How to remediate – Multiple Web Server printenv CGI Information Disclosure

1. Introduction

The Multiple Web Server printenv CGI Information Disclosure vulnerability involves a default test script on some web servers revealing sensitive information. This is because the ‘test-cgi’ script returns its environment variables, potentially exposing details like installation paths, server IP addresses and administrator email addresses. Successful exploitation could lead to limited confidentiality compromise of system configuration data.

2. Technical Explanation

The vulnerability exists due to the presence of a default CGI script (‘test-cgi’) that is often left enabled on web servers after installation. This script, when accessed, outputs its environment variables. An attacker can request this script and analyse the output for sensitive information. There are no known CVEs currently associated with this specific issue but similar disclosures exist under CWE-200 (Information Disclosure). An example attack involves simply browsing to http://example.com/cgi-bin/test-cgi.

• Root cause: The ‘test-cgi’ script is included by default and outputs environment variables without restriction.
• Exploit mechanism: An attacker sends an HTTP request to the vulnerable CGI script, retrieving its output which contains sensitive information.
• Scope: Web servers that include the ‘test-cgi’ script by default are affected. This includes Apache, IIS and other web server software.

3. Detection and Assessment

Confirming vulnerability involves checking for the presence of the test CGI script and its ability to return environment variables. A thorough method is to attempt access it directly.

• Quick checks: Check if a test-cgi file exists in the /cgi-bin directory using your web server’s filesystem browser or command line tools.
• Scanning: Nessus plugin ID 10384 may identify this issue, but results should be manually verified.
• Logs and evidence: Web server access logs will show requests to the test-cgi script if it has been accessed. Look for HTTP GET requests targeting /cgi-bin/test-cgi.

ls -l /cgi-bin/test-cgi

4. Solution / Remediation Steps

Removing the printenv CGI script is the primary solution to this vulnerability.

4.1 Preparation

• Ensure you have appropriate permissions to modify files in the /cgi-bin directory. A roll back plan is simply restoring the backed-up configuration.
• A change window may be required depending on your organisation’s policies. Approval from a senior administrator might be needed.

4.2 Implementation

1. Step 1: Delete the ‘test-cgi’ file from the /cgi-bin directory using the command line or filesystem browser.
2. Step 2: Restart your web server service to ensure the changes take effect.

4.3 Config or Code Example

Before

ls -l /cgi-bin/test-cgi

After

ls -l /cgi-bin/test-cgi  # Should return "No such file or directory"

4.4 Security Practices Relevant to This Vulnerability

Several security practices can help prevent this type of issue.

• Practice 1: Least privilege – limit access to sensitive files and directories, reducing the impact if a script is compromised.
• Practice 2: Safe defaults – ensure new installations are configured with secure settings by default, disabling unnecessary scripts or services.

4.5 Automation (Optional)

#!/bin/bash
# Check if test-cgi exists
if [ -f /cgi-bin/test-cgi ]; then
  # Remove the file
  rm /cgi-bin/test-cgi
  echo "Removed test-cgi from /cgi-bin"
else
  echo "test-cgi does not exist in /cgi-bin"
fi

5. Verification / Validation

Confirm the fix by verifying that the ‘test-cgi’ script is no longer accessible and does not return environment variables.

• Post-fix check: Attempt to access http://example.com/cgi-bin/test-cgi in a web browser. The server should return a “404 Not Found” error or similar.
• Re-test: Re-run the `ls -l /cgi-bin/test-cgi` command from the detection phase; it should report that the file does not exist.
• Monitoring: Check web server access logs for any attempts to access /cgi-bin/test-cgi, which should now result in 404 errors.

curl -I http://example.com/cgi-bin/test-cgi # Expected output: HTTP/1.1 404 Not Found

6. Preventive Measures and Monitoring

Several measures can help prevent this type of vulnerability.

• Baselines: Update your web server security baseline to include a check for unnecessary CGI scripts like ‘test-cgi’.
• Pipelines: Incorporate static analysis tools into your CI/CD pipeline to identify potentially insecure files or configurations.
• Asset and patch process: Implement a regular review cycle for web server configuration, ensuring that default scripts are removed or secured.

7. Risks, Side Effects, and Roll Back

Removing ‘test-cgi’ is generally safe but could impact other applications if they depend on it (unlikely).

• Roll back: Restore the backed-up web server configuration to return the ‘test-cgi’ file if necessary.

8. References and Resources

• Vendor advisory or bulletin: Check your web server vendor’s documentation for information on default CGI scripts.
• NVD or CVE entry: While no specific CVE exists, search the NVD database for similar information disclosure vulnerabilities.
• Product or platform documentation relevant to the fix: Refer to your web server’s official documentation for details on managing CGI scripts.