When I first started seeing CORS bugs, I thought… “Okay, cool, but this is probably nothing serious.”
Wrong.

In this breakdown, we’ll hit the 4 most dangerous CORS mistakes I keep finding, how to spot them fast, and why they can lead to account takeover, data leaks, and even complete system compromise.

1. The Wildcard Disaster

When Access-Control-Allow-Origin is set to * and Access-Control-Allow-Credentials is true, you’ve basically given any origin full access to your authenticated data.
What to check:

• Send an Origin header with any random domain — if the response allows it and credentials, you’re in.
• Look for sensitive API responses that return user data, tokens, or configuration.
• Test both GET and POST requests — some endpoints are protected differently.

2. The Reflection Trap

The server takes whatever Origin you send… and reflects it back in the Access-Control-Allow-Origin header.
With Access-Control-Allow-Credentials: true, this becomes dangerous: your malicious site can make the request as the victim, and the browser will happily send their cookies.
What to check:

• Can you grab an authenticated API response from your own domain?
• Does the site cache the response headers? If yes, think cache poisoning — you can poison it once, and every visitor gets the bad header.
• Test across subdomains — sometimes only part of the app is vulnerable.

3. The WordPress API Trap

A lot of WP sites leave /wp-json/ open to the world with permissive CORS rules.
Why it’s bad:

• Many plugins and themes expose sensitive or admin-only endpoints through this API.
• Some allow reading draft posts, user emails, or even nonce tokens.
  What to check:
• See if authenticated-only data is exposed when cookies are sent.
• Check plugin-specific API routes for private data leaks.

4. The Ruby Breakdown

Rails apps often use the rack-cors gem, and if it’s misconfigured, you can access internal or authenticated endpoints from anywhere.
What to check:

• Look for origins '*' with credentials: true.
• See if specific controllers or environments override CORS rules inconsistently.
• Always test staging environments — dev teams often forget to lock them down.

5. Tools to Catch It Fast

• Corsy — quick CORS scanner for broad sweeps.
• Scanner (Caido Plugin) — catch CORS along with other bug classes during recon.

The Real Hacker Mindset
Most hunters shrug at CORS — they see it as “informational” and move on.
But combine it with XSS, CSRF, or cache poisoning, and you’re looking at account takeover, password resets, or sensitive data exfiltration.

Final Words
The next time you see a “boring” CORS issue… stop. Test it. Chain it. Escalate it.
The boring bugs are often the ones with the biggest payouts.

Stay curious, and stay secure. 🔐🔥