openssh bugs - Dec 2015 - [Bug 2516] New: ssh client shouldn't trust the DNS AD bit blindly

https://bugzilla.mindrot.org/show_bug.cgi?id=2516

            Bug ID: 2516
           Summary: ssh client shouldn't trust the DNS AD bit blindly
           Product: Portable OpenSSH
           Version: 7.1p1
          Hardware: All
                OS: All
            Status: NEW
          Severity: security
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: scott-mindrot at shambarger.net

I've been working on getting DNSSEC local validation working on OSX
with ldns (see bug 2119), and I see that the code for libresolv and
libldns both trust the AD bit in DNS responses for the SSHFP by
default.
>From RFC 4035 section 4.6,
   A resolver MUST disregard the meaning of the CD and AD bits in a
   response unless the response was obtained by using a secure channel
   or the resolver was specifically configured to regard the message
   header bits without using a secure channel.

My coffee house here happily sets the AD bit on DNSSEC answers, but I
wouldn't trust that they did the correct validation... which means I
could easily be auto-accepting server fingerprints for MITM hosts into
which I may type my password (ie "Bad(tm)")

AD bit might be useful, but not in an untrusted environment, and
currently openssh doesn't have any way to know if it can trust the AD
bit it gets, so per the RFC, it should probably ignore it unless it's
explicitly configured to do otherwise.

I suggest only treating the SSHFP as secure if the DNS response is
locally validated (eg with ldns and a locally stored root anchor), and
perhaps supporting the AD bit only if a (new) host config indicates
that it's ok.

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https://bugzilla.mindrot.org/show_bug.cgi?id=2516

Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |djm at mindrot.org

--- Comment #1 from Damien Miller <djm at mindrot.org> ---
That RFC advice is irrelevant to OpenSSH because OpenSSH isn't a
resolver. OpenSSH talks to a resolver via libc interfaces (or
optionally ldns) and its up to the resolver in use to determine the
trustworthyness of AD.

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https://bugzilla.mindrot.org/show_bug.cgi?id=2516

--- Comment #2 from scott-mindrot at shambarger.net ---
The ssh process, (through libresolv, libldns or whatever), is
processing a DNS packet from an untrusted third party resolver (it
sends and receives DNS packets directly with the resolv.conf server, eg
coffee shop router)... not sure who else is supposed to decide that the
AD bit is untrusted at that point?

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https://bugzilla.mindrot.org/show_bug.cgi?id=2516

Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|NEW                         |RESOLVED

--- Comment #3 from Damien Miller <djm at mindrot.org> ---
Like I said, it's up to the resolver code. OpenSSH doesn't implement
name resolution.

We couldn't implement this even if we wanted to*, as the libc resolver
doesn't expose the AD information to its callers. 

* short of implementing a full resolver in OpenSSH, which isn't going
to happen.

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Damien Miller <djm at mindrot.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |CLOSED

--- Comment #4 from Damien Miller <djm at mindrot.org> ---
Close all resolved bugs after 7.3p1 release

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