Bug 1265785 (CVE-2015-7327)

Security researchers Yossef Oren, Vasileios P. Kemerlis, Simha Sethumadhavan,
Angelos D. Keromytis of Columbia University's Network Security Lab reported a
method of using the High Resolution Time API for side channel attacks. This
attack uses JavaScript loaded through a hostile web page to track access to
the last-level cache over a period of time as a user engages in other browser
activity. This attack takes advantage of the performance.now() API's use of
single nanosecond resolution for timing.

Security researcher Amit Klein independently reported use of the
performance.now() API on Windows systems to extract the Windows counter
frequency as an avenue for side channel attacks.

Both of these flaws allow for the disclosure of private information, user
fingerprinting, and data leakage. They have been addressed by reducing the
resolution of the performance.now()API to 5 microseconds to remove the
precision in resolution available to attackers.

The Windows counter frequency issue does not affect Linux or OS X systems.

External Reference:

https://www.mozilla.org/en-US/security/advisories/mfsa2015-114/