How to remediate – SMTP Service STARTTLS Command Support

1. Introduction

The SMTP Service STARTTLS Command Support vulnerability means a mail service allows traffic encryption using the ‘STARTTLS’ command. This is not inherently a flaw, but it introduces complexity and potential for downgrade attacks if not managed correctly. Systems affected are typically mail servers running software like Postfix, Sendmail, Exchange, or similar.

2. Technical Explanation

• Root cause: The service allows STARTTLS negotiation without strict enforcement of secure cipher suites or certificate validation.
• Exploit mechanism: An attacker connects to the SMTP server, initiates a connection, requests STARTTLS, and then attempts to negotiate an insecure encryption method. If successful, they can intercept traffic.
• Scope: Mail servers running Postfix, Sendmail, Microsoft Exchange Server, Dovecot, or other mail transfer agents (MTAs) are affected.

3. Detection and Assessment

Confirming a system is vulnerable involves checking if STARTTLS is supported and reviewing the configured cipher suites. A thorough method includes analysing the server’s TLS configuration.

• Quick checks: Use telnet to connect to port 25, send the ‘EHLO’ command, and look for ‘STARTTLS’ in the response.
• Scanning: Nessus plugin ID 10874 can identify STARTTLS support. OpenVAS also has relevant scans. These are examples only.
• Logs and evidence: Check mail server logs for successful STARTTLS negotiations. Look for entries indicating encryption method used.

telnet your_mail_server 25
EHLO example.com
STARTTLS

4. Solution / Remediation Steps

Fixing this issue involves ensuring strong TLS configurations and regularly reviewing certificate validity.

4.1 Preparation

• A change window may be needed for larger environments. Approval from a senior administrator might be required.

4.2 Implementation

1. Step 1: Review your mail server’s TLS configuration file (e.g., main.cf in Postfix).
2. Step 2: Ensure only strong cipher suites are enabled, disabling weak or outdated algorithms like SSLv3 and TLS 1.0.
3. Step 3: Verify the server is configured to use a valid certificate from a trusted Certificate Authority (CA).
4. Step 4: Restart the mail service to apply the changes.

4.3 Config or Code Example

Before

# In Postfix main.cf (example)
smtp_tls_ciphers = all

After

# In Postfix main.cf (example)
smtp_tls_ciphers = HIGH:!aNULL:!MD5
smtp_tls_security_level = may

4.4 Security Practices Relevant to This Vulnerability

List only practices that directly address this vulnerability type. Use neutral wording and examples instead of fixed advice.

• Practice 1: Least privilege for mail server accounts reduces the impact if compromised.
• Practice 2: Regular certificate validation ensures encryption remains effective.

4.5 Automation (Optional)

# Example Ansible task to update Postfix TLS ciphers
- name: Update Postfix TLS ciphers
  lineinfile:
    path: /etc/postfix/main.cf
    regexp: '^smtp_tls_ciphers ='
    line: 'smtp_tls_ciphers = HIGH:!aNULL:!MD5'
  notify: Restart Postfix

5. Verification / Validation

Confirm the fix works by checking the TLS configuration and attempting a connection with a secure cipher suite.

• Post-fix check: Use openssl s_client -connect your_mail_server:25 to verify supported ciphers. Look for strong algorithms in the output.
• Re-test: Re-run the telnet command from Detection and Assessment, ensuring STARTTLS is still supported but only secure ciphers are negotiated.
• Monitoring: Monitor mail server logs for any TLS negotiation errors or attempts to use insecure cipher suites.

openssl s_client -connect your_mail_server:25

6. Preventive Measures and Monitoring

Suggest only measures that are relevant to the vulnerability type.

• Baselines: Update security baselines or policies to require strong TLS configurations for mail servers (for example, CIS control 14).
• Pipelines: Add checks in CI/CD pipelines to validate mail server configuration against a secure baseline.
• Asset and patch process: Implement a regular review cycle for mail server certificates and TLS settings.

7. Risks, Side Effects, and Roll Back

List known risks or service impacts from the change.

• Roll back: Restore the original mail server configuration file from backup. Restart the mail service.

8. References and Resources

Link only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: Check your mail server vendor’s documentation for specific guidance on TLS configuration.
• NVD or CVE entry: No specific CVE is associated with STARTTLS support itself, but related vulnerabilities may exist depending on the implementation.
• Product or platform documentation relevant to the fix: Refer to Postfix documentation (https://www.postfix.org/tls_policy/) for TLS configuration details.