How to remediate – Anonymous SMTP Authentication Enabled

1. Introduction

Anonymous SMTP Authentication Enabled allows anyone to send emails through your mail server without a username and password. This can turn your server into an open relay, used for spam or malicious activity. This affects servers running SMTP services, typically Microsoft Exchange, Postfix, Sendmail, or similar. Impact on confidentiality is low; integrity is high due to potential message forgery; availability may be impacted by blacklisting.

2. Technical Explanation

• Root cause: Missing or disabled authentication requirements on the SMTP service.
• Exploit mechanism: An attacker connects to the SMTP port, issues commands like MAIL FROM, RCPT TO, and DATA, sending arbitrary email content.
• Scope: All systems running SMTP services with anonymous authentication enabled (e.g., Microsoft Exchange Server, Postfix, Sendmail).

3. Detection and Assessment

• Quick checks: Use telnet or netcat to connect to port 25 and try sending a test message without authentication.
• Scanning: Nessus plugin ID 30748 can detect this issue, but results should be verified.
• Logs and evidence: Check SMTP service logs for connections originating from unexpected sources without valid credentials. Log file locations vary by platform (e.g., Exchange transport logs).

telnet your.mail.server 25
Trying 192.168.1.10...
Connected to your.mail.server.
EHLO test.example.com
250 your.mail.server Hello test.example.com [your_IP_address]
MAIL FROM: 
250 2.1.0 Sender OK
RCPT TO: 
250 2.1.5 Recipient OK
DATA
250 Go ahead with mail input
Subject: Test Message
This is a test message sent without authentication.
.
250 Ok: queued as ABCDEF123456
QUIT
221 Bye.

4. Solution / Remediation Steps

Fix the issue by requiring authentication for all SMTP connections. Make steps small and testable, ensuring you can roll back if needed.

4.1 Preparation

• Ensure you have a valid authentication method configured (e.g., username/password, TLS certificates). A roll back plan is to restore the original configuration files and restart the service.
• A change window may be needed for production systems; approval from IT security or system owners might be required.

4.2 Implementation

1. Step 1: Configure your SMTP server to require authentication for all incoming connections. This is usually done in the service’s configuration file (e.g., Exchange transport rules, Postfix main.cf).
2. Step 2: Restart the SMTP service to apply the new configuration.
3. Step 3: Verify that unauthenticated relay attempts are now rejected.

4.3 Config or Code Example

Before (Example Postfix main.cf – allowing relay)

# Allow unauthenticated relay from any host
# my_network = 127.0.0.0/8
relayhost =

After (Example Postfix main.cf – requiring authentication)

# Disallow unauthenticated relay
my_network = 127.0.0.0/8
relayhost = [your.smtp.server]
smtpd_authentication_required = yes

4.4 Security Practices Relevant to This Vulnerability

Practices that directly address this vulnerability type include least privilege, secure defaults, and regular configuration reviews. Least privilege limits the impact if an attacker gains access; secure defaults prevent misconfigurations; regular reviews identify issues early.

• Practice 1: Least privilege – limit SMTP service accounts to only necessary permissions.
• Practice 2: Secure Defaults – configure SMTP services with authentication enabled by default.

4.5 Automation (Optional)

If using configuration management tools, automate the SMTP configuration update for consistent enforcement across servers.

# Example Ansible task to ensure smtpd_authentication_required is set in Postfix main.cf
- name: Ensure SMTP authentication is required
  lineinfile:
    path: /etc/postfix/main.cf
    regexp: '^smtpd_authentication_required'
    line: 'smtpd_authentication_required = yes'
    state: present
  notify: Restart Postfix

5. Verification / Validation

• Post-fix check: Use telnet or netcat to connect to port 25 and try sending a test message without authentication; expect a rejection error (e.g., “503 Authentication required”).
• Re-test: Re-run the earlier detection method (telnet attempt) to confirm it now fails.
• Smoke test: Send an email from a valid user account through the server and verify successful delivery.
• Monitoring: Monitor SMTP service logs for authentication failures or unexpected relay attempts. Example query: search for “503 Authentication required” errors.

telnet your.mail.server 25
Trying 192.168.1.10...
Connected to your.mail.server.
EHLO test.example.com
250 your.mail.server Hello test.example.com [your_IP_address]
MAIL FROM: 
503 Authentication required

6. Preventive Measures and Monitoring

Update security baselines to include SMTP authentication requirements, add checks in CI/CD pipelines to prevent misconfigurations, and establish a regular patch or configuration review cycle.

• Baselines: Update your security baseline (e.g., CIS benchmark) to require SMTP authentication.
• Pipelines: Add automated checks in your deployment pipeline to verify that SMTP authentication is enabled.
• Asset and patch process: Implement a regular configuration review cycle (e.g., quarterly) to identify and address misconfigurations.

7. Risks, Side Effects, and Roll Back

Potential risks include disruption of email flow if the configuration is incorrect or authentication methods are unavailable. Roll back by restoring the original SMTP service configuration files.

• Risk or side effect 2: Authentication issues can prevent users from sending emails; ensure authentication methods are working correctly.