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by Retr0id 113 days ago
I'm not sure if you understand what makes passkeys phishing-resistant?

The backdoored version of the app would need to have a different app ID, since the attacker does not have the legitimate publisher's signing keys. So the OS shouldn't let it access the legitimate app's credentials.
2 comments
Tharre 113 days ago
I understand how passkeys work. You don't need the legitimate app's credentials, we're talking about phishing attacks, you're trying to bring the victim to giving you access/control to their account without them realizing that that's what is happening.

A simple scenario adapted from the one given in the android blog post: the attacker calls the victim and convinces them that their banking account is compromised, and they need to act now to secure it. The scammer tells the victim, that their account got compromised because they're using and outdated version of the banking app that's no longer suppported. He then walks them through "updating" their app, effectively going through the "new device" workflow - except the new device is the same as the old one, just with the backdoored app.

You can prevent this with attestation of course, essentially giving the bank's backend the ability to verify that the credentials are actually tied to their app, and not some backdoored version. But now you have a "blessed" key that's in the hands of Google or Apple or whomever, and everyone who wants to run other operating systems or even just patched versions of official apps is out of luck.

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tadfisher 113 days ago
> He then walks them through "updating" their app, effectively going through the "new device" workflow - except the new device is the same as the old one, just with the backdoored app.

This is where the scheme breaks down: the new passkey credential can never be associated with the legitimate RP. The attacker will not be able to use the credential to sign in to the legitimate app/site and steal money.

The attacker controls the fake/backdoored app, but they do not control the signing key which is ultimately used to associate app <-> domain <-> passkey, and they do not control the system credentials service which checks this association. You don't even need attestation to prevent this scenario.

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Tharre 113 days ago
> do not control the signing key which is ultimately used to associate app <-> domain <-> passkey, and they do not control the system credentials service which checks this association.

You're assuming the attacker must go through the credential manager and the backing hardware, but that is only the case with attestation. Without it, the attacker can simply generate their own passkey in software, because the backend on the banks side would have no way of telling where the passkey came from.

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tadfisher 113 days ago
How did the service authenticate the user in order to create the new credential within the attacker-controlled app?
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Tharre 113 days ago
With banks, typically a combination of your account number, pin and some confirmation code sent via email or SMS. And of course unregistering your previous device. Not sure where you're going with this though?
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tadfisher 113 days ago
I am just pointing out that you are essentially saying passkeys can be phished because banks can allow phishable credentials to bypass passkeys.
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microtonal 113 days ago
I understand how passkeys work. You don't need the legitimate app's credentials, we're talking about phishing attacks, you're trying to bring the victim to giving you access/control to their account without them realizing that that's what is happening.

That doesn't work, because the scammer's app will be signed with a different key, so the relying party ID is different and the secure element (or whatever hardware backing you use), refuses to do the challenge-response.

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tadfisher 113 days ago
Correction: nothing prevents the attacker from using the app's legit package ID other than requiring the uninstall of the existing app.

The spoofed app can't request passkeys for the legit app because the legit app's domain is associated with the legit app's signing key fingerprint via .well-known/assetlinks.json, and the CredentialManager service checks that association.

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mwwaters 113 days ago
If the side loaded app does not have permission to use the passkeys and cannot somehow get the user to approve passkey access of the new app, that would be a good alternative to still allow custom apps.
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tadfisher 113 days ago
I don't think you understand. This exists _today_, regardless of how you install apps, because attackers can't spoof app signatures. If I don't have Bank of America's private signing key, I cannot make an app that requests passkeys for bankofamerica.com, because bankofamerica.com publishes a file [0] that says "only apps signed with this key fingerprint are allowed to request passkeys for bankofamerica.com" and Android's credential service checks that file.

No need for locking down the app ecosystem, no need to verify developers. Just don't use phishable credentials and you are not vulnerable to malware trying to phish credentials.

0: https://www.bankofamerica.com/.well-known/assetlinks.json

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