API CORS Check: a beginner's guide
Analyze CORS configuration
CORS: the browser rule that decides which sites can talk to which APIs
CORS (Cross-Origin Resource Sharing) is the security rule that browsers enforce to control whether JavaScript on one website is allowed to make HTTP requests to a different website's API. By default, browsers refuse: it is called the same-origin policy, and it has been part of every web browser since the 1990s. CORS is the exception mechanism that lets a server explicitly say "yes, JavaScript on these specific other origins is allowed to call me." The mechanism is a small set of HTTP headers (`Access-Control-Allow-Origin`, `Access-Control-Allow-Methods`, `Access-Control-Allow-Credentials`, etc.) that the API server returns, and the browser enforces them on the client side.
You should care because CORS is one of the most-misunderstood and most-blamed parts of modern web development. Every developer who has ever built a single-page application has hit a CORS error and assumed it was a bug. It is not a bug; it is the browser working correctly to prevent a malicious site from making requests to your bank on your behalf. The error is the intended behavior. Understanding CORS — really understanding it — is the difference between debugging it in five minutes and debugging it in two hours. It is also a real security control: a misconfigured CORS policy can quietly expose an API to the entire internet in a way that defeats the entire same-origin protection.
The five things every CORS check looks at:
Is `Access-Control-Allow-Origin` set? If not, no cross-origin requests are allowed at all.
**Is it set to `*` (allow any origin), to a specific list, or dynamically reflected from the request?** The dynamic-reflection pattern is the dangerous one — if combined with `Access-Control-Allow-Credentials: true`, it effectively disables the same-origin policy.
Are the allowed methods (`Access-Control-Allow-Methods`) appropriate? Allowing `DELETE` and `PUT` cross-origin is almost always a sign of a misconfiguration.
Are credentials (cookies, auth headers) allowed cross-origin? `Access-Control-Allow-Credentials: true` is the riskiest setting and should only be combined with a specific origin allowlist, never `*`.
Is the preflight (`OPTIONS`) request handled correctly? Browsers send a preflight before any non-trivial cross-origin request; getting this wrong breaks legitimate use cases.
Three questions a CORS check answers:
Is this API's CORS policy reasonable, or does it expose the API to any website on the internet?
For a security review, is the policy correctly restricted to the origins that should actually have access?
For an integration project, will my front-end be allowed to call this API from the browser?
The cost of misconfigured CORS is either (a) a security vulnerability that lets any website call this API as the user, or (b) endless debugging from developers who can't figure out why their request is being blocked. The fix is to read the CORS headers, understand them, and configure them deliberately. The CORS specification is in the Fetch standard and the canonical reference is Mozilla's CORS guide on MDN.
The API CORS Check endpoint, in plain language
In one sentence: Analyze CORS configuration
Tests and analyzes CORS (Cross-Origin Resource Sharing) configuration. Identifies permissive configurations that could lead to security vulnerabilities.
Don't worry if some of the words above are still unfamiliar — there's a plain-language glossary at the bottom of this page, and most of the terms link to their own beginner guides if you want to learn more.
What is actually happening when you call it
Here's what's actually happening behind the scenes when you call this endpoint:
Tests CORS configuration by sending OPTIONS requests with multiple Origin headers: a standard domain (example.com), a malicious domain (evil.attacker.com to detect origin reflection), and `null` (to detect null origin acceptance). Analyzes Access-Control-Allow-Origin, Access-Control-Allow-Credentials, and other CORS headers. Identifies overly permissive configurations like wildcard origins with credentials, origin reflection, and null origin acceptance.
If you're using an AI assistant through MCP, you don't need to understand any of the technical details — the assistant calls the tool and translates the result for you.
Why this specific tool matters
Let's skip the marketing fluff and answer the only question that actually matters: why should you, a real human with a real to-do list, care about the API CORS Check tool? Here's the plain-English version, written the way you'd hear it from a friend who happens to do this for a living.
Misconfigured CORS is a common security vulnerability (per OWASP Web Security Testing Guide) that can allow unauthorized cross-origin access. Testing CORS configuration helps identify potential data exposure risks.
Picture this in real life. Imagine a security engineer. Here's the situation they're walking into: Audit API (Application Programming Interface) CORS configuration for overly permissive settings. Without the right tool, that person would be stuck copy-pasting between five browser tabs, reading documentation written for engineers, and crossing their fingers that the answer they cobble together is correct. With the API CORS Check tool, the same person gets a clear answer in seconds — no spreadsheets, no guessing, no waiting for someone on the infrastructure team to free up.
Three questions this tool answers in plain English. If any of these have ever crossed your mind, the API CORS Check tool is built for you:
What is this website actually built with, layer by layer?
