Input validation is the practice of checking user-supplied data to ensure it conforms to expected formats, ranges, and safety requirements before the application processes it. Proper input validation is a fundamental security measure that prevents a wide range of attacks including SQL injection, XSS, and command injection.
Input validation can be implemented on both the client side (for user experience) and the server side (for security). Client-side validation provides immediate feedback but can be bypassed, so server-side validation is always required as the authoritative check. The two approaches are complementary: client-side for usability, server-side for security. Secure development books on Amazon cover implementation patterns.
For URL shortening services, input validation is critical in several areas: validating submitted URLs (checking format, protocol, and domain), validating custom aliases (checking length, allowed characters, and uniqueness), and validating API parameters (checking types, ranges, and required fields).
Best practices include using allowlists rather than blocklists, validating data type and format before processing, encoding output appropriately for the context, and using parameterized queries for database operations. Software quality books on Amazon discuss comprehensive validation strategies.