What is Input Validation?
1 min read
Updated 2026-02-05
The process of ensuring that user-supplied data meets expected formats and constraints before processing. A critical defense against injection attacks.
Understanding Input Validation
Input validation is the first line of defense against many attacks. It should check data type, length, format, and range. However, validation alone isn't sufficient - it should be combined with output encoding and parameterized queries for defense in depth.
Examples
- Validating email format with regex
- Ensuring numeric fields contain only numbers
- Checking file uploads for allowed extensions and MIME types
- Limiting string length to prevent buffer overflows
How to Prevent
- Validate on the server side (client-side validation can be bypassed)
- Use allowlists over denylists when possible
- Validate data type, length, format, and range
- Reject invalid input rather than trying to sanitize it
- Combine with output encoding for defense in depth
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