[Bug 2077576] Re: SSH client doesn't handle properly non-ASCII chars

[Steve Langasek]

Ok that's correct, at the time of review Tobias was a member of the
security team so I think we can take that as the necessary sign-off.

-- 
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to openssh in Ubuntu.
https://bugs.launchpad.net/bugs/2077576

Title:
  SSH client doesn't handle properly non-ASCII chars

Status in openssh package in Ubuntu:
  Fix Released
Status in openssh source package in Focal:
  Incomplete
Status in openssh source package in Jammy:
  Incomplete
Status in openssh source package in Noble:
  Fix Released

Bug description:
  [ Impact ]

  Non-ascii visible chars (including back-slashes, new lines and so) are
  not properly rendered by clients, showing their octal visualization.

  Such as:

    Hello SSHD \\ We love \360\237\215\225!

  Instead of:

    Hello SSHD \ We love 🍕!

  This is particularly an issue when a server has configured keyboard
  interactive authentication and a PAM module wants to show non-ASCII
  characters such as a QR code for web authentication:

  When using an ubuntu server running authd for web authentication we
  may end up having the login qrcode rendered such as

  \210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210
                          https://ubuntu.com
                                 1337

  Which is clearly unreadable.

  [ Test case ]

  ## Server preparation

  Enable PAM and keyboard interactive authentication in a ssh server:

  Add a configuration file such as:
   /etc/ssh/sshd_config.d/test-ssh-pam.conf

  Containing:

   UsePAM yes
   KbdInteractiveAuthentication yes# This was working already; here to check potential regressions
   ForceCommand bash -c "echo Hello from SSHD \ We also love 🍕!; $SHELL"

  It's also suggested to check for regressions using a `Banner` option
  in sshd, pointing to a file with utf-8 contents:

   echo "Hello" | qrencode -t ansiutf8 > /tmp/ssh-banner
   Banner /tmp/ssh-banner

  Restart the server:

    sudo systemctl restart ssh.service

  Edit the sshd PAM configuration file, adding as first line:

    auth    requisite pam_echo.so Hello SSHD \ We love 🍕!

  Can be done with the command:
    sudo sed '1 iauth    requisite pam_echo.so Hello SSHD! \\ We love 🍕!' \
     -i /etc/pam.d/sshd

  ## Client test

  In the same host:

   ssh -o PubkeyAuthentication=no \
       -o PasswordAuthentication=no \
       -o PreferredAuthentications=keyboard-interactive \
       $USER at localhost

  The client should show:

  Hello SSHD \ We love 🍕!
  ($USER at localhost) Password:
  ...
  Hello from SSHD \ We also love 🍕!
  █████████████████████████████
  █████████████████████████████
  ████ ▄▄▄▄▄ █▀█ █ █ ▄▄▄▄▄ ████
  ████ █   █ █▄█▄▄▀█ █   █ ████
  ████ █▄▄▄█ █   ▄▄█ █▄▄▄█ ████
  ████▄▄▄▄▄▄▄█ █▄█▄█▄▄▄▄▄▄▄████
  ████  █▄▀▀▄ █▀▄ ▄▀▄ ▄█▄ ▀████
  ████ ██▀▀ ▄▀▀▄▀▄▀▀    ▄▀ ████
  ████▄▄▄▄██▄▄  █▄█   ▀█▀██████
  ████ ▄▄▄▄▄ █▄▀▀▄▄█ ▀ ▄▄ ▀████
  ████ █   █ █▀█▀█▄  ▀▄▀▀▀ ████
  ████ █▄▄▄█ █▀ ▄ ▀▄▄█▄█▄█▄████
  ████▄▄▄▄▄▄▄█▄▄███▄█▄█▄█▄█████
  █████████████████████████████
  █████████████████████████████

  
  Retry the same with another host and without keyboard authentication enabled in the server side.

  To verify the fix in more complex scenario it's possible to follow the instructions of configuring authd:
   - https://github.com/ubuntu/authd/wiki/05--How%E2%80%90to-log-in-over-SSH

  Once authd is configured, the user should be able to scan a QrCode
  from a ssh session.

  ## Cleanup

  Revert the changes done in the cleanup phase, after test is done

  sudo sed '/pam_echo\.so/d' -i /etc/pam.d/sshd
  sudo rm /etc/ssh/sshd_config.d/test-ssh-pam.conf

  # Further testing

  It's also required to check if other configurations using keyboard
  interactive such as TOTP/HOTP access still work:

  https://ubuntu.com/server/docs/openssh-server#two-factor-
  authentication-with-totphotp

  # Authd testing

  - Configurand install authd and MsEntraID broker as described at:
    https://github.com/ubuntu/authd/wiki/01---Get-started-with-authd

  - Configure SSHd and try to login using the Qrcode as documented at
    https://github.com/ubuntu/authd/wiki/05--How%E2%80%90to-log-in-over-SSH

  - Accessing via SSH to a machine via QrCode should show a properly rendered
    qrcode

  [ Regression potential ]

  SSH info messages are not shown by the client. Even though those
  aren't covered by this change, it's important to check for regressions
  in any output that SSH exposes to the user. So banners and other
  messages should be checked for regressions.

  These kind of messages are normally shown only when PAM *and* keyboard
  interaction are enabled in the server side, so it should not affect
  the default ubuntu servers behavior.

To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/openssh/+bug/2077576/+subscriptions