FIDO2 user verification impossible when using the ssh agent

[Andrea Ieri]

I am having trouble setting up FIDO2 ssh keys with my yubikey 5C NFC if I want to enable user verification (user presence works fine).

Steps to reproduce:

= Prep work =

Client (kinetic):
* generate a key that requires user verification:
  $ ssh-keygen -t ed25519-sk -f ~/.ssh/id_ed25519_verify_sk -O verify-required -C "this key requires UV"
  [provide your authenticator PIN, touch it, and add an encryption password]

Server (jammy):
* add id_ed25519_verify_sk.pub to authorized_keys

= Symptoms =

Shell 1 (w/ssh agent):

$ eval $(ssh-agent)
Agent pid 3279738

$ ssh-add ~/.ssh/id_ed25519_verify_sk
Enter passphrase for /home/aieri/.ssh/id_ed25519_verify_sk:
Identity added: /home/aieri/.ssh/id_ed25519_verify_sk (this key requires UV)

$ ssh ubuntu@10.35.202.231 -i ~/.ssh/id_ed25519_verify_sk "echo FIDO2 fails\!"
sign_and_send_pubkey: signing failed for ED25519-SK "/home/aieri/.ssh/id_ed25519_verify_sk" from agent: agent refused operation
ubuntu@10.35.202.231: Permission denied (publickey).

[note that the above is printed immediately, and that the yubikey does not light up]

Shell 2 (no ssh agent):

$ ssh ubuntu@10.35.202.231 -i ~/.ssh/id_ed25519_verify_sk "echo FIDO2 works\!"
Enter passphrase for key '/home/aieri/.ssh/id_ed25519_verify_sk':
Enter PIN for ED25519-SK key /home/aieri/.ssh/id_ed25519_verify_sk:
Confirm user presence for key ED25519-SK SHA256:nhgS4c2rGtE7XKeez3rAsofrjJvsL6rmBLShZxfTXIY
User presence confirmed
FIDO2 works!

NOTE:
* user _presence_ can be validated correctly with or without the ssh-agent: keys generated without `-O verify-required` work as expected (aside from bug 1869897)

[Lena Voytek (lvoytek)]

Thank you for submitting this report. I attempted to verify on a fresh install of Kinetic as a client and Jammy as a server using a Yubikey Bio. ssh login worked for me both with and without ssh-agent active. I unfortunately don't have a 5c to test with and the issue may lie specifically with that. Have you tried this with multiple new ssh keys to confirm the issue?

[Andrea Ieri (aieri)]

hi Lena, actually yes, I can reproduce the same issue with a nitrokey 3A NFC. I also have an older yubikey 5A but I cannot test with it as its firmware does not support user verification, only user presence.

[Andreas Hasenack (ahasenack)]

I believe this issue is correct. I noticed this when writing up the documentation[1] on how to use openssh with fido2 resident keys:

"""
NOTE
If you used the -O verify-required option when generating the keys, or if that option is set on the SSH server via /etc/ssh/sshd_config’s PubkeyAuthOptions verify-required, then using the agent currently in Ubuntu 22.04 LTS won’t work.
"""

I remember I found an upstream bug about this, but it is a bit muddy because there is ssh-agent from ssh, and there is one from gnome (and I think from gpg-agent as well; again, muddy).

If I find those pointers again, I'll post them here.

1. https://ubuntu.com/server/docs/service-openssh

[Andreas Hasenack (ahasenack)]

Some references I found back then:

https://gitlab.gnome.org/GNOME/gnome-keyring/-/issues/101

https://github.com/openssh/openssh-portable/commit/39d17e189f8e72c34c722579d8d4e701fa5132da

From my chat messages:
plain ssh-agent on kinetic worked with verify-required keys
but I had to install ssh-askpass (ugly X11 interface)
without it, it fails
gnome-keyring's ssh-agent doesn't seem to support PIN entry for verify-required keys
that's a lot of exceptions to list in the docs: a) newer openssh-client; b) ssh-askpass-gnome installed; c) use ssh-agent, not gnome-keyring (gnome-keyring is our default)

So IIRC, it worked with kinetic's openssh ssh-agent, not ssh-agent from elsewhere (that would be the gnome-keyring bug linked above).

[denos (shane-systemnexus)]

Just a note that the pin prompt works under Ubuntu 23.04 after installing ssh-askpass-gnome. Yubico - Security Key C NFC used.