![]() ![]() The path “/shop/product.html” is not considered as a part of the origin The path “/about” is not considered as a part of the origin Let’s look at the following example.Īssuming our origin is the requests can be categorized into same-origin or cross-origin requests as follows: Origin Same-origin requests are essentially those requests whose scheme, domain, and port match. Similarly, the port can also be any valid port number. The scheme could be HTTP, HTTPS, FTP, or anything else. It’s the combination of a scheme, domain, and port. In simple terms, the same-origin policy is the web version of “don’t talk to strangers” incorporated by the browser.Īll modern web browsers available today follow the same-origin policy that restricts how XMLHttpRequest and fetch requests from one origin interact with a resource from another origin. To understand CORS, let us first understand the same-origin policy and its need. ![]() Cross-origin resource sharing, or CORS, is the mechanism through which we can overcome this barrier. Our web browsers enforce the same-origin policy, which restricts resource sharing across different origins. ![]() Interestingly, this is not an error as we portray it, but rather the expected behavior. So, what exactly is the CORS policy and why do we face this error often? What is Cross-Origin Resource Sharing (CORS)? Seem familiar? With over 10,000 questions posted under the cors tag on StackOverflow, it is one of the most common issues that plague frontend developers and backend developers alike. You open up the console and see either “No Access-Control-Allow-Origin header is present on the requested resource,” or “The Access-Control-Allow-Origin header has a value that is not equal to the supplied origin” written in red text, indicating that your request was blocked by CORS policy. We can do this by installing a CORS library ( ) and telling the server to expect requests from 127.0.0.1:3000 app.Consider the following situation: you’re trying to fetch some data from an API on your website using fetch() but end up with an error. Just like our earlier example, 127.0.0.1: 3000 and 127.0.0.1: 4000 are treated as two separate domains, so you cannot make requests across them yet. If our React application made a fetch request to our backend like this: // Fetch request made from const express = require('express')Ĭonsole.log(`Example app listening on port $`) Since our backend cannot also run on port 3000 locally, we’ll setup Express to run on port 4000. Our frontend is running on port 3000 - a common default for React. Let’s say we have a React application with an Express backend. Most server frameworks have a library for configuring your CORS headers, but if you want to see the underlying headers themselves, here’s a good resource. The important thing to note about CORS is that the configuration/settings are done on the server and enforced by both the server and your browser. It can reject all POST requests but allow GETs and PUTs. It can reject requests from but accept requests from. It can reject requests that need cookies. With CORS, the server is allowed to specify which cross-origin requests it will accept vs reject. If this request was allowed and your cookie was included, the owner of malicious.site would be able to make requests on your behalf and read your account details. While you are browsing, you accidentally visit malicious.site, which makes a request to that looks like this: // Fetch request made from Let’s say you are logged in to and you have a cookie set with indicating that you are logged in. Your browser holds a lot of state about you for every website you visit. If you’ve written any frontend code, you’ve likely encountered a CORS error before, like this:Īccess to _ has been blocked by CORS policyĬross-Origin Resource Sharing (CORS) is a protocol that defines how web requests should be handled when they are across different URLs. ![]()
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