How to remediate – Sendmail RCPT TO Command Arbitrary File Overwrite

1. Introduction

The Sendmail RCPT TO Command Arbitrary File Overwrite vulnerability allows an attacker to bypass SMTP authentication and potentially overwrite files on a remote server running Sendmail. This is a critical issue because it could lead to complete system compromise, data loss or denial of service. Systems commonly affected are servers running vulnerable versions of the Sendmail mail transfer agent (MTA). Impact: Confidentiality – potential data theft; Integrity – file corruption/modification; Availability – denial of service.

2. Technical Explanation

The vulnerability occurs because Sendmail does not properly validate input when handling the RCPT TO command in SMTP transactions. This allows an attacker to inject arbitrary commands directly into the mail stream, which can be interpreted as file system operations. An attacker needs network access to port 25 (SMTP) on the target server. There is no known CVE associated with this specific test case, but similar vulnerabilities exist and are documented under CWE-94: Improper Control of Generation of Code (‘Code Injection’).

• Root cause: Lack of input validation for the RCPT TO command allows arbitrary commands to be executed.
• Exploit mechanism: An attacker sends a specially crafted SMTP command containing ‘MAIL FROM’ and ‘RCPT TO’ directives designed to write data to an arbitrary file on the server. For example, sending ‘RCPT TO: /tmp/nessus_test’ attempts to create or overwrite this file.
• Scope: Sendmail versions prior to 8.15.2 are likely affected. Specific configurations may also be vulnerable.

3. Detection and Assessment

• Quick checks: Check the Sendmail version using sendmail -v.
• Scanning: Nessus plugin ID 35864 may detect this vulnerability. Other scanners with SMTP protocol testing capabilities might also identify it.

sendmail -v

4. Solution / Remediation Steps

Fixing this vulnerability requires upgrading Sendmail, reconfiguring it to properly validate input, or changing your SMTP Server (MTA).

4.1 Preparation

• Back up the /etc/mail directory and any custom configuration files before making changes. Stop the sendmail service if possible: systemctl stop sendmail.
• Ensure you have a tested rollback plan in case of issues, such as restoring from backup.
• Changes should be made during a scheduled maintenance window with appropriate approval.

4.2 Implementation

1. Step 1: Upgrade Sendmail to version 8.15.2 or later using your distribution’s package manager (e.g., apt update && apt upgrade sendmail on Debian/Ubuntu, or yum update sendmail on CentOS/RHEL).
2. Step 2: If upgrading is not immediately possible, review and tighten Sendmail configuration to enforce strict input validation for RCPT TO commands. This may involve modifying the mcmap file.
3. Step 3: Restart the Sendmail service after applying changes: systemctl start sendmail.

4.3 Config or Code Example

Before

# No specific RCPT TO validation rules in mcmap file

After

FEATURE(relay_control_list)
FEATURE(local_escape_ccs)
FEATURE(safe_pop3_uids)
... other config ...

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this type of vulnerability.

• Practice 1: Least privilege – run Sendmail with the minimum necessary permissions to reduce the impact if exploited.
• Practice 2: Input validation – rigorously validate all user-supplied input, including SMTP commands, to block malicious data.

4.5 Automation (Optional)

# Example Ansible task to upgrade Sendmail
- name: Upgrade Sendmail
  apt:
    name: sendmail
    state: latest
  become: true

5. Verification / Validation

Confirm the fix by re-attempting the exploit and verifying that the test file is not created, or checking server logs for blocked attempts.

• Post-fix check: Run ls -l /tmp/nessus_test. The command should return “No such file or directory”.
• Re-test: Re-run the exploit attempt (sending ‘RCPT TO: /tmp/nessus_test’) and confirm that the file is not created.
• Smoke test: Verify that legitimate email sending and receiving functionality continues to work as expected.
• Monitoring: Monitor mail logs for blocked RCPT TO attempts or any errors related to input validation.

ls -l /tmp/nessus_test

6. Preventive Measures and Monitoring

Update security baselines and implement checks in CI/CD pipelines to prevent similar vulnerabilities.

• Baselines: Update your server security baseline to include the latest Sendmail version or configuration recommendations (e.g., CIS benchmarks).
• Pipelines: Add static analysis tools (SAST) to your CI pipeline to identify potential code injection flaws in custom configurations.
• Asset and patch process: Implement a regular patch review cycle for all servers, including timely application of security updates for Sendmail.

7. Risks, Side Effects, and Roll Back

Upgrading Sendmail may introduce compatibility issues with existing applications or custom configurations.

• Risk or side effect 1: Compatibility issues – test the upgrade in a non-production environment first to identify any potential conflicts.
• Risk or side effect 2: Service interruption – ensure you have a rollback plan in case of unexpected errors during the upgrade process.
• Roll back: 1) Restore the /etc/mail directory from backup. 2) Downgrade Sendmail to the previous version using your distribution’s package manager. 3) Restart the sendmail service.

8. References and Resources

• Vendor advisory or bulletin: Sendmail Security Advisories
• NVD or CVE entry: No specific CVE is associated with this test case, but similar vulnerabilities are documented on the NVD website.
• Product or platform documentation relevant to the fix: Sendmail Documentation