PAM_U2F(8)

NAME
pam_u2f - Module for U2F authentication

SYNOPSIS

pam_u2f [...]

DESCRIPTION

The module provides U2F authentication against Yubikeys and other compliant authenticators.

OPTIONS

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 FIDO 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. The authfile format is <username>:<KeyHandle1>,<UserKey1>,<CoseType1>,<Options1>:<KeyHandle2>,<UserKey2>,<CoseType2>,<Options2>:...

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

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 FIDO authenticators allowed per user (default is 24). FIDO authenticators 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 authenticator. Recommended if your authenticator doesn’t have 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 FIDO 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 FIDO authenticator.

[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
    Skip detecting if a suitable FIDO authenticator is inserted before performing a full authentication. See NOTES below.

userpresence=int
    If 1, require 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, require 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, require 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 authenticator. It is not possible to mix native credentials and SSH credentials. Once this option is enabled all credentials will be parsed as SSH.

conf=path/to/pam_u2f.conf
    Set an alternative location for the configuration file. The supplied path must be absolute and must correspond to an existing regular file. See CONFIGURATION FILE.

EXAMPLES

Second factor authentication deferring user verification configuration to the authorization mapping file.

    auth required pam_u2f.so authfile=/etc/u2f_mappings cue

Passwordless authentication enforcing PIN verification.

    auth sufficient pam_u2f.so authfile=/etc/u2f_mappings cue pinverification=1 userverification=0

Passwordless authentication using built-in UV (e.g. biometrics) with a fallback to PIN.

    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

CAVEATS

By default, relative paths to the authfile will be opened as the target user, whereas absolute paths 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.

CONFIGURATION FILE

A configuration file can be used to set the default module arguments.

    • The file has a name = value format, with comments starting with the # character.

    • White spaces at the beginning of line, end of line, and around the = sign are ignored.

    • Any conf argument in the configuration file is ignored.

    • The maximum size for the configuration file is 4 KiB.

    • The default path for the configuration file is /usr/local/etc/security/pam_u2f.conf. The default file is allowed to not exist. An alternative path may be set in the module command line options.

    • The options specified on the module command line override the values from the configuration file.

NOTES

Nodetect

The "nodetect" option should be used with caution. pam_u2f checks that a key configured for the user is inserted before performing the full tactile authentication. This detection is done by sending a "check-only" authentication request to all inserted tokens to so see if at least one of them responds affirmatively to one or more of the keyhandles configured for the user. By doing this, pam_u2f can avoid emitting the "cue" prompt (if configured), which can cause some confusing UI issues if the cue is emitted followed by the underlying library immediately failing the tactile authentication. This option is also useful to avoid an unintended 1-second delay prior to the tactile authentication caused by versions of libu2f-host <= 1.1.5.

If pam_u2f is configured to "cue" and "nodetect", an attacker can determine that pam_u2f is part of the authentication stack by inserting any random U2F token and performing an authentication attempt. In this scenario, the attacker would see the cue message followed by an immediate failure, whereas with detection enabled, the authentication will fail silently. Understand that an attacker could choose an authenticator that alerts him or her in some way to the "check-only" authentication attempt, so this precaution only pushes the issue back a step.

In summary, the detection feature was added to avoid confusing UI issues and to prevent leaking information about the authentication stack in very specific scenario when "cue" is configured. The "nodetect" option was added to avoid buggy sleep behavior in older versions of libu2f-host and for hypothetical tokens that do not tolerate the double authentication. Detection is performed, and likewise "nodetect" honored, regardless of whether "cue" is also specified.

SELinux

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/authfile

should be used.

For more information see https://access.redhat.com/security/cve/CVE-2020-24612.

BUGS

Report pam-u2f bugs in the issue tracker: https://github.com/Yubico/pam-u2f/issues

SEE ALSO

pam(7)

The pam-u2f home page: https://developers.yubico.com/pam-u2f/

YubiKeys can be obtained from Yubico: http://www.yubico.com/