Firefox 40+: what does the "Found hi-entropy localStorage" message mean?

Question

I updated to Firefox 40 today, and I see a neat new message in my Firebug console:

Found hi-entropy localStorage: 561.0263282209031  bits http://localhost:8080/my_app_path itemName

...where itemName is the name of a particular item I've stuck in localStorage.

The referenced line number is always unhelpful: the last one of the main HTML document (it is a single-page app).

Why does this happen? If you'd like an example of my "hi-entropy localStorage", here are the data in question:

Object {
 id: "c9796c88-8d22-4d33-9d13-dcfdf4bc879a",
 userId: 348,
 userName: "admin"
}

Answer 1

Your browser has the Privacy Badger plugin (1.0), which can detect some types of super-cookies and browser fingerprinting. It identified your local storage item as a false positive and produced those cryptic logs.

A high-entropy string can be vaguely defined as complicated, hard to guess/repeat, or likely to contain meaningful information. If there's such a string in your local storage (in your example, the item id), it's possible that advertisers put it there to uniquely identify you. Privacy Badger has rough methods to estimate a string's entropy, which the developers discuss here.

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You should check out the paper The Web never forgets: Persistent tracking mechanisms in the wild, particularly the section on cookie-syncing:

Cookie synchronization or cookie syncing is the practice of tracker domains passing pseudonymous IDs associated with a given user, typically stored in cookies, amongst each other.