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

[Marco Trevisan (Treviño)]

Mh, yeah looks like the terminal filtering is something we should do in
openssh anyways, at various levels though.

Fact is that if a server is malicious, nothing prevents to do the same
through a simpler sshd banner or command, that is still able to act on
remote terminal.

It's true that PAM modules are sneakier than that, as they may be
checked less than the pure openssh server configuration, but still it's
all server duty, and if compromised (intentionally or not), it can be
still a driver for such issues in the current state.

ASCII based solutions for qr code are acceptable, they can just quite
big though. Being a problem if you use them from a tty. However that's
another possibility we were considering already.

Regarding upstream diversion, I totally agree (and I would have loved to
avoid it)... Ideally we had a plan, and the fix was proposed way earlier
upstream than downstream. So we were just waiting...

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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.

  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

  [ 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.

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