1. Introduction
The Web mirroring stub vulnerability occurs when a Nessus scan identifies that a website is being mirrored. This means an attacker could create a copy of your site, potentially exposing sensitive information or using it for malicious purposes like phishing. Websites are commonly affected, especially those with publicly accessible content and CGIs. A successful exploit could lead to data theft, service disruption, or reputational damage.
2. Technical Explanation
This vulnerability isn’t a direct flaw in your systems but indicates reconnaissance activity. Nessus uses a script to create a mirror of the website and identify Common Gateway Interface (CGI) scripts present on the remote host. Attackers use this information to map out potential attack vectors, focusing on vulnerable CGI applications. There is no CVE associated with simply *having* a web mirroring stub; it’s an indicator of exposure. An attacker could exploit identified CGIs by sending crafted requests designed to trigger vulnerabilities within those scripts.
- Root cause: The presence of publicly accessible website content allows for easy mirroring.
- Exploit mechanism: Attackers use tools like `wget` or custom scripts to copy the website and then analyse CGI scripts for weaknesses, such as buffer overflows or command injection vulnerabilities.
- Scope: Any web server hosting a public website is potentially affected.
3. Detection and Assessment
Confirming whether your system is vulnerable involves checking if Nessus has flagged the issue and reviewing accessible content. A quick check is to browse your website yourself, looking for publicly exposed CGIs.
- Quick checks: Use a web browser to visit URLs ending in `.cgi`, `.pl`, or similar extensions on your public website.
- Scanning: Nessus plugin ID 10428 (Web Mirroring Stub) identifies this condition. This is an example only, as scan results can vary based on configuration.
- Logs and evidence: Web server access logs may show requests from the Nessus scanner IP address attempting to crawl your website. Look for patterns of repeated requests across multiple pages.
curl -I https://yourwebsite.com/cgi-bin/script.cgi4. Solution / Remediation Steps
Fixing this issue involves reducing the number of publicly accessible pages and securing any exposed CGIs.
4.1 Preparation
- Ensure you have access to restore the previous version of your website in case of issues. A roll back plan involves restoring from the backup.
- Changes should be made during a scheduled maintenance window with appropriate approval from IT management.
4.2 Implementation
- Step 1: Review all CGI scripts on your server and identify any that are no longer needed.
- Step 2: Remove unnecessary CGI scripts from the web server’s document root or CGI directory.
- Step 3: For remaining CGIs, ensure they have robust input validation to prevent command injection or other attacks.
- Step 4: Configure your web server to restrict access to CGI directories if possible.
4.3 Config or Code Example
Before
# Apache httpd.conf - allowing access to cgi-bin directory
<Directory /var/www/cgi-bin>
AllowOverride All
Options ExecCGI
Require all granted
</Directory>After
# Apache httpd.conf - restricting access to cgi-bin directory
<Directory /var/www/cgi-bin>
AllowOverride None
Options -ExecCGI
Require all denied
</Directory>4.4 Security Practices Relevant to This Vulnerability
Several security practices can help prevent this issue.
- Practice 1: Least privilege – restrict access to sensitive directories and files, limiting the impact of a potential compromise.
4.5 Automation (Optional)
No specific automation script is recommended for this vulnerability, as it requires manual review and configuration changes.
5. Verification / Validation
Confirm the fix by re-running the Nessus scan and verifying that the Web Mirroring Stub issue no longer appears. Also check your website to ensure remaining CGIs function correctly.
- Post-fix check: Re-run the Nessus scan; the plugin ID 10428 should not report any findings.
- Re-test: Browse your website and confirm that no unnecessary CGI scripts are accessible.
- Smoke test: Test key functionality that relies on remaining CGIs to ensure they still work as expected.
- Monitoring: Monitor web server access logs for unusual activity or attempts to access removed CGI scripts.
nessuscli scan your_scan_id --report-format html6. Preventive Measures and Monitoring
Preventing this vulnerability involves ongoing security assessments and configuration management.
- Baselines: Update a web server security baseline to include restrictions on CGI access and removal of unnecessary scripts.
- Asset and patch process: Regularly review website content and remove outdated or unused files, including CGIs.
7. Risks, Side Effects, and Roll Back
Removing necessary CGI scripts could break functionality on your website.
- Risk or side effect 2: Incorrect configuration of web server access controls could inadvertently block legitimate traffic. Mitigation: Carefully review and test any changes to web server configuration files.
8. References and Resources
Links only to sources that match this exact vulnerability.
- Vendor advisory or bulletin: No specific vendor advisory exists for this general issue, as it is a reconnaissance finding.
- NVD or CVE entry: No specific CVE entry exists for the Web Mirroring Stub itself.
- Product or platform documentation relevant to the fix: Apache HTTP Server documentation on CGI configuration: https://httpd.apache.org/docs/2.4/howto/cgi.html