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Allowlist Validation

An input-handling defense that checks user input against a list of explicitly permitted values or patterns and rejects everything else, rather than trying to block known-bad input. It is the safer default for validating untrusted data.

Allowlisting (accepting only what is known-good) is more reliable than denylisting (blocking known-bad), because attackers constantly find new bad inputs that a denylist misses. It is one layer of input handling and works alongside output encoding and parameterized queries to prevent injection.

Introduced in: Defensive Web Fundamentals

Examples

  • Accepting only digits for a numeric ID field and rejecting anything else.
  • Permitting only a fixed set of country codes from a dropdown value.
  • Validating an uploaded file's type against an approved list before processing.

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