Pluggable Authentication Module (PAM) Universal 2nd Factor (U2F)

This module implements PAM over U2F and FIDO2, providing an easy way to integrate the YubiKey (or other U2F/FIDO2 compliant authenticators) into your existing infrastructure.

Building

You may get signed release tarballs from Yubico’s release page.

This project uses 'autoconf', 'automake', 'pkg-config' and 'libtool' to achieve portability and ease of use.

In addition, 'libfido2' (>= 1.3.0) is needed. Versions of this project up to 1.0.8 used libu2f-host and libu2f-server. On Ubuntu, the necessary dependencies can be installed using

# apt install --no-install-recommends autoconf automake libtool pkg-config \
    libfido2-dev libpam-dev libssl-dev

If you downloaded a tarball, build it as follows.

$ ./configure
$ make

Building from Git

You may check out the sources using Git with the following command:

$ git clone https://github.com/Yubico/pam-u2f.git

This will create a directory 'pam-u2f'. Enter the directory:

$ cd pam-u2f

autoconf, automake, libtool, and libpam must be installed. AsciiDoc and xsltproc are used to generate the manpages. On Ubuntu, the necessary dependencies can be installed using

# apt install --no-install-recommends autoconf automake libtool \
    pkg-config libfido2-dev libpam-dev libssl-dev asciidoc xsltproc \
    libxml2-utils docbook-xml

On Fedora, the necessary dependencies can be installed using

# dnf -y install autoconf automake libtool libfido2-devel pam-devel \
    openssl-devel asciidoc

Generate the build system using:

$ autoreconf --install

Then build as usual, see above under Building.

Installation

Once the module is built, copy the file pam_u2f.so to the correct directory for your system. Typically /lib/security/ or /lib/x86_64-linux-gnu/security/. This is automated by make install assuming that the pam directory chosen by configure is correct. If that is not the case it can be specified with ./configure --with-pam-dir=.

Service Configuration

Create a file for a new service in /etc/pam.d/ or edit an already existing one by adding a line similar to this:

auth sufficient pam_u2f.so debug

For more information about the syntax of PAM configuration files, please see the manual page for pam.conf(5). Additional example configurations can be found below.

Important

An erroneous PAM configuration may lock some or all users out of the system or prevent you from gaining root privileges. It is recommended that you start a separate shell with root privileges while configuring PAM to be able to revert changes if something goes wrong. Test your configuration thoroughly before closing the root shell.

