How to remediate – OpenFTPD SITE MSG FTP Command Format String

1. Introduction

The OpenFTPD SITE MSG FTP Command Format String vulnerability affects file transfer servers running OpenFTPD, a program used for sharing files. A successful exploit could allow an authenticated attacker to run commands on the server. This impacts confidentiality, integrity and availability of the affected system.

2. Technical Explanation

This vulnerability stems from improper handling of user-supplied data within the SITE MSG command in OpenFTPD. An attacker can send a specially crafted string via the FTP protocol that is interpreted as format string specifiers, potentially allowing them to execute arbitrary code. The Nessus scanner does not actively check for this flaw and may report false positives. CVE-2004-2523 details this issue.

• Root cause: Lack of input validation on the data received through the SITE MSG command.
• Exploit mechanism: An attacker sends a malicious string containing format specifiers (e.g., %s, %x) to the server via FTP. This can overwrite memory and redirect execution flow. For example, sending “SITE MSG %1024x%1024x” could reveal parts of the stack.
• Scope: OpenFTPD versions prior to a specific patch are affected; exact vulnerable ranges are not specified in available documentation.

3. Detection and Assessment

Confirming vulnerability requires checking the running version of OpenFTPD and assessing its configuration. A direct check is preferred, but scanning can provide initial indicators.

• Quick checks: Use `netstat -an | grep ftp` to identify FTP services listening on standard ports (21). Then use a command like `ps aux | grep openftpd` to confirm the process is running and check its version if possible.
• Scanning: Nessus vulnerability ID 10830 may detect this issue, but be aware of potential false positives.
• Logs and evidence: Examine FTP server logs for unusual activity or errors related to the SITE MSG command. Log locations vary by distribution; check `/var/log/syslog` or similar system logs.

netstat -an | grep ftp

4. Solution / Remediation Steps

The recommended solution is to disable the OpenFTPD service, as it’s a known vulnerable application with limited support. If continued use is required, patching or replacement are necessary but outside the scope of this document.

4.1 Preparation

• Ensure alternative file transfer methods are available if disabling OpenFTPD impacts business operations. Roll back plan: restart the OpenFTPD service.
• A change window may be required depending on your organisation’s policies. Approval from a system owner might be needed.

4.2 Implementation

1. Step 1: Stop the OpenFTPD service using `systemctl stop openftpd` (or equivalent command for your operating system).
2. Step 2: Disable the OpenFTPD service from starting automatically on boot with `systemctl disable openftpd`.

4.3 Config or Code Example

Before

# No specific config example available, as the vulnerability is in the application itself.  The service will be running.

After

systemctl stop openftpd
systemctl disable openftpd

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help mitigate this type of vulnerability. Least privilege limits the impact of exploitation, while input validation prevents malicious data from being processed.

• Practice 1: Implement least privilege principles; run services with minimal necessary permissions.
• Practice 2: Enforce strict input validation on all user-supplied data to block potentially harmful characters or commands.

4.5 Automation (Optional)

#!/bin/bash
# Stop and disable OpenFTPD service
systemctl stop openftpd || true # Ignore errors if service is already stopped
systemctl disable openftpd || true # Ignore errors if service is already disabled
echo "OpenFTPD service stopped and disabled."

5. Verification / Validation

Confirm the fix by checking that the OpenFTPD service is no longer running and attempting a connection to the FTP port fails. A smoke test should verify other services are unaffected.

• Post-fix check: Run `systemctl status openftpd`. Expected output: “inactive (dead)”.
• Re-test: Use `netstat -an | grep ftp` again; no processes should be listening on the FTP port.
• Smoke test: Verify other network services are functioning as expected, such as SSH or web servers.
• Monitoring: Monitor system logs for any attempts to start the OpenFTPD service unexpectedly.

systemctl status openftpd

6. Preventive Measures and Monitoring

Regular security baselines and patch management are crucial. Consider adding checks in your CI/CD pipeline to identify vulnerable software.

• Baselines: Update your server baseline or hardening guide to exclude OpenFTPD, or require a specific patched version if continued use is essential.
• Asset and patch process: Implement a regular patch review cycle for all servers and applications.

7. Risks, Side Effects, and Roll Back

Disabling OpenFTPD may disrupt file transfer operations. Ensure alternative solutions are available. Rolling back involves restarting the service.

• Risk or side effect 1: Disruption of existing file transfer workflows. Mitigation: Communicate changes to users and provide alternative methods for file sharing.
• Roll back: Step 1: `systemctl start openftpd`. Step 2: Verify the service is running with `systemctl status openftpd`.

8. References and Resources

• Vendor advisory or bulletin: https://seclists.org/bugtraq/2004/Aug/21
• NVD or CVE entry: CVE-2004-2523
• Product or platform documentation relevant to the fix: https://seclists.org/bugtraq/2004/Jul/361