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

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


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

In addition, both the Yubico U2F libu2f-host-dev and libu2f-server-dev libaries are needed.

Debian:   apt-get install autoconf automake libtool pkg-config libu2f-host-dev libu2f-server-dev --no-install-recommends

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 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.

Debian:   apt-get install autoconf automake libtool libpam-dev libu2f-host-dev libu2f-server-dev asciidoc xsltproc libxml2-utils docbook-xml --no-install-recommends

Generate the build system using:

  $ autoreconf --install

Then build as usual, see above under Building.


Once the module is built copy the file 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=.

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 debug

Supported parameters for the module are:


Enables debug output


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".


Set the origin for the U2F authentication procedure. If no value is specified, the origin "pam://$HOSTNAME" is used.


Set the application ID for the U2F authentication procedure. If no value is specified, the same value used for origin is taken ("pam://$HOSTNAME" if also origin is not specified).


Set the location of the file that holds the mappings of user names to keyHandles and user keys. The format is username:first_keyHandle,first_public_key: second_keyHandle,second_public_key:…​ the default location of the file is $XDG_CONFIG_HOME/Yubico/u2f_keys. If the environment variable is not set, $HOME/.config/Yubico/u2f_keys is used. (more on Authorization Mapping Files). An individual (per user) file may be configured, see Individual authorization mapping by user.


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. Set an empty value in order to disable this functionality, like so: authpending_file=. Default value: /var/run/user/$UID/pam-u2f-authpending


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.


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 or XDG_CONFIG_HOME environment variable has been set.


Set to enable all authentication attempts to succeed (aka presentation mode).


Maximum number of devices allowed per user (default is 24). Devices specified in the authentication file that exceed this value will be ignored.


Set to prompt a message and wait before testing the presence of a U2F 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.


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.


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.


Set to skip detecting if a suitable U2F 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-user 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.


If 1, request user presence during authentication. If 0, do not request user presence during authentication. Otherwise, fallback to the authenticator’s default behaviour.


If 1, request user verification during authentication. If 0, do not request user verification during authentication. Otherwise, fallback to the authenticator’s default behaviour.


If 1, request PIN verification during authentication. If 0, do not request PIN verification during authentication. Otherwise, fallback to the authenticator’s default behaviour.

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.

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, the number of registered Yubikeys and the information obtained during the registration procedure.

The mappings should look like this, one per line:


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

auth sufficient 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.

On dynamics 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.

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 dir), 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:


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. During the U2F registration, the user is required to touch the token. 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)

Multiple Devices

Multiple devices are supported. If more than one device is specified, authentication against them is attempted sequentially as they are defined in the configuration file of the module. 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.

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