[tor-bugs] #26037 [Core Tor/Tor]: DirAuths should check vote signatures before parsing

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#26037: DirAuths should check vote signatures before parsing
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     Reporter:  isis          |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.3.5.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-security, tor-crypto
Actual Points:                |  Parent ID:
       Points:  2             |   Reviewer:
      Sponsor:                |
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 teor pointed out that vote parsing occurs before checking the votes
 signature (both verifying the signature and ensuring that it comes from a
 known valid directory authority). dgoulet confirmed this is the case:

 > See dirvote.c, function dirvote_add_vote(). You will notice that the
 very first thing is parsing the whole thing with
 networkstatus_parse_vote_from_string(). Now, as far as I can tell, the
 voter signature check happens in that function. However, by the time we
 check it out, we've tokenized the votes and parsed _many_ parts of the
 vote already. (If you look for check_signature_token() in that function).
 >
 > And then once we are done parsing, we do have a valid signature for the
 vote which then make us check if we know the authority with
 trusteddirserver_get_by_v3_auth_digest().

 The issue of anyone being able to trigger a hypothetical vulnerability in
 one of the parsing functions aside, it's also just simply not efficient to
 do all the parsing work and then chuck the results at the end of
 `networkstatus_parse_vote_from_string()` if the signature wasn't from a
 valid sig from a known authority.

 This issue has been apparently been present since f4ce7f9c9b4 in
 tor-0.2.0.3-alpha.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26037>
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