Building Validation Libraries and Reusable Components

Create centralized validation libraries that can be reused across your application. This ensures consistency and makes security updates easier. Design validators to be composable, allowing complex validation rules to be built from simpler components. For example, a username validator might compose length validation, character set validation, and uniqueness checking.

Implement logging and monitoring for validation failures. Unusual patterns of validation failures might indicate attack attempts. For example, repeated attempts to submit scripts or SQL injection payloads warrant investigation. However, be careful not to log sensitive data like passwords, even when they fail validation. Log the fact that validation failed and the type of failure, but not the actual invalid input if it might contain sensitive information.

Document your validation rules clearly and keep them synchronized with client-side validation. Consider generating client-side validation rules from server-side definitions to ensure consistency. This might involve creating a validation schema that can be used by both server and client code. Regular security reviews should verify that validation rules remain appropriate and haven't been weakened over time.

Input validation and sanitization are essential components of a comprehensive XSS prevention strategy. While they cannot replace output encoding, they provide valuable defense-in-depth, catching many attacks before they can do damage. The key is implementing validation that's strict enough to be secure but flexible enough to accommodate legitimate use cases. Regular testing and updates ensure your validation remains effective against evolving attack techniques. Remember that security is not about perfection but about raising the bar high enough that attackers move on to easier targets.