How to remediate – Web mirroring

1. Introduction

Web mirroring is when a tool creates a copy of a website on a local system. Nessus identifies this capability, meaning it can map the site’s structure and discover CGI scripts used by the server. This matters because exposed CGIs could allow attackers to run commands on the webserver. Systems running web servers are usually affected. Likely impact is information disclosure if CGIs handle sensitive data, potential integrity compromise if writable CGIs exist, and limited availability disruption through resource exhaustion.

2. Technical Explanation

The vulnerability occurs because Nessus can successfully crawl the website without restriction. This reveals details about its internal structure, specifically the location of CGI scripts. Attackers use this information to identify potential entry points for exploitation. A simple attack involves sending a crafted request to a vulnerable CGI script with malicious input. The plugin extracts CGIs that are used by the remote host.

• Root cause: unrestricted access allowing website crawling and discovery of CGI scripts.
• Exploit mechanism: an attacker identifies a CGI script, then sends a specially crafted request to execute arbitrary code or gain unauthorized access. For example, sending a malicious payload through a vulnerable form handler.
• Scope: Web servers running any operating system that supports CGI scripting are potentially affected. Specific versions aren’t directly relevant as the issue is configuration-based.

3. Detection and Assessment

Confirming vulnerability involves checking if Nessus successfully mapped the website structure. A thorough assessment requires reviewing the discovered CGIs for potential weaknesses.

• Quick checks: Review Nessus scan results for findings related to “Web Mirroring”.
• Scanning: Nessus plugin ID 10423 can identify this issue. This is an example only, as other scanners may also detect it.
• Logs and evidence: Web server access logs might show requests from the scanning IP address during the crawl. Look for unusual activity targeting CGI scripts.

curl -I https://example.com/cgi-bin/script.cgi

4. Solution / Remediation Steps

Fixing this issue involves restricting access to website content and reviewing exposed CGIs.

4.1 Preparation

• Ensure you have documented the current CGI script locations for rollback purposes. A roll back plan involves restoring the original configuration file.
• Changes may require a planned maintenance window and approval from the system owner.

4.2 Implementation

1. Step 1: Configure your web server to restrict access to the CGI directory. For example, using .htaccess files or server configuration settings.
2. Step 2: Review each exposed CGI script for potential vulnerabilities.
3. Step 3: Remove any unnecessary CGIs.

4.3 Config or Code Example

Before

<Directory /var/www/cgi-bin>
    Options Indexes FollowSymLinks
    AllowOverride None
    Require all granted
</Directory>

After

<Directory /var/www/cgi-bin>
    Options -Indexes FollowSymLinks
    AllowOverride None
    Require all denied
</Directory>

4.4 Security Practices Relevant to This Vulnerability

List only practices that directly address this vulnerability type. Use neutral wording and examples instead of fixed advice. For example: least privilege, input validation, safe defaults, secure headers, patch cadence. If a practice does not apply, do not include it.

• Practice 1: Least privilege access to the web server’s CGI directory reduces impact if exploited.
• Practice 2: Input validation on any data passed to CGI scripts prevents malicious code execution.

4.5 Automation (Optional)

# Example Bash script to deny access to CGI directory using .htaccess
echo "Order Deny,Allow" > /var/www/.htaccess
echo "Deny from all" >> /var/www/.htaccess
chmod 644 /var/www/.htaccess # Ensure correct permissions.

5. Verification / Validation

Confirm the fix by verifying that Nessus no longer identifies web mirroring and that access to CGI scripts is blocked.

• Post-fix check: Run a new Nessus scan; the “Web Mirroring” finding should be absent.
• Re-test: Re-run the original Nessus plugin (ID 10423) and confirm it does not detect the vulnerability.
• Monitoring: Check web server logs for any failed attempts to access the restricted CGI directory.

curl -I https://example.com/cgi-bin/script.cgi # Should return a 403 Forbidden error

6. Preventive Measures and Monitoring

Suggest only measures that are relevant to the vulnerability type. Use “for example” to keep advice conditional, not prescriptive.

• Baselines: Update your web server security baseline to include restrictions on CGI directory access. For example, a CIS control related to web server configuration.
• Asset and patch process: Regularly review the list of installed CGIs and remove any unnecessary ones. A quarterly review is sensible.

7. Risks, Side Effects, and Roll Back

• Roll back: Restore the original web server configuration file and restart the service.

8. References and Resources

• Vendor advisory or bulletin: No specific vendor advisory available for this general configuration issue.
• NVD or CVE entry: No specific CVE entry exists for web mirroring itself, but related vulnerabilities in CGI scripts may have entries.
• Product or platform documentation relevant to the fix: Refer to your web server’s documentation on configuring directory access restrictions (e.g., Apache .htaccess documentation).