Input validation is the practice of examining all data entering an application to ensure it conforms to expected formats, types, ranges, and lengths before processing. As the first line of defense against injection attacks, data corruption, and application logic abuse, input validation establishes a security boundary between untrusted external data and trusted internal application logic.
Effective input validation follows the allowlist (positive validation) approach -- defining exactly what is permitted rather than attempting to block known bad patterns. Each input field has a validation specification: data type, minimum and maximum length, allowed character set, expected format (regex pattern), and valid range for numeric values. Server-side validation is mandatory because client-side validation exists only for user experience and can be bypassed entirely. Validation occurs at every trust boundary -- between the browser and server, between microservices, between the application and database, and when processing files or API responses. Canonicalization normalizes input (resolving encoding, path traversal sequences, and Unicode variations) before validation to prevent bypass through alternative representations. Structured data formats use schema validation (JSON Schema, XML Schema) to enforce structural requirements. Validation failures return generic error messages to users while logging detailed information for security monitoring. Frameworks and libraries provide built-in validation capabilities that centralize rules and reduce implementation errors.
Insufficient input validation is the root cause of the most dangerous vulnerability classes: injection attacks, buffer overflows, path traversal, and format string vulnerabilities. Every data entry point that lacks validation is a potential attack vector. While input validation alone does not prevent all attacks (output encoding and parameterized queries are still necessary), it dramatically reduces the attack surface by rejecting malformed data before it reaches vulnerable code paths.
CDA treats input validation as a VSD fundamental. Theater missions establish validation standards for each input type, implement centralized validation libraries, and audit applications for unvalidated data paths -- particularly in areas where validation is commonly overlooked such as HTTP headers, file metadata, and inter-service communication.