1. Introduction
The CGI Generic Open Redirection vulnerability allows a web application to redirect users to unintended websites. This can happen when crafted parameters are sent to Common Gateway Interface (CGI) scripts, potentially leading to phishing attacks and credential theft. Systems using CGIs without proper input validation are at risk. A successful attack could compromise the confidentiality of user credentials.
2. Technical Explanation
The vulnerability occurs because CGI scripts do not properly validate or escape arguments passed in URLs. An attacker can manipulate these parameters to redirect users to a malicious website. This redirection is often seamless, making it difficult for users to detect the attack. The Common Weakness Enumeration (CWE) IDs associated with this issue are 601, 722, 801, 819, 928 and 938. For example, an attacker could craft a URL that redirects a user to a fake login page designed to steal their credentials.
- Root cause: Missing or inadequate input validation in CGI scripts when handling redirection parameters.
- Exploit mechanism: An attacker crafts a malicious URL with specially encoded characters in the redirect parameter, causing the application to redirect to an external website controlled by the attacker. Example payload:
http://example.com/cgi-script?redirect=https://evil.com - Scope: Web servers and applications using CGI scripts are affected. Specific versions depend on the implementation of the CGIs themselves.
3. Detection and Assessment
To confirm vulnerability, check for redirect parameters in URLs used by your web application. Thorough assessment involves testing with various encoded characters to see if redirection occurs to unintended websites.
- Quick checks: Examine the configuration of CGI scripts for any hardcoded redirects or default values.
- Scanning: Nessus vulnerability ID 16874 can detect this issue, but results should be verified manually.
- Logs and evidence: Check web server logs for redirect requests originating from CGIs. Look for unusual destination URLs.
# Example command to check CGI script configuration (Linux)
grep -r "redirect=" /path/to/cgi-scripts
4. Solution / Remediation Steps
4.1 Preparation
- Ensure you have a rollback plan to revert to the original scripts in case of issues. A change window may be needed for production systems.
4.2 Implementation
- Step 1: Identify all CGI scripts that handle URL redirection.
- Step 2: Implement input validation on any parameters used in redirect URLs, ensuring they are properly escaped and sanitized.
- Step 3: Test the modified scripts thoroughly to ensure they no longer allow unintended redirects.
- Step 4: Deploy the updated CGI scripts to your web server.
4.3 Config or Code Example
Before
#!/bin/bash
redirect_url=$QUERY_STRING
echo "Location: $redirect_url"
echo "Content-type: text/html"
echo ""
After
#!/bin/bash
redirect_url=$(cgi-escape "$QUERY_STRING") # Escape the URL parameter
echo "Location: $redirect_url"
echo "Content-type: text/html"
echo ""
4.4 Security Practices Relevant to This Vulnerability
Practices like input validation and least privilege can help prevent this issue. Least privilege limits the impact if an attack succeeds, while input validation blocks malicious data from being processed.
- Practice 2: Safe Defaults – Avoid hardcoding default redirect URLs or values that could be exploited.
4.5 Automation (Optional)
Automated scanning tools can help identify vulnerable CGI scripts. However, manual verification is always recommended.
# Example Bash script to scan for potentially vulnerable CGIs
find /path/to/cgi-scripts -name "*.cgi" -exec grep -q "redirect=" {} ; -print
5. Verification / Validation
Confirm the fix by attempting to redirect to a malicious website using crafted URLs. Verify that the application no longer redirects to unintended destinations. Perform a smoke test to ensure core functionality remains intact.
- Post-fix check: Attempt to access
http://example.com/cgi-script?redirect=https://evil.comand confirm it does not redirect. - Re-test: Re-run the Nessus scan (ID 16874) and verify that the vulnerability is no longer detected.
- Monitoring: Monitor web server logs for any unexpected redirect requests or errors related to CGI scripts.
# Example command to check redirection (Linux)
curl -I http://example.com/cgi-script?redirect=https://evil.com | grep Location
6. Preventive Measures and Monitoring
Update security baselines to include input validation requirements for CGI scripts. Implement checks in CI/CD pipelines to prevent vulnerable code from being deployed. A regular patch review cycle is also important.
- Baselines: Update your web server security baseline to require strict input validation for all CGI scripts.
- Asset and patch process: Review and update CGI scripts regularly, applying any necessary patches or security updates.
7. Risks, Side Effects, and Roll Back
- Risk or side effect 1: Incorrectly implemented input validation may prevent valid redirects from working.
- Risk or side effect 2: Changes to CGI scripts could introduce compatibility issues with existing applications.
- Roll back: Restore the original, unpatched CGI scripts from your backup. Restart the web service if necessary.
8. References and Resources
- Vendor advisory or bulletin: N/A – This is a general CGI vulnerability, so specific vendor advisories may not exist.
- NVD or CVE entry: https://nvd.nist.gov/vuln/detail/CVE-2017-8295
- Product or platform documentation relevant to the fix: https://www.owasp.org/index.php/Open_redirect