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Firefox Privacy Notice

Mozilla Foundation Security Advisory 2010-33

User tracking across sites using Math.random()

June 22, 2010
Amit Klein
Firefox, SeaMonkey
Fixed in
  • Firefox 3.5.10
  • Firefox 3.5.12
  • Firefox 3.6.4
  • Firefox 3.6.9
  • 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.