Don't Snore on CORS

Let's talk CORS – everyone's favorite (or least favorite) web security feature. I've been wrestling with it lately, and that's my cue to write a blog post!

The core concept of CORS is simple: prevent cross-origin code execution. If css-tricks.com tries to fetch JavaScript from any-other-website.com, the browser blocks it by default. Console error? Yep, "Not allowed."

The exception? The target website sends a header explicitly granting permission. Your domain gets whitelisted, or a wildcard allows access. There are nuances (preflighting, credentials, etc.), but the MDN documentation covers those well.

My biggest headaches with CORS stem from its seemingly inconsistent behavior. Two requests succeed, the third fails – reproducible, yet baffling. (Maybe a load balancer with half-cached headers? Who knows!) Or a proxy suddenly stops working. I've lost count of the times I've debugged CORS issues, easily over 100.

Recent CORS encounters:

• A popular "Learn CORS In 6 Minutes" video (10,000 likes!) highlighted a common solution: npm install cors.
• You must configure servers with the correct headers. I demonstrated this in a Cloudflare Workers video (though Cloudflare Workers offer a cool cross-origin bypass).
• Jake Archibald's excellent "How to win at CORS" article, complete with a playground.
• Browser extensions (Firefox, Chrome) exist to inject CORS headers – a questionable development workaround, but understandable in a pinch.
• My previous post on easily proxying any content (including third-party JavaScript) to make it first-party. The comments rightly pointed out this bypasses CORS protection – risky unless you fully control the third-party resource.

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