What is Input Validation?
The practice of checking and sanitizing all data received from users or external sources before processing it. It ensures only expected, safe data enters your system.
In plain English
Input validation is like airport security screening. Every piece of luggage gets checked before it's allowed on the plane, regardless of who owns it. You don't just trust that passengers packed safely — you verify everything because the consequences of missing something are catastrophic.
How it works
Input validation involves checking data against expected formats, types, ranges, and lengths before processing it. Validation should happen on both the client side (for user experience) and server side (for security). Sanitization goes further by removing or escaping potentially dangerous characters from input, preventing injection attacks.
Why it matters for AI-built apps
AI-generated code frequently processes user input with zero validation — directly inserting form data into database queries, HTML output, or system commands. This is the root cause of SQL injection, XSS, and command injection attacks. Without input validation, your app is essentially trusting every user to send perfectly safe data.
Common issues
Client-side-only validation (easily bypassed), not validating API request bodies, allowing unlimited input length (enabling denial-of-service), not sanitizing HTML/JavaScript in user-generated content, trusting data from 'internal' APIs without validation, and not validating file uploads (type, size, content).
Best practices
Validate all input on the server side — client-side validation is only for UX. Use allowlists (specifying what IS allowed) over denylists (specifying what ISN'T). Validate type, length, range, and format. Use parameterized queries instead of string concatenation for database operations. Sanitize HTML output to prevent XSS. Use schema validation libraries like Zod or Joi for structured validation.
Frequently asked questions
Is client-side form validation enough to protect my app?
No — client-side validation exists solely for user experience (showing errors before submission). Attackers can bypass it entirely using browser dev tools or direct API calls. Every input must be validated on the server side. Think of client-side validation as a courtesy and server-side validation as the security gate.
What library should I use for input validation?
For JavaScript/TypeScript apps, Zod is excellent — it provides runtime validation with TypeScript type inference. For APIs, use express-validator or joi. For forms, pair Zod with react-hook-form. The key is having a consistent validation layer rather than ad-hoc checks scattered throughout your code.
How do I validate file uploads securely?
Check file size limits, validate the MIME type by reading file headers (not just the extension), use an allowlist of accepted types, rename files to prevent path traversal, scan for malware if possible, and store uploads outside your web root or in a dedicated storage service like S3.
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