pam_u2f(8)

Updated: 2025/Nov/16

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PAM_U2F(8) PAM U2F Module Manual 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 U2F 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. 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 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. 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 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
U2F device. Recommended if your device 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 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
Skip detecting if a suitable key 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 devices. It is not possible to mix native credentials and
SSH credentials. Once this option is enabled all credentials will
be parsed as SSH.

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.

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 U2F
authentication will fail silently. Understand that an attacker could
choose a U2F token 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