Missing Input Validation on API Endpoints
Your API endpoints generated by Claude Code accept any input without validation, allowing malformed data, oversized payloads, or malicious content to reach your business logic and database. There are no checks on field types, lengths, formats, or required fields.
Without input validation, attackers can submit negative prices, inject SQL through string fields, send payloads that crash your server, or store garbage data that breaks your application later. Even non-malicious users can accidentally submit invalid data that causes downstream errors.
This often becomes apparent when your database contains impossible values, when your app crashes on unexpected input, or when a security audit flags every endpoint as vulnerable.
Error Messages You Might See
Common Causes
- No validation library configured — The generated project doesn't include Joi, Zod, class-validator, or equivalent validation middleware
- Trust in client-side validation only — Form validation exists in the frontend but the API accepts anything directly
- Missing type coercion — String values like '0' or 'null' are not converted or rejected, causing type confusion
- No payload size limits — The server accepts arbitrarily large JSON bodies or file uploads
- Incomplete schema definitions — Some fields are validated but others (especially nested objects and arrays) are passed through unchecked
How to Fix It
- Add a validation library — Install Zod (TypeScript), Joi (Node.js), or Pydantic (Python) and define schemas for every API endpoint
- Validate at the controller layer — Parse and validate request bodies before they reach your service or database layer
- Define strict schemas — Specify types, min/max lengths, regex patterns, enums, and required fields for every input
- Set payload size limits — Configure body-parser or equivalent to reject oversized requests (e.g., 1MB max)
- Return clear validation errors — Send 400 Bad Request with specific field-level error messages so the client can correct the input
- Test with fuzzing — Submit random, empty, oversized, and malicious inputs to verify your validation catches them
Real developers can help you.
Richard McSorley
Full-Stack Software Engineer with 8+ years building high-performance applications for enterprise clients. Shipped production systems at Walmart (4,000+ stores), Cigna (20M+ users), and Arkansas Blue Cross. 5 patents in retail/supply chain tech. Currently focused on AI integrations, automation tools, and TypeScript-first architectures.
Omar Faruk
As a Product Engineer at Klasio, I contributed to end-to-end product development, focusing on scalability, performance, and user experience.
My work spanned building and refining core features, developing dynamic website templates, integrating secure and reliable payment gateways, and optimizing the overall system architecture.
I played a key role in creating a scalable and maintainable platform to support educators and learners globally.
I'm enthusiastic about embracing new challenges and making meaningful contributions.
Luca Liberati
I work on monoliths and microservices, backends and frontends, manage K8s clusters and love to design apps architecture
Mehdi Ben Haddou
- Founder of Chessigma (1M+ users) & many small projects
- ex Founding Engineer @Uplane (YC F25)
- ex Software Engineer @Amazon and @Booking.com
Costea Adrian
Embedded Engineer specilizing in perception systems. Latest project was a adas camera calibration system.
Anthony Akpan
Developer with 8 years of experience building softwares fro startups
AUXLE
I am a Full Stack Developer experienced in building Websites, Web apps and Cross Platform Mobile Apps for Startups and Companies.
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I'm a Senior Android developer, open to work in various fields
Stanislav Prigodich
15+ years building iOS and web apps at startups and enterprise companies. I want to use that experience to help builders ship real products - when something breaks, I'm here to fix it.
Matthew Butler
Systems Development Engineer @ Amazon Web Services
You don't need to be technical. Just describe what's wrong and a verified developer will handle the rest.
Get HelpFrequently Asked Questions
Why is client-side validation not enough?
Anyone can bypass frontend validation by sending requests directly to your API using curl or Postman. Server-side validation is the only reliable way to ensure data integrity and security.
What validation library should I use?
For TypeScript projects, Zod is the most popular choice. For plain Node.js, use Joi. For Python, Pydantic is standard. All three provide schema definition, type coercion, and clear error messages.