How to remediate – Sendmail Service Detection

1. Introduction

The Sendmail Service Detection vulnerability means someone can find out what version of Sendmail email software is running on a server. This isn’t directly damaging, but knowing the version helps attackers look for known weaknesses in that specific release. Systems running Sendmail are usually affected, especially those publicly accessible or handling sensitive emails. A successful exploit could lead to information disclosure and potentially allow an attacker to gain control of the system.

2. Technical Explanation

The vulnerability occurs because Sendmail responds to connection attempts with a banner that includes its version number. An attacker can simply connect to the service to read this information. There is no CVE associated with just detecting the version, but specific versions may have known vulnerabilities. For example, an attacker could use `telnet` or `netcat` to connect to port 25 and view the banner. Affected systems include those running Sendmail versions prior to security updates addressing banner hiding.

• Root cause: The Sendmail service includes a version string in its connection banner by default.
• Exploit mechanism: An attacker connects to the SMTP port (usually 25) and reads the banner message. For example, using `telnet 25`.
• Scope: All systems running Sendmail are potentially affected, regardless of platform. Specific versions with known vulnerabilities are at higher risk.

3. Detection and Assessment

You can confirm if a system is vulnerable by checking for the presence of the Sendmail service and its version number. A quick check involves connecting to port 25. More thorough methods include network scanning tools.

• Quick checks: Use `telnet 25` and look for a banner containing “Sendmail”.
• Scanning: Nessus plugin ID 30819 or OpenVAS script smtp-version can identify Sendmail versions. These are examples only, results may vary.
• Logs and evidence: Check system logs for connections to port 25. The banner message might be logged depending on the logging configuration.

telnet  25
Trying ... Connected to .
Escape character is '^]'.
220  ESMTP Sendmail 8.14.7/8.13.8
quit
221 1: Goodbye

4. Solution / Remediation Steps

The best way to fix this is to configure Sendmail to hide its version number in the banner. This reduces information available to potential attackers.

4.1 Preparation

• Take a backup of your Sendmail configuration file (usually `/etc/mail/sendmail.cf` or similar). Stop the Sendmail service if possible, depending on business needs.
• Ensure you have access to edit the Sendmail configuration file with root privileges. A roll back plan is to restore the original configuration file and restart the service.
• A change window may be needed for planned downtime. Approval from a senior system administrator might be required.

4.2 Implementation

1. Step 1: Edit the Sendmail configuration file (e.g., `/etc/mail/sendmail.cf`).
2. Step 2: Add or modify the line `define(`confSMTP_banner’, ‘ ‘)`. This sets an empty banner string.
3. Step 3: Save the changes to the configuration file.
4. Step 4: Restart the Sendmail service (e.g., `systemctl restart sendmail`).

4.3 Config or Code Example

Before

define(`confSMTP_banner', `${hostname} ESMTP Sendmail ${version}')

After

define(`confSMTP_banner', ' ')

4.4 Security Practices Relevant to This Vulnerability

Least privilege can limit the impact if an attacker gains access. Regularly patching Sendmail ensures you have the latest security fixes. Input validation is less directly relevant here, but important for overall email security.

• Practice 1: Least privilege – run Sendmail with minimal necessary permissions to reduce potential damage from compromise.
• Practice 2: Patch cadence – regularly update Sendmail to the latest version to address known vulnerabilities.

4.5 Automation (Optional)

Ansible can automate configuration changes. Be careful when modifying system files automatically.

---
- hosts: mailservers
  tasks:
    - lineinfile:
        path: /etc/mail/sendmail.cf
        regexp: 'define(`confSMTP_banner', .*')
        line: define(`confSMTP_banner', ' ')
      become: true
    - service:
        name: sendmail
        state: restarted
      become: true

5. Verification / Validation

Confirm the fix by checking that the Sendmail banner no longer displays the version number. Re-run the earlier detection method to verify. Perform a basic service smoke test.

• Post-fix check: Use `telnet 25`. The output should *not* contain “Sendmail” or any version information.
• Re-test: Re-run the `telnet` command from step 3 in Detection and Assessment. No version information should be displayed.
• Smoke test: Send a test email to confirm that email sending and receiving still work as expected.

telnet  25
Trying ... Connected to .
Escape character is '^]'.
220  ESMTP 
quit
221 1: Goodbye

6. Preventive Measures and Monitoring

Update your security baseline to include a requirement for hiding the Sendmail version number. Consider adding checks in your CI/CD pipeline to validate configuration settings. A regular patch review cycle is essential.

• Baselines: Update your server hardening baseline or policy to require an empty `confSMTP_banner` setting in Sendmail.
• Asset and patch process: Implement a regular patch review cycle for all servers, including Sendmail.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 1: Incorrect configuration may prevent Sendmail from starting. Mitigation is to restore the original configuration file.
• Roll back:
  1. Stop the Sendmail service.
  2. Restore the original `sendmail.cf` file from your backup.
  3. Restart the Sendmail service.

8. References and Resources

• Vendor advisory or bulletin: http://www.sendmail.org/