> Avoiding it is fairly easy, by isolating all of the extension’s user interface in an element. Right, but if the social network website can modify the HTML that the Keybase extension is injecting, then surely it can also modify the iframe's URL to an attacker-controlled one? Or, for that matter, replace the event handler on the "Keybase Chat" button itself before it even gets clicked? I'm not an extension developer,…
Keybase could minimize that by showing the user's name and/or logo in the iframe. Barring another vulnerability, the site shouldn't know who is logged in into the extension, so they shouldn't be able to fake that.