How to remediate – WFTPD APPE Command Buffer Overflow

1. Introduction

The WFTPD APPE Command Buffer Overflow is a flaw in the Windows FTP server, WFTPD. This vulnerability allows an attacker to crash the application or potentially run code on the server by sending a specially crafted command. It affects systems running vulnerable versions of WFTPD and could lead to denial of service or compromise of confidentiality, integrity, and availability.

2. Technical Explanation

The vulnerability is caused by insufficient bounds checking when handling the APPE command in WFTPD. An authenticated user, possibly even an anonymous one, can send a long string as part of this command which overflows a buffer on the stack. This overflow can overwrite critical data and lead to application crashes or arbitrary code execution. The vulnerability is tracked as CVE-2006-5826.

• Root cause: Lack of input validation when processing the length of the filename provided in the APPE command.
• Exploit mechanism: An attacker sends a crafted APPE command with an excessively long filename, exceeding the buffer size allocated for it on the stack. This overwrites adjacent memory regions.
• Scope: Windows systems running WFTPD versions prior to a currently unknown patched version are affected.

3. Detection and Assessment

Confirming vulnerability requires identifying the WFTPD version in use. A thorough assessment involves attempting to trigger the overflow, but this is best done in a test environment.

• Quick checks: Use ftp then enter user anonymous password password and finally ver to attempt to retrieve the WFTPD version.
• Scanning: Nessus plugin ID 30861 may detect this vulnerability, but results should be verified.
• Logs and evidence: Look for application crashes or unexpected errors in Windows Event Logs following attempts to use the APPE command with long filenames.

ftp 

4. Solution / Remediation Steps

Currently, a specific patch is not known. Mitigation focuses on disabling WFTPD if possible or restricting access.

4.1 Preparation

• Dependencies: No external dependencies are required for disabling the service. Roll back involves restarting the WFTPD service.
• Change window: A standard change window is recommended, with approval from the IT security team.

4.2 Implementation

1. Step 1: Stop the WFTPD service using the Services control panel (services.msc) or PowerShell command Stop-Service -Name "WFTPD".
2. Step 2: If possible, uninstall WFTPD to remove the vulnerability entirely. Use the Control Panel’s Programs and Features section.

4.3 Config or Code Example

Before

WFTPD service is running.

After

WFTPD service is stopped or uninstalled.

4.4 Security Practices Relevant to This Vulnerability

Practices that reduce the impact of this vulnerability include least privilege and input validation.

• Practice 1: Least privilege – restrict access to the FTP server to only authorized users, limiting the potential attack surface.
• Practice 2: Input validation – implement strict input validation on all data received by the server to prevent malicious payloads from being processed.

4.5 Automation (Optional)

# PowerShell example to stop the WFTPD service on multiple servers
# Requires appropriate permissions
$servers = @("server1", "server2")
foreach ($server in $servers) {
  try {
    Stop-Service -ComputerName $server -Name "WFTPD" -Force
    Write-Host "Stopped WFTPD on $server"
  } catch {
    Write-Host "Failed to stop WFTPD on $server: $($_.Exception.Message)"
  }
}

5. Verification / Validation

Confirm the fix by verifying that the service is stopped or uninstalled and attempting a connection using the APPE command.

• Post-fix check: Run services.msc to confirm WFTPD is not running.
• Re-test: Attempt to connect via FTP with an anonymous user and use the APPE command. The connection should fail or be refused if the service is stopped/uninstalled.
• Monitoring: Monitor Windows Event Logs for any errors related to failed FTP connections.

services.msc

6. Preventive Measures and Monitoring

Update security baselines and consider adding checks in deployment pipelines.

• Baselines: Update your Windows server baseline to include a requirement for disabling or removing unnecessary services like WFTPD.
• Pipelines: Implement static code analysis (SAST) tools during development to identify potential buffer overflow vulnerabilities in custom applications.
• Asset and patch process: Review the list of installed software regularly, identifying and removing unused or vulnerable components.

7. Risks, Side Effects, and Roll Back

Disabling WFTPD may impact users who rely on it for file transfer. Rolling back involves restarting the service.

• Risk or side effect 2: Uninstalling WFTPD may require reconfiguring other applications that depend on it.
• Roll back: Restart the WFTPD service using the Services control panel (services.msc) or PowerShell command Start-Service -Name "WFTPD".

8. References and Resources

• Vendor advisory or bulletin: No specific vendor advisory is available at this time.
• NVD or CVE entry: CVE-2006-5826
• Product or platform documentation relevant to the fix: No specific documentation is available at this time.