Module Arguments

debug	Enables debug output
debug_file	Filename to write debugging messages to. If this file is missing, nothing will be logged. This regular file has to be created by the user or must exist and be a regular file for anything getting logged to it. It is not created by pam-u2f on purpose (for security considerations). This filename may be alternatively set to "stderr" (default), "stdout", or "syslog".
origin=origin	Set the relying party ID for the FIDO authentication procedure. If no value is specified, the identifier "pam://$HOSTNAME" is used.
appid=appid	Set the application ID for the FIDO authentication procedure. If no value is specified, the same value used for origin is taken ("pam://$HOSTNAME" if also origin is not specified). This setting is only applicable for U2F credentials created with pamu2fcfg versions v1.0.8 or earlier. Note that on v1.1.0 and v1.1.1 of pam-u2f, handling of this setting was temporarily broken if the value was not the same as the value of origin.
authfile=file	Set the location of the file that holds the mappings of user names to keyHandles and user keys. An individual (per user) file may be configured relative to the users' home dirs, e.g. ".ssh/u2f_keys". If not specified, the location defaults to $XDG_CONFIG_HOME/Yubico/u2f_keys. If $XDG_CONFIG_HOME is not set, $HOME/.config/Yubico/u2f_keys is used.
expand	Enables variable expansion within the authfile path: %u is expanded to the local user name (PAM_USER) and %% is expanded to %. Unknown expansion sequences result in an authentication error. See also openasuser.
authpending_file=file	Set the location of the file that is used for touch request notifications. This file will be opened when pam-u2f starts waiting for a user to touch the device, and will be closed when it no longer waits for a touch. Use inotify to listen on these events, or a more high-level tool like yubikey-touch-detector. Note that yubikey-touch-detector v1.5.1 and later no longer rely on the authpending file for its detection mechanism. Set an empty value in order to disable this functionality, like so: authpending_file=. Default value: /var/run/user/$UID/pam-u2f-authpending
nouserok	Set to enable authentication attempts to succeed even if the user trying to authenticate is not found inside authfile or if authfile is missing/malformed.
openasuser	Setuid to the authenticating user when opening the authfile. Useful when the user’s home is stored on an NFS volume mounted with the root_squash option (which maps root to nobody which will not be able to read the file). Note that after release 1.0.8 this is done by default when no global authfile (path is absolute) or XDG_CONFIG_HOME environment variable has been set.
alwaysok	Set to enable all authentication attempts to succeed (aka presentation mode).
max_devices=n_devices	Maximum number of devices (credentials) allowed per user (default is 24). Devices specified in the authorization mapping file that exceed this value will be ignored.
interactive	Set to prompt a message and wait before testing the presence of a FIDO device. Recommended if your device doesn’t have a tactile trigger.
[prompt=your prompt here]	Set individual prompt message for interactive mode. Watch the square brackets around this parameter to get spaces correctly recognized by PAM.
manual	Set to drop to a manual console where challenges are printed on screen and response read from standard input. Useful for debugging and SSH sessions without U2F-support from the SSH client/server. If enabled, interactive mode becomes redundant and has no effect.
cue	Set to prompt a message to remind to touch the device.
[cue_prompt=your prompt here]	Set individual prompt message for the cue option. Watch the square brackets around this parameter to get spaces correctly recognized by PAM.
nodetect	Set to skip detecting if a suitable FIDO token is inserted before performing the full tactile authentication. This detection was created to avoid emitting the "cue" message if no suitable token exists, because doing so leaks information about the authentication stack if a token is inserted but not configured for the authenticating user. However, it was found that versions of libu2f-host 1.1.5 or less has buggy iteration/sleep behavior which causes a 1-second delay to occur for this initial detection. For this reason, as well as the possibility of hypothetical tokens that do not tolerate this double authentication, the "nodetect" option was added.
userpresence=int	If 1, request user presence during authentication. If 0, do not request user presence during authentication. If omitted, fallback to the authenticator’s default behaviour.
userverification=int	If 1, request user verification during authentication (e.g. biometrics). If 0, do not request user verification during authentication. If omitted, fallback to the authenticator’s default behaviour. If enabled, an authenticator with support for FIDO2 user verification is required.
pinverification=int	If 1, request PIN verification during authentication. If 0, do not request PIN verification during authentication. If omitted, fallback to the authenticator’s default behaviour. If enabled, an authenticator with support for a FIDO2 PIN is required.
sshformat	Use credentials produced by versions of OpenSSH that have support for FIDO devices. It is not possible to mix native credentials and SSH credentials. Once this option is enabled all credentials will be parsed as SSH.

Important

On dynamic networks (e.g. where hostnames are set by DHCP), users should not rely on the default origin and appid ("pam://$HOSTNAME") but set those parameters explicitly to the same value.

Example Service Configurations

Second Factor Authentication

Configure pam-u2f as a required module after your primary authentication module(s) for use as a second factor. Make sure that the primary authentication method is not sufficient or uses other control values that may preempt execution of pam-u2f.

# Primary authentication method(s) above this line.
auth required pam_u2f.so authfile=/etc/u2f_mappings cue

Passwordless Authentication

For a passwordless experience, where the authenticator PIN can be used in place of the user password, you can insert the below line towards the top of your service configuration. You may need to change the control value to something else if you’d like to execute additional authentication modules after pam-u2f.

auth sufficient pam_u2f.so authfile=/etc/u2f_mappings cue pinverification=1
# Fallback modules go below this line (e.g. password authentication).

Passwordless Authentication using Biometrics

Similar to the previous configuration but capable of built-in user verification, e.g. fingerprint matching using the YubiKey Bio. This example falls back to using PIN verification if the fingerprint does not match or is otherwise blocked.

auth sufficient pam_u2f.so authfile=/etc/u2f_mappings cue pinverification=0 userverification=1
auth sufficient pam_u2f.so authfile=/etc/u2f_mappings cue pinverification=1 userverification=0
# More fallback modules go below this line (e.g. password authentication).

Authorization Mapping Files

A mapping must be made between the YubiKey token and the user name, see here for details on how to perform the registration using the bundled tool.

There are two ways to do this, either centrally in one file, or individually, where users can create the mapping in their home directories. If the central authorization mapping file is being used, user home directory mappings will not be used and the opposite applies if user home directory mappings are being used, the central authorization mappings file will not be used.

