1. Introduction
The FTP Privileged Port Bounce Scan vulnerability affects FTP servers, allowing attackers to force them to connect to third-party hosts. This can be used to scan other networks from your infrastructure, potentially hiding the attacker’s origin and disrupting network services. Systems running an FTP server are typically affected. A successful exploit could compromise confidentiality, integrity, and availability of systems on your network.
2. Technical Explanation
The vulnerability occurs because the FTP server does not properly validate user input when handling the PORT command. This allows attackers to specify arbitrary IP addresses and ports for data connections. Attackers can exploit this by sending a malicious PORT command, causing the server to initiate a connection to an attacker-controlled host. CVE-1999-0017 describes this issue.
- Root cause: Lack of input validation on the PORT command allows arbitrary IP addresses and ports to be specified for data connections.
- Exploit mechanism: An attacker sends a malicious PORT command with an IP address they control, causing the FTP server to connect to that address.
- Scope: FTP servers are affected. Specific versions were not identified in the provided context.
3. Detection and Assessment
You can confirm vulnerability by checking your FTP server configuration and monitoring network connections. A quick check is to see if your FTP server allows external PORT commands.
- Quick checks: Check your FTP server’s configuration file for settings related to the PORT command and allowed IP address ranges.
- Scanning: Nessus plugin 126 can detect this vulnerability. This is an example only, other scanners may also be able to identify it.
- Logs and evidence: Look for unusual outbound connections from your FTP server in system logs. Check firewall logs for connections to unexpected IP addresses or ports.
# Example command placeholder:
netstat -an | grep ftp
4. Solution / Remediation Steps
The solution is to apply the workarounds and solutions outlined in the CERT advisory.
4.1 Preparation
- Ensure you have a rollback plan in case of issues. A simple rollback would be to restore the backed-up configuration file and restart the FTP service.
- Consider a change window for this update, especially on production systems. Approval from a system administrator may be required.
4.2 Implementation
- Step 1: Consult the CERT advisory (CA-1997-27) for specific recommendations based on your FTP server software.
- Step 2: Disable or restrict external PORT commands if possible.
- Step 3: Implement firewall rules to block outbound connections from your FTP server to untrusted IP addresses and ports.
4.3 Config or Code Example
Before
#Example FTP server configuration allowing all PORT connections
port-command enable
After
#Example FTP server configuration disabling external PORT commands
port-command disable
4.4 Security Practices Relevant to This Vulnerability
Several security practices can help prevent this vulnerability.
- Practice 1: Least privilege – Limit the permissions of the FTP service account to reduce the potential impact if exploited.
- Practice 2: Input validation – Validate all user input, including commands and parameters, to block malicious requests.
4.5 Automation (Optional)
# Put a short PowerShell, Bash, Ansible, or similar snippet here
5. Verification / Validation
Confirm the fix by checking your FTP server configuration and attempting to exploit the vulnerability.
- Post-fix check: Verify that external PORT commands are disabled in your FTP server’s configuration file.
- Re-test: Attempt to send a malicious PORT command with an IP address you control. The connection should be blocked or rejected.
- Monitoring: Monitor your FTP server logs for any failed connection attempts related to the PORT command.
# Post-fix command and expected output
grep port-command /etc/ftp/ftpd.conf #Expected output should show "port-command disable" or similar
6. Preventive Measures and Monitoring
Update security baselines and implement input validation checks.
- Baselines: Update your FTP server security baseline to include the recommended configuration settings from the CERT advisory.
- Pipelines: Implement static analysis tools in your CI/CD pipeline to identify potential vulnerabilities related to input validation.
- Asset and patch process: Establish a regular patch review cycle for all servers, including FTP servers.
7. Risks, Side Effects, and Roll Back
Disabling external PORT commands may impact some legitimate use cases.
- Roll back: Restore the backed-up FTP server configuration file and restart the service if issues occur.
8. References and Resources
- Vendor advisory or bulletin: https://seclists.org/bugtraq/1995/Jul/46
- NVD or CVE entry: CVE-1999-0017
- Product or platform documentation relevant to the fix: https://www.cert.org/historical/advisories/CA-CA-1997-27.cfm