Pluggable Authentication Module (PAM) Universal 2nd Factor (U2F) ================================================================ image:https://github.com/yubico/pam-u2f/workflows/linux/badge.svg["Linux Build Status (github actions)", link="https://github.com/Yubico/pam-u2f/actions"] image:https://github.com/yubico/pam-u2f/workflows/macos/badge.svg["macOS Build Status (github actions)", link="https://github.com/Yubico/pam-u2f/actions"] image:https://github.com/yubico/pam-u2f/workflows/fuzzer/badge.svg["Fuzz Status (github actions)", link="https://github.com/Yubico/pam-u2f/actions"] 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]] Building -------- This project uses 'autoconf', 'automake', 'pkg-config' and 'libtool' to achieve portability and ease of use. In addition, https://developers.yubico.com/libfido2['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 [source, console] ---- # 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. [source, console] ---- $ ./configure $ make ---- Building from Git ----------------- You may check out the sources using Git with the following command: [source, console] ---- $ git clone git://github.com/Yubico/pam-u2f.git ---- This will create a directory 'pam-u2f'. Enter the directory: [source, console] ---- $ cd pam-u2f ---- `autoconf`, `automake`, `gengetopt`, `libtool`, and `libpam` must be installed. `AsciiDoc` and `xsltproc` are used to generate the manpages. On Ubuntu, the necessary dependencies can be installed using [source, console] ---- # apt install --no-install-recommends autoconf automake libtool gengetopt \ pkg-config libfido2-dev libpam-dev libssl-dev asciidoc xsltproc \ libxml2-utils docbook-xml ---- On Fedora, the necessary dependencies can be installed using [source, console] ---- # dnf -y install autoconf automake libtool gengetopt libfido2-devel pam-devel \ openssl-devel asciidoc ---- Generate the build system using: [source, console] ---- $ autoreconf --install ---- Then build as usual, see above under <>. 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=`. 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). 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. Supported parameters for the module are: [horizontal] 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://$HOSTNAME" is used. appid=appid:: Set the https://developers.yubico.com/U2F/App_ID.html[application ID] for the FIDO authentication procedure. If no value is specified, the same value used for origin is taken ("pam://$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 <>. An <> 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, $HOME/.config/Yubico/u2f_keys is used. authpending_file=file:: Set the location of the file that is used for touch request notifications. This file will be opened when pam-u2f starts waiting for a user to touch the device, and will be closed when it no longer waits for a touch. Use inotify to listen on these events, or a more high-level tool like https://github.com/maximbaz/yubikey-touch-detector[yubikey-touch-detector]. Set an empty value in order to disable this functionality, like so: `authpending_file=`. Default value: /var/run/user/$UID/pam-u2f-authpending 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 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. Otherwise, fallback to the authenticator's default behaviour. userverification=int:: If 1, request 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, request 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. [[authMappingFiles]] Authorization Mapping Files --------------------------- A mapping must be made between the YubiKey token and the user name, see <> 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: :,,,:,,,:... :,,,:,,,:... 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. 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. [[individualAuth]] 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: :,,,:,,,:... 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. [[registration]] 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: [source, console] ---- $ 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`). SSH Credentials --------------- To generate SSH credentials OpenSSH version 8.2 or later is required. It is then possible to generate a credential file with: [source, console] ---- $ 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`. [[devices]] 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. 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 [source, console] ---- # chcon -R -t auth_home_t /path/to/target ---- should be used. For more information see https://access.redhat.com/security/cve/CVE-2020-24612[HERE].