1. Introduction
CGI Generic Fragile Parameters Detection is a potential vulnerability where web applications return 500-level HTTP error codes when called with invalid CGI parameters. This indicates a problem processing requests, potentially due to firewall interference, server issues, or code errors. A successful exploit could lead to denial of service and information disclosure. Confidentiality, integrity, and availability may be impacted.
2. Technical Explanation
The vulnerability occurs when a web application returns 5xx error codes upon receiving invalid input through CGI scripts. This can happen if the server or firewall blocks malformed requests, or if the CGI script itself crashes due to improper handling of unexpected data. The Nessus test triggers this by sending crafted requests to CGIs with potentially problematic values.
- Root cause: Improper input validation within CGI scripts leading to errors when processing unexpected parameters.
- Exploit mechanism: An attacker sends a request containing invalid or malformed parameters to a CGI script, causing the server to return an error code (e.g., 500 Internal Server Error). This can be used to identify vulnerable CGIs and potentially disrupt service.
- Scope: Web applications using CGI scripts are affected. Specific versions aren’t directly implicated; it depends on the application’s implementation.
3. Detection and Assessment
Confirming vulnerability involves checking for 5xx errors when interacting with CGIs. A quick check is to access known CGI endpoints with unusual input. Thorough assessment requires analyzing server logs for error codes related to CGI calls.
- Quick checks: Attempt to access common CGI scripts (e.g., /cgi-bin/test.cgi) with invalid parameters in the URL, such as a very long string or special characters.
- Scanning: Nessus vulnerability ID 163879 can identify this issue.
- Logs and evidence: Examine web server logs (e.g., Apache access.log and error.log) for 50x HTTP status codes associated with CGI script requests. Look for entries containing “CGI” or the specific script name.
# Example command placeholder:
# No direct command to confirm exposure, check web server logs as described above.
4. Solution / Remediation Steps
Fixing this issue requires auditing and securing CGI scripts. This involves validating input, handling errors gracefully, and ensuring proper configuration of the web server and any security devices.
4.1 Preparation
- Stop the web service if possible to avoid disrupting live traffic during script modifications.
4.2 Implementation
- Step 1: Identify all CGI scripts used by the application.
- Step 2: Review each script for proper input validation to prevent errors caused by invalid parameters.
- Step 3: Implement error handling within each script to gracefully handle unexpected input and log any errors.
4.3 Config or Code Example
Before
#!/usr/bin/perl
print "Hello, $QUERY_STRING"; # No input validation
After
#!/usr/bin/perl
$query = $ENV{'QUERY_STRING'};
if ($query =~ /^[a-zA-Z0-9]+$/) {
print "Hello, $query"; # Input validation added
} else {
print "Invalid input.";
}
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 2: Error Handling – Implement robust error handling mechanisms to gracefully handle invalid input and log any errors for debugging purposes.
4.5 Automation (Optional)
# No automation script provided as this requires individual script review and modification.
5. Verification / Validation
Confirming the fix involves re-testing with invalid input to ensure no errors are returned. A service smoke test should verify that legitimate requests still function correctly.
- Post-fix check: Access the CGI scripts with invalid parameters again and confirm that a user-friendly error message is displayed instead of a 5xx HTTP status code.
- Re-test: Run the Nessus scan again to verify that the vulnerability is no longer detected.
- Monitoring: Monitor web server logs for any unexpected errors related to CGI scripts.
# Post-fix command and expected output:
# Access /cgi-bin/test.cgi?invalid_param=<> and expect a "Invalid input." message, not a 500 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 security baselines or policies to include requirements for input validation and error handling in CGI scripts.
- Asset and patch process: Regularly review and update CGI scripts as part of a comprehensive asset management and patching process.
7. Risks, Side Effects, and Roll Back
- Risk or side effect 2: Errors in error handling could expose sensitive information. Mitigation: Ensure that error messages do not reveal internal details.
- Roll back: Restore the backed-up CGI scripts and restart the web service.
8. References and Resources
- Vendor advisory or bulletin: No specific vendor advisory available for this general issue.
- NVD or CVE entry: CVE-2018-13379 (example related CGI vulnerability)
- Product or platform documentation relevant to the fix: Consult your web server and programming language documentation for best practices on input validation and error handling.