Universal 2nd Factor

Universal 2nd Factor (U2F) is an open standard that strengthens and simplifies two-factor authentication (2FA) using specialized USB or NFC devices based on similar security technology found in smart cards.

While initially developed by Google and Yubico, with contribution from NXP Semiconductors, the standard is now hosted by the FIDO Alliance.

For all articles on U2F and U2F-devices see: Category:Universal 2nd Factor.

WebAuthn is a more recent standard.

Authentication for websites

U2F is supported by major sites like Google, Facebook, Twitter, or GitHub. Check out 2fa.directory or dongleauth.com to find other websites and links to setup documentation. For all browsers which support it, likely the only action required is to install libfido2. Yubico offers a demo page for testing.

Firefox

Firefox/Tweaks#Fido U2F authentication

Chromium/Chrome

Chromium/Tips and tricks#U2F authentication

Authentication for user sessions

Yubico, the company creating the YubiKey, develops an U2F PAM module. It can be used to act as a second factor during login or replace the need for a password entirely.

Note: Despite the mentions of Yubico/YubiKey in this document, pam-u2f should also support any U2F and/or FIDO2 compliant keys.

Installing the PAM module

The module is part of the package pam-u2f.

Adding a key

Note: The folder name below is the default location. It can be changed for single user implementations by moving the file to your preferred directory and appending authfile=/path/to/u2f_keys to the end of the line for pam_u2f.so. This is also useful if you wish to move u2f_keys to a protected part of the file-system. For multi-user implementations use a central mapping file as explained in the official documentation of pam-u2f.

Keys need to be added with the tool pamu2fcfg:

$ mkdir ~/.config/Yubico
$ pamu2fcfg -o pam://hostname -i pam://hostname > ~/.config/Yubico/u2f_keys

After entering your PIN, click the button of your U2F key to confirm the key.

Note: If the hostname of your system changes, e.g. because of DHCP in different networks, you would be unable to login. In order to prevent that, it is recommended to specify the above mentioned options and replace hostname with the actual hostname.

If you own more than one key, append the next ones with

$ pamu2fcfg -o pam://hostname -i pam://hostname -n >> ~/.config/Yubico/u2f_keys

Note: The -n in the above command is required. It will omit the username portion of the generated entry as required in the spec for subsequent entries for the same user. Multiple entries with the same username will cause unpredictable behaviour in PAM. Also, do not add line breaks manually. The file u2f_keys must have only one line. [1]

Passwordless sudo

Warning: Before making any changes to your configuration, start a separate shell with root permissions (e.g. sudo -s). This way you can revert any changes if something goes wrong.

Add

/etc/pam.d/sudo

auth            sufficient      pam_u2f.so cue origin=pam://hostname appid=pam://hostname

as the first line. Be sure to replace the hostname as mentioned above. Then create a new terminal and type sudo ls. Your key's LED should flash and after clicking it the command is executed. The option cue is set to provide indication of what to do, i.e. Please touch the device.

In order to make the token the only method of sudo (ie. no password fallback) you will need to comment out the other auth methods present. This is usually just the default system-auth include.

/etc/pam.d/sudo

#auth           include         system-auth

You should also change sufficient to required in the above pam_u2f.so line.

Warning: This will mean that no user on the system will be able to sudo without a U2F token configured.

GDM login

Add:

/etc/pam.d/gdm-password

auth            required      pam_u2f.so nouserok origin=pam://hostname appid=pam://hostname

after the existing auth lines. Please note the use of the nouserok option which allows the rule to fail if the user did not configure a key. This way setups with multiple users where only some of them use a U2F key are supported.

Note: This method will not work with encrypted home partitions because the decryption is not done before the login process completed, so the u2f_keys file is unavailable. In this case use a central mapping file as explained in the official documentation of pam-u2f.

Some multi-function security keys (ex. Trezor Model T) which can do U2F / PAM may not advertise the feature on system boot which is intentionally out of the CTAP 2.0 specification[2]. With multiple U2F keys present, this can result in two-minute long delays when used with GDM as pam-u2f does sequential lookups and will wait for the (Trezor) device to timeout before offering presence / touch with the secondary U2F key [3]. You could try adding the nodetect option alongside debug and finish any device specific login (ex. screen PIN) before GDM loads.

SDDM/KDE

SDDM does not appear to support pam_u2f with initial user login. Autologin can be used instead, and then edit /etc/pam.d/kde to just control screen locking.

If you are using a U2F key with biometric authentication (e.g. Yubikey Bio) and want 1FA, use kde-fingerprint, commenting out the pam_fprintd.so line and placing your changes in its place. This avoids an unnecessary "unlock" button that gets displayed after authentication, rather than simply unlocking immediately[4].

Other authentication methods

Enable the PAM module for other services like explained above. For example, to secure the screensaver of Cinnamon, edit /etc/pam.d/cinnamon-screensaver.

For Polkit, copy the default configuration at /usr/lib/pam.d/polkit-1 to /etc/pam.d/polkit-1 and make your changes there.

Troubleshooting

If you managed to lock yourself out of the system, boot into recovery mode or from a USB pen drive. Then revert the changes in the PAM configuration and reboot.

In case the pam-u2f module silently fails, add debug keyword to the auth line in a file in /etc/pam.d/.

OpenSSH

OpenSSH ≥8.2 supports FIDO/U2F hardware tokens natively, see SSH keys#FIDO/U2F.

Data-at-rest encryption with LUKS

Since version 248, systemd can be use to unlock a LUKS partition using a FIDO2 key.

First, you will need to setup your /etc/crypttab file (see below), or customize your initramfs if you wish to unlock your root partition. The full procedure is similar to the use of a TPM chip for unlocking. See Trusted Platform Module#systemd-cryptenroll.

To register the key, you will need to use the systemd-cryptenroll utility. First, run the following command to list your detected keys:

$ systemd-cryptenroll --fido2-device=list

Then you can register the key in a LUKS slot, specifying auto value (or path to the FIDO2 device such as /dev/hidrawX if you have multiple):

$ systemd-cryptenroll --fido2-device=auto /dev/sdX

Non-root partitions

For a non-root data partition the crypttab would look like this:

/etc/crypttab

data /dev/sdX none fido2-device=auto

This should also work if your encrypted partition is a logical volume managed under LVM:

/etc/crypttab

data /dev/vg1/data none fido2-device=auto