How to remediate – Web Application Information Disclosure

1. Introduction

Web Application Information Disclosure means a web application is revealing details about its internal file paths when it shouldn’t. This can help attackers understand how the application works and find weaknesses to exploit. Systems running custom web applications, or those with poorly configured third-party apps, are most at risk. A successful attack could compromise confidentiality by exposing source code or sensitive data.

2. Technical Explanation

The vulnerability happens when a web application doesn’t properly filter error messages. These messages can include the full path to files on the server. An attacker sends specially crafted requests designed to trigger these errors and reveal the paths. No specific CVE is currently associated with this general issue, but it relates to CWE-201: Information Leakage. For example, an attacker might request a non-existent file to see if the error message shows its location on disk.

• Root cause: Missing or inadequate filtering of error messages generated by the web application.
• Exploit mechanism: An attacker sends requests that intentionally cause errors, hoping the resulting response includes path information. Example payload: requesting a file named ‘../../../../../etc/passwd’ to see if the server reveals its location.
• Scope: Web applications running on various platforms (Linux, Windows) and using languages like PHP, Python, Java, or .NET are affected. Specific versions depend on the application code.

3. Detection and Assessment

You can check for this vulnerability by manually testing web application inputs and looking at error responses. Automated scanning tools can also help.

• Quick checks: Examine web server logs for unusual file requests or errors containing path information.
• Scanning: Burp Suite, OWASP ZAP, and Nessus may identify this issue using signatures related to path disclosure (examples only).
• Logs and evidence: Look in application logs for error messages that include full file paths. Common log locations vary by platform but often reside in /var/log on Linux or C:inetpublogs on Windows.

curl -v 'https://example.com/nonexistentfile' 2>&1 | grep "path"

4. Solution / Remediation Steps

4.1 Preparation

• Ensure you have access to the application’s source code or configuration settings. A roll back plan is to restore the original code/config from your backup.
• Changes should be deployed during a maintenance window with approval from the IT security team.

4.2 Implementation

1. Step 1: Identify all locations in the application code where error messages are generated and displayed.
2. Step 2: Implement filtering logic to remove path information from these error messages. This might involve using regular expressions or string manipulation functions.
3. Step 3: Test the changes thoroughly to ensure that legitimate errors are still handled correctly, but no path information is revealed.

4.3 Config or Code Example

Before

echo "Error: File not found at /var/www/html/sensitive_directory/file.txt"

After

echo "Error: File not found."

4.4 Security Practices Relevant to This Vulnerability

• Practice 1: Least privilege – limit the application’s access to only the files and directories it needs, reducing the impact if path information is leaked.
• Practice 2: Input validation – sanitize all user inputs to prevent attackers from crafting malicious requests that trigger errors.

4.5 Automation (Optional)

If using a configuration management tool like Ansible, you can automate the process of updating error handling code.

# Example Ansible task - replace with your application's specific logic
- name: Filter path information in error messages
  replace:
    path: /var/www/html/error_handling.php
    regexp: 'Error: File not found at (.+)'
    replace: 'Error: File not found.'
  notify: Restart web service

5. Verification / Validation

Confirm the fix by attempting to trigger an error and verifying that no path information is revealed in the response or logs.

• Post-fix check: Run the same curl command as before (curl -v 'https://example.com/nonexistentfile' 2>&1 | grep "path") and confirm it returns no results.
• Re-test: Repeat the manual testing steps from the detection phase to ensure that path information is no longer disclosed.
• Monitoring: Monitor application logs for any new errors containing path information (example query: search for “file not found” AND “path”).

curl -v 'https://example.com/nonexistentfile' 2>&1 | grep "path" # Should return no output

6. Preventive Measures and Monitoring

• Baselines: Update security baselines to include requirements for error message filtering in web applications (for example, CIS control 8).
• Pipelines: Add Static Application Security Testing (SAST) tools to your CI/CD pipeline to identify potential information disclosure vulnerabilities during development.
• Asset and patch process: Review application code changes regularly to ensure that new features or updates do not introduce information disclosure issues. A monthly review cycle is sensible.

7. Risks, Side Effects, and Roll Back

• Risk or side effect 2: Changes to error handling code may introduce unexpected behaviour in the application. Thorough testing is essential.
• Roll back: Restore the original web application code and configuration files from your backup. Restart the web service if it was stopped during implementation.

8. References and Resources

• Vendor advisory or bulletin: Check your web server/application vendor’s security advisories for specific guidance.
• NVD or CVE entry: While no single CVE covers this general issue, search the NVD database for related information leakage vulnerabilities (https://nvd.nist.gov/).
• Product or platform documentation relevant to the fix: Consult your web application framework’s documentation on error handling and security best practices.