My web site uses magic links for login, however, I have a problem on mobile (not sure about Android - haven't tried yet, but the problem exists at least on iOS): when a user receives the email say in the GMail app, the link opens in the embedded browser, meaning that cookies will not be passed to the "real" browser.
Is there a way to ensure the link in the email opens in the real system browser and therefore cookies are stored permanently?
(Essentially browser session isolation on iOS breaks a lot of things on the Internet, so surely there is a workaround?)
After some research: no, it is not possible to detect you are in an embedded browser, neither is it possible to enforce opening a link in the system one. Makes sense from security point of view.
However, I was asking the wrong question. The problem of a magic link login is solved differently: when starting a login process you can set a session cookie and create an associated DB record for it, marking it as blocked, i.e. not logged in.
At next step, when the magic link is opened in whatever browser you unblock the session in the DB. At this stage you can replace the login cookie with a real one, e.g. JWT, or continue using it as your main auth token.
If the user then returns to the real browser, you check the login cookie and act accordingly, keeping in mind that it may not be the browser where they validated the magic link. At this stage you can, again, replace the login cookie with your real auth cookie knowing that the session has been validated already.
I'm not entirely sure this is 100% safe, need to think about it more but at first glance it does look safe and seems to be pretty much the only way of handling magic links on mobile.