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CORS (Cross-Origin Resource Sharing) is a way for a server to tell a web browser which other websites’ scripts may read a response. It creates a controlled exception to the browser’s same-origin policy—the default restriction that prevents a page from freely reading data from a different origin.
What counts as a different origin?
An origin is the combination of a URL’s scheme (such as https), host (such as example.com), and port. If any of those differ, the origins differ. A different path by itself does not create a different origin. For example, https://example.com/news and https://example.com/account are same-origin, while http://example.com and https://example.com are not. MDN explains the same-origin policy as a security mechanism restricting how documents or scripts from one origin interact with another origin’s resources.
Why does a browser restrict cross-origin reads?
Imagine you are signed in to a bank in one tab and visit a malicious page in another. If that page’s JavaScript could freely read the bank’s responses through your browser, it might try to access private account information using your existing session. The same-origin policy helps prevent that kind of cross-site data exposure.
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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →The restriction is specifically about what a script can read. It does not mean browsers block every cross-origin request: navigation, embedding, and some cross-origin writes are governed by other browser rules. A request may reach a server even when the browser later refuses to expose its response to the page’s JavaScript.
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How CORS lets a browser share a response
When JavaScript on https://site-a.example uses fetch() to request a resource from https://site-b.example, the browser includes an Origin header identifying the requesting origin. The resource’s server can respond with CORS headers, including Access-Control-Allow-Origin. The browser checks those headers and, if the response allows the requesting origin, makes the response available to the script. MDN’s CORS guide describes this browser-and-server exchange.
For a public resource that does not use credentials, a server can allow broad access with Access-Control-Allow-Origin: *. For a restricted resource, it can name a permitted origin, such as Access-Control-Allow-Origin: https://site-a.example. The permission comes from the server’s response; JavaScript on the requesting page cannot grant itself access by adding a request header.
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When does the browser send an OPTIONS preflight?
Some cross-origin requests require a preliminary permission check, called a preflight. Before sending the intended request, the browser sends an OPTIONS request describing the planned method and any non-safelisted headers. The server’s response can approve the method and headers. If the check succeeds, the browser sends the actual request; if it fails, the browser does not proceed with that request.
Not every cross-origin request is preflighted. Whether a check is needed depends on the request’s method, headers, and other characteristics defined by the Fetch standard. A preflight is a browser protocol check, not authentication and not proof that the eventual operation is harmless. MDN’s preflight documentation covers the exchange.
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How credentials change the rules
Requests that include credentials—such as cookies or HTTP authentication—need a more specific policy. A server cannot authorize a credentialed cross-origin response using Access-Control-Allow-Origin: *. It must return an explicit allowed origin and, when appropriate, Access-Control-Allow-Credentials: true. The client request must also be configured to include credentials. MDN documents the credential requirements.
Only enable credentialed access for origins you trust and need. Do not blindly reflect whatever value arrives in the Origin request header: that would turn a request from an unapproved site into an approved one.
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Configure CORS without widening access unnecessarily
- Allow only needed origins and resources. Use an explicit allowlist for private or application-specific data.
- Reserve
*for public, non-credentialed resources. It is not a shortcut for sharing credentialed responses. - Account for caches when the allowed origin varies. If the server returns different CORS headers depending on the requesting origin, include
Vary: Originso caches distinguish those responses. MDN’s header reference explains this behavior. - Avoid
Access-Control-Allow-Origin: null. Some sandboxed or opaque origins serialize asnull, so allowing it may grant access more broadly than intended. MDN details the risks of this value. - Keep security controls separate. CORS governs whether browser scripts can read a response. It does not replace authentication, authorization, or defenses against cross-site request forgery (CSRF).
What a CORS error means—and how to troubleshoot it
A browser often reports a CORS failure to JavaScript as a generic network error, so the page’s code may not reveal which header or policy is wrong. The browser console and Network panel usually provide more useful details. A CORS error also does not prove that the server never received the request; the browser may block access to the response after the request has been sent.
- Identify both origins. Note the page’s origin and the full URL of the requested resource, including scheme, host, and port.
- Inspect the browser’s console and Network panel. Find the failing request and determine whether it was an
OPTIONSpreflight or the actual request. - Check the server’s response headers. Verify that
Access-Control-Allow-Originmatches the requesting origin or is appropriately set to*for public, non-credentialed access. For a preflight, check the server’s allowed methods and headers as well. - Check credentials and caching. If the request includes credentials, ensure the response uses a specific permitted origin and the appropriate credentials header. If the allowed origin is selected dynamically, ensure the response varies by
Origin. - Change the server or the architecture if needed. If you do not control the remote server, your frontend cannot add permission headers to its response. The server owner must allow your origin, or your application may use a server-side intermediary that is legitimately permitted to access the resource.
Adding mode: 'no-cors' to fetch() does not make a protected response readable. That mode restricts the request and returns an opaque response, which prevents JavaScript from inspecting its body and headers. MDN’s CORS error guide explains why client-side workarounds cannot supply missing server permission.
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