Definition
Input validation is the process of verifying that data entered into a system or application meets predefined criteria for correctness, format, and type. It acts as a crucial gatekeeper, ensuring that only legitimate and expected information is processed, thereby preventing unexpected behavior or security vulnerabilities. This is achieved by checking incoming data against rules, such as expected data types, length limits, allowed characters, and valid ranges. Implementing input validation is essential for protecting systems from malicious attacks, maintaining data integrity, and ensuring reliable application performance. For instance, a web form validating an email address ensures it follows the standard format before submission. It is often confused with sanitization, which focuses on removing or neutralizing potentially harmful characters, whereas validation checks if the input is acceptable in the first place. This concept is fundamental to cybersecurity and appears on exams like Security+, CySA+, and CISSP.