Web Application Security Best Practices

Input Validation and Output Encoding

[Critical] Implement server-side input validation for all user inputs OWASP A1
[High] Use parameterized queries or prepared statements to prevent SQL injection
[High] Implement proper output encoding to prevent Cross-Site Scripting (XSS) OWASP A7
[Medium] Validate and sanitize all file uploads
[Medium] Implement Content Security Policy (CSP) headers

Authentication and Session Management

[Critical] Implement strong password policies OWASP A2
[High] Use multi-factor authentication (MFA) for sensitive operations
[High] Implement secure session management (e.g., using HttpOnly and Secure flags for cookies)
[Medium] Implement proper logout functionality
[Medium] Use secure password storage (e.g., bcrypt, Argon2)

Access Control

[Critical] Implement proper authorization checks for all sensitive operations OWASP A5
[High] Use the principle of least privilege for user roles
[High] Implement proper access controls for API endpoints
[Medium] Use re-authentication for sensitive actions (e.g., changing password)
[Medium] Implement proper CORS (Cross-Origin Resource Sharing) policies

Data Protection and Encryption

[Critical] Use HTTPS for all connections OWASP A3
[High] Implement proper key management for encryption
[High] Encrypt sensitive data at rest
[Medium] Use secure protocols (TLS 1.2+) and disable older, insecure protocols
[Medium] Implement proper certificate validation

Security Headers and Configurations

[High] Implement HTTP Strict Transport Security (HSTS)
[High] Set proper X-Frame-Options header to prevent clickjacking
[Medium] Use X-XSS-Protection header
[Medium] Implement proper X-Content-Type-Options header
[Low] Use Referrer-Policy header

Error Handling and Logging

[High] Implement proper error handling to avoid information disclosure
[High] Use centralized logging for security events
[Medium] Implement proper log rotation and retention policies
[Medium] Monitor and alert on suspicious activities
[Low] Implement proper debug mode handling

Third-Party Components and Dependencies

[High] Regularly update third-party libraries and frameworks OWASP A9
[High] Use dependency scanning tools to identify vulnerable components
[Medium] Maintain an inventory of third-party components
[Medium] Verify the integrity of third-party components
[Low] Review licenses of third-party components

Client-Side Security

[High] Implement proper Cross-Site Request Forgery (CSRF) protection OWASP CSRF
[High] Use Subresource Integrity (SRI) for external scripts and stylesheets
[Medium] Implement proper DOM-based XSS prevention
[Medium] Use secure flag for cookies containing sensitive information
[Low] Implement proper client-side input validation (in addition to server-side)

Security Testing and Monitoring

[High] Conduct regular security assessments and penetration testing
[High] Use automated security scanning tools
[Medium] Implement a bug bounty program
[Medium] Use real-time monitoring and alerting for security events
[Low] Conduct regular security training for development teams

Note: This checklist provides a comprehensive overview of web application security best practices.
The specific measures you need to implement may vary depending on your application's architecture, technology stack, and specific requirements.
Always consult with security professionals and stay informed about the latest security trends and threats in web application development.