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Cross-Origin Resource Sharing with Connect and gRPC-Web

CORS is a mechanism that allows servers to restrict requests from JavaScript running in a web browser. For example, consider a malicious web page making requests to an API on a different domain. Simply visiting this page could run arbitrary actions on your behalf.

To avoid this problem, browsers ask the target for permission to issue a request, if the request originates from somewhere else than the target. They send a preflight request, and the server replies with CORS information. The browser decides whether to issue or deny the actual request, and will only expose response headers to JavaScript that are explicitly allowed.

Both Connect and gRPC-Web require CORS preflights for cross-origin requests. Note that we highly recommend to use a library like rs/cors for Go to implement the logic. This document gives you the necessary information to configure CORS appropriately using any library.

Preflight request

Here's an example for a CORS preflight request with the Connect protocol:

OPTIONS /connectrpc.greet.v1.GreetService/Greet HTTP/1.1
Access-Control-Request-Method: POST
Access-Control-Request-Headers: Connect-Protocol-Version,Content-Type
[other headers elided...]

Note that the browser transmits the origin, as well as the HTTP methods and header names of the actual request.

Preflight response

Here is an example for a CORS preflight response for the preflight above that allows all requests with the Connect and gRPC-Web protocols, and asks the browser to cache the information:

HTTP/1.1 204 No Content
Access-Control-Allow-Methods: POST,GET
Access-Control-Allow-Headers: Content-Type,Connect-Protocol-Version,Connect-Timeout-Ms,Grpc-Timeout,X-Grpc-Web,X-User-Agent
Access-Control-Max-Age: 7200
Vary: Origin,Access-Control-Request-Method,Access-Control-Request-Headers
[other headers elided...]

Let's take a look at the details:

Allowing origins

Access-Control-Allow-Origin: allows browsers to make requests from via https with the default port 443.

Allowing methods and headers

Access-Control-Allow-Methods: POST,GET is required to support gRPC-Web and Connect. gRPC-Web will only use POST. Connect uses POST and optionally GET.

Access-Control-Allow-Headers: Content-Type,Connect-Protocol-Version,Connect-Timeout-Ms,Grpc-Timeout,X-Grpc-Web,X-User-Agent allows all necessary request headers for Connect and gRPC-Web.

Note: If your application uses custom request headers, you must explicitly allow them. This includes commonly used headers such as Authorization.

Caching preflight responses

Access-Control-Max-Age: 7200 lets browsers cache CORS information for longer, which reduces the number of preflight requests. The value is given in seconds. Any changes to your CORS setup won't take effect until the cached data expires. Firefox caps this value at 24h, and modern Chrome caps it at 2h (7200 seconds).

Vary: Origin,Access-Control-Request-Method,Access-Control-Request-Headers is used by the browser to cache CORS information based on the given header names. If the CORS preflight response is dynamic, all request headers resulting in conditional preflight responses must be added to the Vary response header to avoid poisoning intermediate caches.

Actual response

Here is what an actual response will look like, after a successful preflight request:

HTTP/1.1 200 OK
Content-Type: application/json
Access-Control-Expose-Headers: Grpc-Status,Grpc-Message,Grpc-Status-Details-Bin
[other headers elided...]

{"greeting": "Hello, Buf!"}

All three exposed headers are necessary to support gRPC-Web. Content-Type does not need to be exposed explicitly, because it is a CORS-safelisted response header.

Note: If your application uses custom response headers, you must explicitly expose them.
If your application uses custom response trailers, you must expose them with a Trailer- prefix, because Connect sends trailers for unary RPCs as headers. If your service sets the trailer Bar, the response must expose Trailer-Bar.

Connect GET

For side-effect-free RPCs such as simple queries, Connect supports GET requests that do not require CORS preflights. Browsers will immediately issue the actual request, but there are two requirements:

  1. No request headers must be set, including Connect-Timeout-Ms or Grpc-Timeout, or any application-specific header.
  2. The response must allow the origin, for example with the response header Access-Control-Allow-Origin:

If the first requirement is not met, a preflight request is issued. If the second requirement is not met, the request will error in the browser.

Avoid wildcards

While many CORS headers allow a wildcard *, they only do for requests without credentials, such as cookies, TLS client certificates, or authorization headers.

It's best to avoid wildcards in production. CORS libraries typically provide mechanism for common use cases like allowing multiple origins.

Private network access

Private network access is an addition to CORS that restricts the ability of websites to send requests to servers on private networks. Chrome has been the first to implement the feature. More information can be found in the accompanying blog post.

If your application uses private network access, you will have to set the preflight response header Access-Control-Allow-Private-Network accordingly, and add Vary: Access-Control-Request-Private-Network in case you cache CORS information.