Input validation is the process of ensuring that data entered into a system meets predefined criteria for format, type, length, and value before it is processed. This critical practice is fundamental in software development, serving both functional and security purposes. Proper input validation prevents invalid, unexpected, or malicious data from causing errors, compromising security, or disrupting system operations.

Input validation can occur on two levels: client-side validation and server-side validation. Client-side validation provides immediate feedback to users, ensuring smoother interaction with forms or applications. For instance, it might highlight that an email address lacks the "@" symbol. However, since client-side validation can be bypassed by malicious users, server-side validation is indispensable. It verifies data at the backend, ensuring the integrity of all incoming data.

There are several key types of input validation:

Type validation: Ensures input matches the expected data type (e.g., integer, string).
Length validation: Confirms the input does not exceed or fall short of the required size.
Format validation: Validates patterns, such as email formats or phone numbers.
Range validation: Ensures numeric inputs fall within acceptable boundaries.
Whitelist validation: Allows only explicitly permitted inputs.
Effective input validation defends against a range of vulnerabilities, including SQL injection, cross-site scripting (XSS), and command injection attacks. Tools and frameworks often provide built-in validation libraries, reducing the risk of human error. Ultimately, robust input validation ensures system reliability, enhances user trust, and protects applications from malicious exploitation.