Add support for having specified domain instead of wildcard

[dani2819]

What
We have a use case where we want to allow all domains * as origin, but we want library to set exact domain value (req.headers.origin) instead of * in Access-Control-Allow-Origin header.

The use-case comes as we also need to send credentials such as Cookies to the server. And, with * as Access-Control-Allow-Origin, you can't send credentials to the server.

It can be done by adding another option such as exactOriginIfMatches: true:

var corsOptions = {
  origin: '*',
  exactOriginIfMatches: true,
}

Current Behaviour*

var corsOptions = {
  origin: '*',
};

Request comes from http://example.com -> Library sets `Access-Control-Allow-Origin: *`

Proposed Behaviour*

var corsOptions = {
  origin: '*',
  exactOriginIfMatches: true,
};

Request comes from http://example.com -> Library sets `Access-Control-Allow-Origin: http://example.com`

var corsOptions = {
  origin: '*',
  exactOriginIfMatches: false,
};

Request comes from http://example.com -> Library sets `Access-Control-Allow-Origin: *`

In that case, the behaviour will be same but the value of Access-Control-Allow-Origin will be req.headers.origin instead of *. It will be helpful in sending credentials to the server.

Let me know how does it sound? Will be happy to open a PR if that makes sense!

[pinko-fowle]

I was digging through MDN docs on CORS, and happened to notice a specific CORS error that suggests an extra-partiuclar reason something like this might be useful:

"Reason: Credential is not supported if the CORS header 'Access-Control-Allow-Origin' is '*"
https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS/Errors/CORSNotSupportingCredentials

That would make something like this handy.

In the meantime, a second handler after the fact should help workaround this:

app.use(function setSpecificOrigin(req, res, next) {
  if (res.get('access-control-allow-origin') === '*') {
    res.set('access-control-allow-origin', req.get('origin'))
  }
  next()
})

[dougwilson]

Hi, yes, it is a dance with the sec researchers, as they do want things to be secure, but also want cves under their name.

And yes, if the response header of access-control-allow-origin is an asterisk to a preflight request, then a cors based client makes the second, real request it will not include credentials (cookie and authentication headers) with it, even if access-control-allow-credentials is true.

[dougwilson]

If you want credentials to work from any origin, this module you can set origin: true to reflect the origin instead of using '*' https://github.com/expressjs/cors?tab=readme-ov-file#configuration-options