Just remember to not use it for non-public APIs.CORS and the Access-Control-Allow-Origin response header And when you need credentials also, you'll see the error. The CORS specification is complicated with many specifics but using the * wildcard won't harm. This is one of the rare cases where disabling a security feature does not harm security most of the time. is different from and that is different from. One important thing is to also set the Vary: Origin header to let proxies know the response might be different for different Origin headers.Īnd also don't forget that an origin contains the scheme and the port also. ![]() setHeader( "Access-Control-Allow-Credentials", "true") setHeader( "Access-Control-Allow-Origin", origin) When you need credentials, which is most of the time, you need to check the origin header against a whitelist and reflect that back to the user: const allowed_origins = [ If you need to specify more than one (for example, your website is accessible under multiple domains) then you need more effort to make it work. But when credentials are sent it requires a stricter check.īut the Access-Control-Allow-Origin can define only one origin. For simple cases where disabling CORS checking means (almost) no harm, the standard allows the wildcard. I find it a very good compromise between security and convenience. But in this case * is not allowed for the Access-Control-Allow-Origin. In this case, the browser checks if the response contains Access-Control-Allow-Credentials: true. with cookies), set credentials: "include" in the request: fetch(, ) Still can't siphon off authenticated responses but it nonetheless opens a way to detect what kind of services are running in the local network. If only you have access to an endpoint then using the wildcard opens a vulnerability. ![]() That means a hacker has no access to information he could not gather just by sending the request himself.īut there is a scenario where it can be a problem. It is like sending the request without logging in. Without Access-Control-Allow-Credentials, another CORS header, no user identifiers, such as cookies, are sent with the request. It seems like the malicious site can steal information.īut in reality, it's usually not a problem. With Access-Control-Allow-Origin: *, evil.example is allowed to read responses from other.example/api. Since that is a cross-origin request, other.example/api sends back an Access-Control-Allow-Origin header. Let's also follow the example above and assume that there is a site.example which is intended to communicate with other.example/api. The site has some javascript that runs a fetch to other.example/api on your behalf. ![]() Let's say you are a user visiting a malicious third-party website at evil.example. This sounds insecure, but let's see the implications! What * allows? In this case, every website can send requests to the target and read the response. This is usually what API developers do when faced with this error. The special value * means "all origins" which essentially disables this security feature. The Access-Control-Allow-Origin specifies the allowed origin that can make cross-origin requests. Has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on Without an Access-Control-Allow-Origin header in the response, the browser throws an exception: Access to XMLHttpRequest at '' from origin '' This is a security feature as it protects the user by not letting random websites fetch data from sites he is logged in. The two important points are that the target server must allow the operation and the client's browser enforces it. In that case, the server must indicate that it allows the cross-origin operation otherwise the browser will reject the request. Want to learn AWS serverless development? Click here CORS headersĬORS headers come into play when a client makes a cross-origin request.
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