[RaisingSpear]

> That's why I qualified it with "certificate-based". The private key never leaves the device

Except that phishing doesn't require the private key - it just needs to echo back the generated token. And even if that isn't possible, what stops it obtaining the session token that's sent back?

[smw]

Doesn't work for FIDO-based tokens, they auth the site as well, so won't send anything to phishing site.

[RaisingSpear]

From my understanding, FIDO isn't MFA though (the authenticator may present its own local challenge, but I don't think the remote party can mandate it).

There's also the issue of how many sites actually use it, as well as how it handles the loss of or inability to access private keys etc. I generally see stuff like 'recovery keys' being a solution, but now you're just back to a password, just with extra steps.

[mr_mitm]

The phisher will not receive a valid token, though, because you sign something that contains the domain you are authenticating to.

[RaisingSpear]

The phisher can just pass on whatever you sign, and capture the token the server sends back.

Sure, you can probably come up with some non-HTTPS scheme that can address this, but I don't see any site actually doing this, so you're back to the unrealistic scenario.