that attackers could use for server spoofing
At the end of March 2024, the German cybersecurity agency, BSI, released research indicating that 12% of the Microsoft Exchange servers in the country are so old that the manufacturer no longer supports security updates and 25% of the email servers are running without recent security patches installed.
Misconfigurations
Since email server implementations can be complex, misconfigurations can create security risks.
For example, some common misconfiguration issues with SMTP include:
- Lack of transport encryption: failure to apply SSL/TLS encryption to SMTP plaintext messages
- Default configurations: failure to appropriately change default settings, like using vendor supplied passwords, failing to apply security patches on installation, or leaving rate limiting and logging disabled
Best Practices for Securing Your Email Server
Email is critical to daily business operations, especially in a distributed work environment. To protect sensitive internal and customer communications, you should consider implementing the following security controls:
- Change all default configurations: Changing default admin passwords mitigates the risk that attackers will use them to gain unauthorized, privileged access to the server.
- Enable Mail Transfer Agent Strict Transport Security (MTA-STS): Encrypting data and verifying the server prevents unauthorized access to data and maintains email communication integrity.
- Implement domain-based message authentication, reporting and conformance (DMARC): Ensuring that only authorized users send emails mitigates risk that malicious actors can spoof the domain and use it to send fraudulent messages.
- Configure Transport Layer Security (TLS) or Secure Sockets Layer (SSL) protocols: Installing an SSL/TLS certificate on the email server enables encryption and authenticates sender identity.
- Implement Sender Policy Framework (SPF): Configuring SPF records for the domain mitigates spoofing risk by specifying email server hostnames or IP addresses.
- Use secure ports for inbound and outbound services: Using port 465 or 587 with SMTPS, port 993 for IMAP, or port 995 for POP3 encrypts inbound and outbound communications.
- Update software and firmware regularly: Scanning email servers for vulnerabilities and monitoring threat intelligence for zero-day attacks make it more difficult for attackers to exploit security weaknesses.
- Implement email server firewalls: Monitoring inbound and outbound traffic helps identify malicious activity like spam, phishing attempts, and malware.
- Set outbound rate limits and size restrictions: Controlling the number of emails sent from the domain helps identify malicious actors potentially using it to send phishing or spamming emails.
- Establish an audit trail: Ensuring that all email servers, email firewalls, and user accounts are appropriately logging activity enables the organization to monitor for abnormal behavior.
Graylog: Threat Detection and Incident Response (TDIR) for Enhanced Email Server Security
With Graylog Security, you can use prebuilt content to map security events to MITRE ATT&CK. By combining Sigma rules and MITRE ATT&CK, you can create high-fidelity alerting rules that enable robust threat detection, lightning-fast investigations, and streamlined threat hunting. For example, with Graylog’s security analytics, you can monitor user activity for anomalous behavior indicating a potential security incident. By mapping this activity to the MITRE ATT&CK Framework, you can detect and investigate adversary attempts at using Valid Accounts to gain Initial Access, mitigating risk by isolating compromised accounts earlier in the attack path and reducing impact.
Graylog’s risk scoring capabilities enable you to streamline your TDIR by aggregating and correlating the severity of the log message and event definitions with the associated asset, reducing alert fatigue and allowing security teams to focus on high-value, high-risk issues.
