[tor-bugs] #25568 [Core Tor/Tor]: hs: Lookup failure cache when introducing to an intro point

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#25568: hs: Lookup failure cache when introducing to an intro point
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     Reporter:  dgoulet       |      Owner:  dgoulet
         Type:  defect        |     Status:  assigned
     Priority:  Medium        |  Milestone:  Tor: 0.3.4.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  security, tor-hs
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 It turns out that if a descriptor contains 10 times the same intro point,
 and that the first introduction attempt fails, we'll try to connect to the
 same failing intro point again for all subsequent remaining intro points.

 The intro point failure cache was introduced to avoid such a situation but
 it is only used between two descriptors that is if an intro point failed
 from the first descriptor and that intro point is still present in the
 second descriptor fetched, we ignore it.

 However, this situation is about the same intro point in the same
 descriptor. In normal circumstances, this can't happen but it is still
 allowed by the protocol.

 One issue with this is that a malicious service would induce many circuits
 out of the client than necessary. This can be used, theoretically, for a
 client guard discovery attack.

 This affects both v2 and v3.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25568>
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