[tbb-dev] The impossible fights on anti-fingerprinting

Tom Ritter tom at ritter.vg
Thu Oct 19 05:33:12 UTC 2017

<mozilla hat>

As we add more and more coverage to privacy.resistFingerprinting in FF
Nightly and Beta, we're getting more and more breakage reports. This
is great. And it's showing us a few places we should think about more
deeply. We have a list we're collecting here:

1) User Agent

We round the user agent of the browser to the previous ESR version. So
FF 57 appears as FF 52.

This breaks Add-On installation:
https://bugzilla.mozilla.org/show_bug.cgi?id=1394448  Addons.Mozilla
uses the User-Agent header to detect if the user is able to install a
given addon and will or will not enable the install button based on

However, does spoofing the major version of the browser actually work?
I would argue: no. A website that wants to learn what version of
Firefox you're using can use feature detection. Every major release
we're adding CSS stuff, creating or enabling DOM apis by default, and
probably changing some subtelties of error messages.

Spoofing the minor version is still valuable; but we're considering
reporting the correct major version. What do you think?

2) OS

We report the OS as Windows on Mac and Linux.

This breaks google apps on mac: keyboard shortcuts are not recognized
because Windows is looking for a key modifier that isn't there.

It also gives desktop pages on mobile:

But is spoofing the OS even possible? You guys don't reward for it in
the bug bounty. I found your list of OS-fingerprinting bugs:

Of those, I'm guessing the Math routines are probably the hardest.
Also, this doesn't affect Tor Browser, but it does affect Firefox: you
can passively (or actively) fingerprint the OS by TCP/IP
characteristics: https://bugzilla.mozilla.org/show_bug.cgi?id=1409269

So I'm wondering, are there other OS-level fingerprinting vectors that
seem unsolvable that don't have tickets for them? What do you think of
reporting the correct OS (in FF at least), since it seems like we
wouldn't be able to hide it anyway?

For both of these Tor Browser will be able to do whatever it wants,
since this data is all controlled by prefs; but we'd value your
thoughts on these things for the FF use case.


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