[level3]

But does the page even need to get you to enter it? Could it be possible to set up a hidden password field for LastPass to monitor, and then run a dictionary attack in the background, waiting for the notification?

(I don't use LastPass, so I don't know anything about how this feature is designed.)

[DrewHintz]

This can be protected against by the extension looking at keyboardEvents and verifying the isTrusted attribute. Here are details about how we do this in Password Alert: https://github.com/google/password-alert/blob/master/SECURIT...

Before Chrome implemented isTrusted, it was a bit more tricky and we had to rely on a variety of attributes that did not have as much of a security guarantee.

[level3]

Thanks for the helpful explanation! Those seem like fair mitigations.

Reading more on it, though, since isTrusted can apparently be spoofed, it looks like the main obstacles are the (2) rate-limiting and the (3) intentional collisions.

For (2), I suspect typical users would have a memorizable master password that's more susceptible to brute forcing, but of course it depends on the actual rate limit and how long you can keep the script running. Alternatively, I suppose a malicious script could overwhelm the rate limit so that the user wouldn't receive a legitimate warning.

For (3), I wonder whether LastPass has a similar mitigation? From what I understand, they don't store the actual password, so all you would need is a matching hash.

I'd be interested to know more details about LastPass's protections.

Edit: I just saw pwman's response above.

[DrewHintz]

> isTrusted can apparently be spoofed

isTrusted cannot be spoofed in this situation, which is its intended use in Chrome. A Chrome extension in the isolated world is receiving events from the main world and checking isTrusted for those events.

[level3]

Thanks for the clarification. I assume the spoofability applies only to JS in the main world then? And extensions can receive the event before a script has a chance to fiddle with it?

Sorry for the questions, just trying to figure out how this all works, and Googling doesn't give me a clear answer.