Who hosts it, who runs analytics on it, who delivers the assets?
Is the company on a stack that fits my product, my pitch, or my integration?
You can either click the tool and get the answer yourself, or ask your AI assistant — connected through MCP (Model Context Protocol) — to ask the question for you and translate the answer into something you can paste into Slack.
Who gets the most out of this. Sales teams qualifying leads, marketers researching competitors, partnership managers scoping integrations, and security teams looking for known-vulnerable software in the wild. If you see yourself in that list, this is one of the EdgeDNS tools you should bookmark today.
What happens if you skip this entirely. Skip it and you're guessing at how a website is built — which kills sales calls, integration scoping, and competitive research. That's why running this check — even once a month — is one of the cheapest forms of insurance you can give your domain.
Available on the developer plan. The technical details: `GET /v1/domain/api-cors`.
When would I actually use this?
If you're still on the fence about whether the API CORS Check tool belongs in your toolbox, this section is for you. Below you'll meet three real people — a security engineer, a backend developer, and a penetration tester — facing three real situations where this tool turns a stressful afternoon into a five-minute task. Read whichever story sounds closest to your week.
Story 1: Security Audit
Imagine you're a security engineer. Audit API (Application Programming Interface) CORS configuration for overly permissive settings.
Why it matters: Find CORS misconfigurations before attackers exploit them.
Story 2: API Development
Imagine you're a backend developer. Verify CORS is properly configured for expected client origins.
Why it matters: Ensure CORS settings match application requirements.
Story 3: Penetration Testing
Imagine you're a penetration tester. Test for CORS vulnerabilities during web application assessments.
Why it matters: Identify exploitable CORS configurations.
Common situations across teams. Beyond the three stories above, here are the everyday workplace moments when people across the company reach for the API CORS Check tool — or one of the tools right next to it in this category. If any of these are on your calendar this month, that's your sign:
During sales prospecting, to qualify a lead by what they are running.
During competitive research, to understand what a rival is built with.
When scoping an integration or partnership.
When you suspect a target is using a known-vulnerable version of something.
If you can see yourself in even one of those bullets, the API CORS Check tool will pay for itself the first time you use it.
Still not sure? Here's the easiest test in the world. Open Claude, ChatGPT, Gemini, or any other AI assistant connected to the EdgeDNS MCP server and ask, in your own words: "Is the API CORS Check tool useful for my job?" The assistant will look at the tool, ask you a couple of follow-up questions about what you're trying to accomplish, and give you a straight answer in plain English. No commitment, no signup forms, no jargon.
The easiest way: just ask your AI assistant
If you've connected the EdgeDNS MCP server to Claude, ChatGPT, Gemini, Cursor, or any other AI assistant, you don't need to write any code. Just ask in plain English:
"Use the API CORS Check tool to check api.example.com and explain anything that looks wrong in plain language."
The AI will figure out which tool to call, fill in the right parameters, run it, and then explain the result back to you. No copy-pasting between tabs. No reading raw JSON. No memorizing endpoint names.
MCP (Model Context Protocol) access is free on every plan, including the free tier. One API key works for both REST and AI — you do not have to choose.
The technical way: call it from code
If you're a developer and want to call the endpoint from a script or your own application, here's the simplest possible example. Replace the placeholder API key with the real one from your dashboard.
# Replace edns_live_YOUR_KEY with your real API key from the dashboard
curl -H "Authorization: Bearer edns_live_YOUR_KEY" \
"https://api.edgedns.dev/v1/domain/api-cors?domain=api.example.com"What you need to provide
There's just one piece of information you need to provide. The table below explains exactly what it is and what a real value looks like.
| Field | Type | Required? | What it means | Example |
|---|---|---|---|---|
domain | string | Yes | The domain to test CORS configuration for | api.example.com |
What you get back
When you call this tool, you'll get back a JSON object with the fields below. If you're talking to it through an AI assistant, the assistant reads these for you and explains them in plain language — you don't need to memorize them.
| Field | Type | What you'll see in it |
|---|---|---|
domain | string | The queried domain |
corsEnabled | boolean | Whether CORS headers are present |
isWildcard | boolean | Whether Access-Control-Allow-Origin is wildcard (*) |
configuration | object | CORS config (allowOrigin, allowMethods, allowHeaders, allowCredentials, maxAge, exposeHeaders) |
security | object | Security assessment (isPermissive, credentialsWithWildcard, reflectsOrigin, allowsNullOrigin) |
recommendations | array | CORS security improvement suggestions |
Words you might be wondering about
If any words on this page felt like jargon, here's a plain-language version. Click any linked term to read a full beginner-friendly guide.
API (Application Programming Interface) — A way for one program to ask another program for something — like a waiter taking your order to the kitchen.
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