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Mozilla Foundation Security Advisory 2010-33

Title: User tracking across sites using Math.random()
Impact: Low
Announced: June 22, 2010
Reporter: Amit Klein
Products: Firefox, SeaMonkey

Fixed in: Firefox 3.6.4, 3.6.9
  Firefox 3.5.10, 3.5.12
  SeaMonkey 2.0.5


Security researcher Amit Klein reported that it was possible to reverse engineer the value used to seed Math.random(). Since the pseudo-random number generator was only seeded once per browsing session, this seed value could be used as a unique token to identify and track users across different web sites.

Update (October 27, 2010): After the Firefox 3.6.4 and Firefox 3.5.10 releases, Amit Klein reported that there was an additional unfixed case where user tracking could occur using the above-mentioned technique and a pop-up window or iframe that was subsequently navigated by the user. This additional variant is identified as CVE-2010-3171.