With the rise of web applications and the ever-expanding cyber-threat landscape, the need for secure apps has never been higher. Cyber attacks are not just a threat to large corporations; they target businesses of all sizes. Whether you're a start-up or an established enterprise, ensuring the security of your web application is paramount.

This guide provides a comprehensive checklist for web application security testing. By following this guide, you'll be well on your way to making your app virtually bulletproof against common security threats.

Table of Contents:

1. Introduction
2. Authentication and Session Management
3. Input Validation
4. Output Encoding
5. Error Handling and Logging
6. Data Protection
7. Business Logic
8. Web Services
9. Miscellaneous Security Controls
10. Conclusion

Introduction

Web application security testing is essential to ensure that your app is free from vulnerabilities that might be exploited by attackers. This guide provides a checklist that covers multiple security domains, ensuring a thorough examination of potential weak points.

Authentication and Session Management

1. Passwords: Ensure that passwords are stored securely, using modern hashing techniques like bcrypt, scrypt, or Argon2. Avoid older hashing algorithms like MD5 or SHA-1.
2. Account Lockout: Implement account lockout mechanisms to prevent brute force attacks. After a certain number of failed login attempts, lock the account for a predefined duration.
3. Session Timeout: Implement session timeouts to ensure that inactive user sessions are terminated after a set period.
4. Session IDs: Ensure session IDs are generated randomly and are hard to predict. They should also be transmitted securely, ideally over HTTPS.
5. Multi-Factor Authentication (MFA): If possible, implement MFA to add an extra layer of security.

Input Validation

1. Whitelisting: Always whitelist input rather than blacklisting. This means only accepting predefined, known-good input.
2. SQL Injection: Ensure all database queries are parameterized to prevent SQL injection attacks.
3. Cross-Site Scripting (XSS): Validate and sanitize all input to prevent malicious scripts from being executed in the user's browser.

Output Encoding

1. HTML Entities: Encode all output that is rendered in HTML to ensure that it's displayed as data and not executed as code.
2. Content Security Policy (CSP): Implement a strict CSP to prevent unauthorized scripts from running.

Error Handling and Logging

1. Informative Errors: Ensure that error messages are generic and don't disclose sensitive information about the system.
2. Logging: Log all security-relevant information, like failed login attempts. Ensure that logs are protected and cannot be tampered with.
3. Monitoring: Regularly monitor logs for any suspicious activity.

Data Protection

1. Data Encryption: Encrypt sensitive data both in transit (using protocols like HTTPS) and at rest (using algorithms like AES).
2. Backups: Regularly backup important data and ensure that backups are encrypted and stored securely.
3. Access Control: Implement strict access controls to ensure that only authorized personnel can access sensitive data.

Business Logic

1. Rate Limiting: Implement rate limiting to prevent abuse of the application's functionality.
2. Resource Limits: Set limits on the amount of resources a user can request or consume.
3. Logic Flaws: Test for logical flaws that might allow users to bypass security controls or gain unauthorized access.

Web Services

1. API Security: Ensure that APIs are protected using authentication and authorization mechanisms.
2. Rate Limiting: Implement rate limiting on API requests to prevent abuse.
3. Data Validation: Just like with web applications, ensure all input to the API is validated.

Miscellaneous Security Controls

1. Cross-Site Request Forgery (CSRF): Implement anti-CSRF tokens to prevent unauthorized actions on behalf of logged-in users.
2. Clickjacking: Use security headers like X-Frame-Options to prevent your site from being embedded in an iframe.
3. Secure Headers: Implement HTTP headers like Strict-Transport-Security to enhance the security of the application.
4. Third-Party Libraries: Regularly update all third-party libraries and components to ensure they are free from known vulnerabilities.
5. CORS Policies: If your application uses Cross-Origin Resource Sharing, ensure that it's configured securely.

Conclusion

Security is not a one-time task; it's an ongoing process. By following this comprehensive checklist, you can ensure that your web application is robust against a myriad of threats. However, always stay updated with the latest security best practices and regularly test your application for any new vulnerabilities.