CORS Security Scanner: Free Guide for Vibe-Coded Apps

Quick answer

A free CORS security scanner checks your web app's Cross-Origin Resource Sharing (CORS) headers for misconfigurations that let malicious sites steal data or perform actions on behalf of your users. For vibe-coded apps, where AI tools often generate permissive CORS policies, running a scanner is a quick, essential step before launch. Use free tools like the security headers checker from OverMCP to detect and fix these issues in minutes.

What to check first

Before diving into a full CORS security scanner, run this quick checklist on your AI-built app. Many vibe-coded apps share the same CORS blind spots, so these checks will catch the most common issues fast.

Step-by-step fix

Once you've run a CORS security scanner and found issues, here's how to fix them. Let's use a Next.js API route as an example, since it's common in vibe-coded apps.

1. Identify the misconfiguration

Run your CORS security scanner and note the exact header values. For instance, you might see:

Access-Control-Allow-Origin: *
Access-Control-Allow-Credentials: true

This is a severe issue: it allows any origin to make authenticated requests, enabling cross-site request forgery (CSRF) and data theft.

2. Replace wildcard with specific origins

Instead of *, allow only your trusted domains. Here's a proper CORS setup in a Next.js API route:

// pages/api/user.ts
import type { NextApiRequest, NextApiResponse } from 'next';

const allowedOrigins = [
  'https://yourapp.com',
  'https://www.yourapp.com',
  process.env.NODE_ENV === 'development' ? 'http://localhost:3000' : ''
].filter(Boolean);

export default function handler(req: NextApiRequest, res: NextApiResponse) {
  const origin = req.headers.origin;
  if (origin && allowedOrigins.includes(origin)) {
    res.setHeader('Access-Control-Allow-Origin', origin);
  }
  res.setHeader('Access-Control-Allow-Credentials', 'true');
  res.setHeader('Access-Control-Allow-Methods', 'GET,POST,PUT,DELETE,OPTIONS');
  res.setHeader('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  
  // Handle preflight requests
  if (req.method === 'OPTIONS') {
    res.status(200).end();
    return;
  }
  // ... rest of your handler
}

3. Use environment variables

Never hardcode origins. Put them in env variables so you can change them without redeploying. In your .env.local:

CORS_ORIGINS=https://yourapp.com,https://www.yourapp.com

Then in your code:

const allowedOrigins = process.env.CORS_ORIGINS?.split(',') || [];

4. Apply globally with middleware

Instead of adding CORS headers to every route, use a global middleware. For Next.js, you can use middleware.ts:

// middleware.ts
import { NextResponse } from 'next/server';
import type { NextRequest } from 'next/server';

const allowedOrigins = process.env.CORS_ORIGINS?.split(',') || [];

export function middleware(request: NextRequest) {
  const origin = request.headers.get('origin') || '';
  const response = NextResponse.next();
  
  if (allowedOrigins.includes(origin)) {
    response.headers.set('Access-Control-Allow-Origin', origin);
  }
  response.headers.set('Access-Control-Allow-Credentials', 'true');
  response.headers.set('Access-Control-Allow-Methods', 'GET,POST,PUT,DELETE,OPTIONS');
  response.headers.set('Access-Control-Allow-Headers', 'Content-Type, Authorization');
  
  return response;
}

export const config = {
  matcher: '/api/:path*',
};

5. Re-scan with your CORS security scanner

After deploying, run the scanner again to confirm the headers are correct. OverMCP's continuous security monitoring can keep an eye on your headers over time, so you'll know if they regress.

Common mistakes

AI coding tools like Cursor, Bolt.new, and v0 often generate CORS configurations that are overly permissive or just wrong. Here are the most common mistakes I've seen in vibe-coded apps:

Why CORS matters for vibe-coded apps

Vibe-coded apps are built fast, and security often takes a backseat. But CORS is one of the most common vulnerabilities in AI-generated code. A single misconfigured header can expose your user's data to any malicious site. The good news is that with a free CORS security scanner, you can catch and fix these issues in minutes.

At OverMCP, we built a free AI app security scanner that includes CORS checks, along with other common vulnerabilities like exposed secrets and missing security headers. It's designed specifically for apps built with AI tools, so you can ship fast and stay secure.

FAQ

What is a CORS security scanner?

A CORS security scanner is a tool that analyzes your web application's CORS headers to detect misconfigurations that could allow unauthorized websites to make cross-origin requests to your API, potentially leading to data breaches or CSRF attacks.

Can I use a free CORS security scanner for my Next.js app?

Yes, there are several free options. OverMCP's security headers checker scans your live site and flags CORS issues. You can also use browser extensions or curl commands to inspect headers manually.

How do I fix CORS errors in a vibe-coded app?

First, identify the issue with a scanner. Then, replace wildcard origins with a whitelist of your trusted domains, ensure Access-Control-Allow-Credentials is set correctly, and handle preflight requests. Use middleware to apply these headers globally, as shown above.

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This article is part of a series on securing AI-built apps. Check out our [Vercel security scanner](https://www.overmcp.com/connect/vercel) for deployment-specific checks.