(bcc security & privacy, please keep discussion on dev-webapi)

In the Idle API bug (https://bugzilla.mozilla.org/show_bug.cgi?id=715041), 
there was discussion around the privacy threat of websites correlating two 
anonymous identities by comparing system idle times. In response a 'fuzz' 
factor was introduced to make this attack less effective. It occurred to me 
that this sort of threat occurs anywhere we fire global events such as screen 
orientation, sensor events, power levels, network connection information etc, 
since a webpage could compare the timing or values of these events to correlate 
 two anonymous identities.

Personally I feel the privacy risk is low (likely, but low impact) - this is 
basically just an extension of fingerprinting, and there isn't a lot we can 
meaningfully do to reduce the attack. Adding 'fuzz' to these sensor events 
often directly reduces their usefulness. But I wanted to put this question to 
the list to get more viewpoints? 

Apart from reducing the resolution of these events, the only other mitigation I 
can't think of is restricting event delivery to foreground content, which may 
impact valid use cases.

Thoughts? 

(PS in any case, I think this is probably too low a risk to be worrying about 
for base camp, but just wanted to have the discussion.)
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