How to remediate – WebWeaver FTP Aborted RETR Command Remote DoS

1. Introduction

The WebWeaver FTP Aborted RETR Command Remote Denial of Service vulnerability allows a remote attacker to crash a vulnerable WebWeaver FTP server by requesting a file that does not exist. This can disrupt service availability, preventing legitimate users from accessing the FTP server. Systems running WebWeaver FTP are typically affected. Impact on confidentiality is low, integrity is low, and availability is high.

2. Technical Explanation

The vulnerability occurs because the WebWeaver FTP server does not handle requests for non-existent files correctly. An attacker can send a RETR command with a file name that doesn’t exist, causing the server to enter an error state and stop responding. This is a remote exploit requiring no prior authentication.

• Root cause: The FTP server lacks proper input validation when processing file requests within the RETR command.
• Exploit mechanism: An attacker connects to the FTP server and sends a ‘RETR’ command followed by a non-existent filename. For example, ‘RETR invalid_file.txt’. This causes the server to attempt retrieval of a nonexistent file, leading to a crash.
• Scope: WebWeaver FTP servers are affected. Specific versions have not been identified in this advisory.

3. Detection and Assessment

Confirming vulnerability requires checking the running version of WebWeaver FTP or attempting to trigger the denial of service.

• Quick checks: Check the WebWeaver FTP server’s banner for version information during connection.
• Scanning: Nessus plugin ID 30865 may detect this vulnerability, but results should be verified manually.
• Logs and evidence: Look for errors or crashes in the FTP server logs following a ‘RETR’ command with an invalid filename. Log file locations vary depending on installation; consult WebWeaver documentation.

ftp 
User anonymous
Password anonymous
retr invalid_file.txt

4. Solution / Remediation Steps

Currently, there is no official patch available for this vulnerability. Mitigation focuses on monitoring and potentially limiting access to the FTP server.

4.1 Preparation

• Dependencies: No dependencies are known, but ensure you have console or remote access in case of issues. Roll back involves restoring from the backup.
• Change window: A standard change window is recommended due to potential service disruption. Approval may be needed depending on internal policies.

4.2 Implementation

1. Step 1: Monitor FTP server logs for errors following RETR commands with invalid filenames.
2. Step 2: Consider restricting access to the FTP server using firewall rules, allowing only trusted clients.

4.3 Config or Code Example

No configuration changes are available at this time.

Before

N/A

After

N/A

4.4 Security Practices Relevant to This Vulnerability

Practices that can help reduce the risk of this type of vulnerability include least privilege and input validation.

• Practice 1: Least privilege – restrict access to the FTP server to only authorized users and systems, minimizing potential impact if exploited.
• Practice 2: Input validation – implement robust input validation on all incoming data, including file names, to prevent malicious requests from being processed.

4.5 Automation (Optional)

No automation is available at this time.

N/A

5. Verification / Validation

• Post-fix check: Monitor FTP server logs for errors following RETR commands with invalid filenames. Expect no crashes if access is restricted.
• Re-test: Attempt the earlier detection method (sending a ‘RETR’ command with an invalid filename) and confirm that it does not crash the server.
• Smoke test: Verify legitimate users can still connect to the FTP server and transfer files successfully.
• Monitoring: Monitor logs for any errors related to file retrieval attempts, indicating potential exploitation attempts.

ftp 
User anonymous
Password anonymous
retr invalid_file.txt
# Check logs for errors; no crash should occur.

6. Preventive Measures and Monitoring

• Baselines: Update your security baseline or policy to require regular review of FTP server configurations and logs.
• Pipelines: Implement static analysis tools (SAST) to scan configuration files for potential vulnerabilities, such as missing input validation rules.
• Asset and patch process: Establish a regular patch review cycle for all servers, including WebWeaver FTP, to ensure timely application of security updates when available.

7. Risks, Side Effects, and Roll Back

Restricting access may impact legitimate users. Rolling back involves restoring the original firewall rules or configuration settings.

• Risk or side effect 1: Restricting access could prevent legitimate users from accessing the FTP server. Mitigation is to carefully configure firewall rules and monitor for false positives.
• Roll back: Restore the previous firewall configuration or revert any changes made to the FTP server settings.

8. References and Resources

Links related to this specific vulnerability are provided below.

• Vendor advisory or bulletin: http://www.securityfocus.com/bid/7425
• NVD or CVE entry: No CVE is currently associated with this vulnerability.
• Product or platform documentation relevant to the fix: Consult WebWeaver FTP documentation for configuration options and security best practices.