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I've tested a lot of websites, and I've found that many sites have a "go.example.com" subdomain that returns the following error message:

404 Not Found
The redirect url is empty

This message makes me think that you can somehow add a header/parameter to make this website redirect to any other site. If that is so, that would be an open redirect, which is a valid vulnerability.

Do these "go." subdomains have a header/parameter that they will redirect to?

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It really depends on the site. Good ones will have you pass an integer in i.e. go.example.com/1234/ which redirects to support.example.com or some such. An open redirect would accept something like go.example.com/badsite.com which would redirect to badsite.

But it is not so cut and dry, some redirect to a page that is passed in... hard to make a generalization here. The behavior I see most often is the passing an int, which checks a DB for where to redirect to.

No way to know but to test or ask the site owner.

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    Sometimes they'll also directly accept a URL and an HMAC to avoid database lookups. – Luke Park Jul 4 '18 at 21:55

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