移动端与Web端跨域（CORS）问题：Web应用调用API报错

Hey there, let's break down this CORS confusion and help you explain it clearly to your backend team!

First, let's get one key point straight: CORS is a security restriction enforced only by web browsers. Other clients like Postman or native mobile apps don't follow this rule, which is why they work fine even without proper CORS configuration.

Let's break down each case:

• Postman: It's an API testing tool, not a browser. It doesn't send automatic OPTIONS preflight requests (unless you explicitly set them) and doesn't check for CORS-related response headers like Access-Control-Allow-Origin. It just sends your request and returns the server's response directly, no extra security checks in between.
• iOS/Android Native Apps: The network libraries used here (like URLSession on iOS or OkHttp on Android) communicate directly with the server without adhering to browser-origin policies. They don't care about CORS headers—they just process the server's response as-is.
• Web Browsers: Browsers follow the Same-Origin Policy to protect users from malicious cross-site requests. When your web app tries to call an API that's on a different domain, port, or protocol, the browser first sends an OPTIONS preflight request to ask the server: "Is this web app allowed to access you?" If the server doesn't respond with headers that explicitly allow your web app's origin, the browser blocks the request and throws the error you saw:

  Failed to load http://xxx.xxx.xxx.xx:8082/external/access_token : Response to preflight request doesn't pass access control check...

You can frame it like this to make it clear:

Hey team, I ran into a CORS error when calling our /external/access_token API from our web app, but Postman, iOS, and Android apps all work fine. This is because browsers have a security rule called CORS that checks if the server allows the web app's origin. Postman and mobile apps aren't browsers, so they don't do this check. Could you add our web app's domain/IP to the server's CORS allowed list? We need the server to return headers like Access-Control-Allow-Origin: [our web app's origin] and allow the OPTIONS request method, which the browser uses for preflight checks.

内容的提问来源于stack exchange，提问作者Pyae Phyoe Shein