How to remediate – Apache on Windows mod_alias URL Validation Canonicalization CG…

1. Introduction

The Apache on Windows mod_alias URL Validation Canonicalization CG… vulnerability is an information disclosure issue affecting Apache web servers running on Windows. It allows attackers to potentially reveal the source code of CGI scripts if the scripts directory is located within the document root. This could lead to sensitive data exposure and compromise system confidentiality. Systems commonly affected are those running Apache HTTP Server on a Windows operating system with mod_alias enabled, particularly when using CGI scripting. The likely impact is information disclosure affecting confidentiality.

2. Technical Explanation

The vulnerability stems from how Apache handles case-sensitive directory names in URLs when the scripts directory resides within the document root. Requests altering the case of the directory name bypass the mod_cgi handler, treating them as regular file requests and potentially disclosing script source code. The CVE associated with this issue is CVE-2006-4110.

• Root cause: Apache does not correctly canonicalize URLs when the CGI directory is within the document root, leading to incorrect routing of requests.
• Exploit mechanism: An attacker can craft a URL request with a modified case for the CGI directory name, bypassing the cgi-script handler and accessing the script source code directly. For example, if the CGI scripts are in /cgi/, an attacker might try /CGI/.
• Scope: Apache HTTP Server versions running on Windows operating systems where the scripts directory is located within the document root are affected.

3. Detection and Assessment

To confirm vulnerability, check your Apache configuration and file system structure. A quick check involves verifying the location of the scripts alias.

• Quick checks: Use the command httpd -V to view Apache’s version and configuration details. Look for the `Alias` directive defining the scripts directory.
• Scanning: Nessus plugin ID 30497 can detect this vulnerability, but results should be verified manually.
• Logs and evidence: Examine Apache access logs for unusual requests targeting CGI script files directly (e.g., .cgi extensions accessed without proper handling). Look in the error log for related errors.

httpd -V

4. Solution / Remediation Steps

The solution involves reconfiguring Apache to locate the scripts directory outside of the document root.

4.1 Preparation

• Ensure you have administrator privileges to modify the Apache configuration. A roll back plan involves restoring the backed-up configuration file and restarting the service.
• A change window may be required depending on your environment and impact assessment. Approval from a system owner might be needed.

4.2 Implementation

1. Step 1: Open the Apache configuration file (typically httpd.conf).
2. Step 2: Locate the `Alias` directive for the scripts directory.
3. Step 3: Change the alias so that the scripts directory is no longer within the document root. For example, change `/cgi/` to `/var/www/scripts/`.
4. Step 4: Save the configuration file.
5. Step 5: Restart the Apache service for the changes to take effect.

4.3 Config or Code Example

Before

Alias /cgi/ c:/Apache24/cgi-bin/

After

Alias /scripts/ c:/var/www/scripts/

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this type of issue.

• Practice 1: Least privilege – configure Apache with the minimum necessary permissions to reduce potential impact if exploited.
• Practice 2: Secure defaults – avoid using default configurations that may be vulnerable.

4.5 Automation (Optional)

Automation is not directly applicable for this specific fix, as it requires modifying the Apache configuration file which varies between installations.

5. Verification / Validation

Confirm the fix by verifying the new scripts directory location and attempting to access a CGI script through the original vulnerable path.

• Post-fix check: Use httpd -V again to confirm the `Alias` directive has been updated correctly.
• Re-test: Attempt to access a CGI script using the old, vulnerable URL (e.g., /cgi/script.cgi). You should receive a “404 Not Found” error or similar.
• Monitoring: Monitor Apache access logs for any unexpected errors related to script handling.

httpd -V

6. Preventive Measures and Monitoring

Update security baselines and implement regular configuration reviews.

• Baselines: Update your server security baseline to include the correct scripts directory location outside of the document root, following CIS benchmarks or similar standards.
• Pipelines: Implement automated configuration checks in CI/CD pipelines to ensure Apache configurations adhere to security best practices.
• Asset and patch process: Establish a regular patch review cycle for Apache and other server software.

7. Risks, Side Effects, and Roll Back

Changing the scripts directory location may require updating application paths or configuration files that rely on the old path.

• Risk or side effect 1: Application compatibility – ensure applications using CGI scripts are updated to reflect the new directory structure.
• Risk or side effect 2: Service interruption – incorrect configuration can lead to service downtime.
• Roll back: Restore the backed-up Apache configuration file and restart the service.

8. References and Resources

Link only to sources that match this exact vulnerability.

• Vendor advisory or bulletin: https://www.securityfocus.com/archive/1/442882/30/0/threaded
• NVD or CVE entry: /cve/CVE-2006-4110
• Product or platform documentation relevant to the fix: Apache HTTP Server documentation on Alias directives.