[tor-bugs] #17207 [Tor Browser]: Testing navigator.mimeTypes for known names can reveal info and increase fingerprinting risk

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#17207: Testing navigator.mimeTypes for known names can reveal info and increase
fingerprinting risk
-------------------------------------------------+-------------------------
 Reporter:  TemporaryNick                        |          Owner:
     Type:  defect                               |  arthuredelstein
 Priority:  High                                 |         Status:
Component:  Tor Browser                          |  assigned
 Severity:  Blocker                              |      Milestone:
 Keywords:  tbb-fingerprinting,                  |        Version:
  TorBrowserTeam201510                           |     Resolution:
Parent ID:                                       |  Actual Points:
  Sponsor:                                       |         Points:
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Changes (by teor):

 * severity:   => Blocker


Comment:

 Replying to [comment:9 arthuredelstein]:
 > Replying to [comment:7 gk]:
 > > Good stuff! I thought we already had a defense against this but I
 could not find anything so far.
 >
 > It's interesting that `navigator.mimeTypes.length == 0`. So one would
 have thought it didn't have any members.

 It seems that a partial anti-enumeration design is in place, but only
 against positional iteration. (As well as setting
 `navigator.mimeTypes.length` to `0`, all indexes of the form
 `navigator.mimeTypes[0]` return `undefined`.)

 But that doesn't stop lookups using an external list of MIME type names.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17207#comment:10>
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