>WebSocket. WebSocket servers are typically built over a few popular opensource frameworks, such as CocoaHTTPServer [3] and QtWebKit [14]. All of them provide a receiver method for getting messages from browser extensions, which is used by Xavus to fingerprint this channel, and a response method for replying to the extensions. The invocations of these two methods are identified as the claim and the use of the channel, respectively. Between them, the server is supposed to access the HTTP header Origin that includes extension IDs attached by the browser and check the signature of the browser through the API SecCodeCheckValidity. If these operations are not found, the app is considered vulnerable to the threat from a malicious extension or app. On the other hand, the attack from a malicious server against an extension cannot be detected through any existing APIs.

This sounds to me like the app validates the extension, but the extension can't validate the app. In that case, using a shared key helps.