By default the mapping file inside a home directory will be opened as the target user, whereas the central file will be opened as root. If the XDG_CONFIG_HOME variable is set, privileges will not be dropped unless the openasuser configuration setting is set.

Important

Using pam-u2f to secure the login to a computer while storing the mapping file in an encrypted home directory, will result in the impossibility of logging into the system. The partition is decrypted after login and the mapping file can not be accessed.

Central Authorization Mapping

Create a file e.g. /etc/u2f_mappings. The file must contain a user name, and the information obtained during the registration procedure.

The mappings should look like this, one per line:

<username1>:<KeyHandle1>,<UserKey1>,<CoseType1>,<Options1>:<KeyHandle2>,<UserKey2>,<CoseType2>,<Options2>:...
<username2>:<KeyHandle1>,<UserKey1>,<CoseType1>,<Options1>:<KeyHandle2>,<UserKey2>,<CoseType2>,<Options2>:...

Now add authfile=/etc/u2f_mappings to your PAM configuration line, so it looks like:

auth sufficient pam_u2f.so authfile=/etc/u2f_mappings

If you do not set the openasuser setting, the authfile will be opened and parsed as root so make sure it has the correct owner and permissions set.

Individual Authorization Mapping by User

Each user creates a .config/Yubico/u2f_keys (default) file inside their home directory and places the mapping in that file. You may want to specify a different per-user file (relative to the users' home directory), i.e. .ssh/u2f_keys. Bear in mind, setting an absolute path here is possible although very likely a fragile setup, and probably not exhibiting the intended behaviour.

The file must have only one line:

<username>:<KeyHandle1>,<UserKey1>,<CoseType1>,<Options1>:<KeyHandle2>,<UserKey2>,<CoseType2>,<Options2>:...

This is much the same concept as the SSH authorized_keys file.

In this case, pam-u2f will drop privileges and read the mapping file as that user. This happens regardless of the openasuser option being set.

Note that if you set the XDG_CONFIG_HOME variable, privileges will not be dropped by default. Consider also setting openasuser in that case.

Obtaining Key Handles and Public Keys

In order to obtain the required information for the authentication procedure, a token should be first registered. This can be done by using the command line configuration tool provided with the module:

$ pamu2fcfg -uusername -opam://myorigin -ipam://myappid

the tool will register a connected token by using the specified origin and appid. If neither are specified they will default to pam://$HOSTNAME. Additionally, it is possible to set other options to require user presence (touch), PIN verification and resident credentials. On success the tool prints to standard output a configuration line that can be directly used with the module. For additional information on the tool read the relative manpage (man pamu2fcfg).

For authenticator management (e.g. setting a PIN, enrolling fingerprints, and more), please refer to fido2-token, yubikey-manager, or some other suitable tool.

SSH Credentials

To generate SSH credentials OpenSSH version 8.2 or later is required. It is then possible to generate a credential file with:

$ ssh-keygen -t ecdsa-sk -f ./credential.ssh

Supported key types are ecdsa-sk and ed25519-sk. Note that passphrase protected credentials are currently not supported.

To use this credential the authfile parameter should be set to the path of the file credential.ssh and the sshformat option should also be set. If the authfile parameter is not set, it defaults to ~/.ssh/id_ecdsa_sk.

Multiple Devices

Multiple devices (credentials) are supported. If more than one credential is specified, authentication against them is attempted sequentially as they are defined in the authorization mapping file. If during an authentication attempt a connected device is removed or a new device is plugged in, the authentication restarts from the top of the list.

SELinux Note

Due to an issue with Fedora Linux, and possibly with other distributions that use SELinux, a system configured with pam-u2f may end up in a situation where access to the credentials file is denied. If the nouserok option is also set, this will result in a successful authentication within the module, without using the FIDO authenticator.

In order to correctly update the security context the command fixfiles onboot should be used on existing installations

Moreover, to allow read access to an authfile or directory placed in a non-standard location, the command

# chcon -R -t auth_home_t /path/to/target

should be used.

For more information see HERE.

FIDO U2F vs FIDO2

Devices that solely support FIDO U2F and not FIDO2, e.g. the YubiKey 4 series, can be used only in conjunction with compatible features. Enabling incompatible features, such as setting the +pin or the +verification flags in the authfile or the corresponding options in the PAM service configuration causes the device to be ignored.