pam_u2f - Module for U2F authentication
The module provides U2F authentication against Yubikeys and other
compliant authenticators.
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 origin for the FIDO authentication procedure. If
no value is specified, the origin "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).
authfile=file
Set the location of the file that holds the mappings of
user names to keyHandles and user keys. The format is
username:keyHandle1,public_key1:keyHandle2,public_key2:... the default
location of the file is $XDG_CONFIG_HOME/Yubico/u2f_keys. If the environment
variable is not set, $HOME/.config/Yubico/u2f_keys is used. An individual (per
user) file may be configured relative to the users' home dirs, i.e.
".ssh/u2f_keys".
authpending_file=file
Set the location of the file that is used for touch
request notifications. This file will be opened when pam-u2f starts waiting
for a user to touch the device, and will be closed when it no longer waits for
a touch. Use inotify to listen on these events, or a more high-level tool like
yubikey-touch-detector. Default value: /var/run/user/$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 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 allowed per user (default is
24). Devices specified in the authentication 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. Otherwise, fallback to the
authenticator’s default behaviour.
userverification=int
If 1, require user verification during authentication. If
0, do not request user verification during authentication. Otherwise, fallback
to the authenticator’s default behaviour.
pinverification=int
If 1, require PIN verification during authentication. If
0, do not request PIN verification during authentication. Otherwise, fallback
to the authenticator’s default behaviour.
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.
auth sufficient pam_u2f.so debug origin=pam://$HOSTNAME
appid=pam://$HOSTNAME
auth required pam_u2f.so origin=http://example.com
appid=http://example.com authfile=/etc/yubikey_mappings
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.
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.
Report pam-u2f bugs in the issue tracker:
https://github.com/Yubico/pam-u2f/issues
pam(7)
The pam-u2f home page: https://developers.yubico.com/pam-u2f/
YubiKeys can be obtained from Yubico: http://www.yubico.com/