[Bug 3320] New: Authentication hangs when SSH_ASKPASS_REQUIRE=prefer and remote host key fingerprint isn't known

[bugzilla-daemon at mindrot.org]

https://bugzilla.mindrot.org/show_bug.cgi?id=3320

            Bug ID: 3320
           Summary: Authentication hangs when SSH_ASKPASS_REQUIRE=prefer
                    and remote host key fingerprint isn't known
           Product: Portable OpenSSH
           Version: 8.6p1
          Hardware: 68k
                OS: Mac OS X
            Status: NEW
          Severity: normal
          Priority: P5
         Component: ssh
          Assignee: unassigned-bugs at mindrot.org
          Reporter: jack+portable-openssh-bugzilla at foys.net

Hello,

I set SSH_ASKPASS_REQUIRE=prefer so that git would use SSH_ASKPASS to
pull passphrases from an external keychain, even when run
interactively. I've unblocked myself from the problem below by setting
SSH_ASKPASS_REQUIRE only from .gitconfig.


Later, I discovered that with SSH_ASKPASS_REQUIRE=prefer, ssh hangs
when connecting to a new host. `ssh -v` shows the following, repeated:


debug1: read_passphrase: requested to askpass
debug1: read_passphrase: requested to askpass
debug1: read_passphrase: requested to askpass


After amending the custom askpass program to show its argv on stderr:


debug1: read_passphrase: requested to askpass
argv: The authenticity of host 'testhost (10.1.10.10)' can't be
established.
ED25519 key fingerprint is SHA256:<deleted>.
This key is not known by any other names
Are you sure you want to continue connecting (yes/no/[fingerprint])?
debug1: read_passphrase: requested to askpass
argv: Please type 'yes', 'no' or the fingerprint:
debug1: read_passphrase: requested to askpass
argv: Please type 'yes', 'no' or the fingerprint:
debug1: read_passphrase: requested to askpass
argv: Please type 'yes', 'no' or the fingerprint:
debug1: read_passphrase: requested to askpass
argv: Please type 'yes', 'no' or the fingerprint:


It looks like all interactive input goes through read_passphrase() in
readpass.c, whether or not it's a credential. Is this intentional?

Possibly related:
https://bugzilla.mindrot.org/show_bug.cgi?id=3314
https://bugzilla.mindrot.org/show_bug.cgi?id=3226

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