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WebAuthn as Second Factor module

The module is implemented as an Authentication Processing Filter. That means it can be configured in the global config.php file or the SP remote or IdP hosted metadata.

Installation

You can install this module with composer:

If you are using PHP 7, you also need to install either the GMP extension (recommended) or the BCMath extension.

How to setup the webauthn module as a second-factor (an authprocfilter)

You need to enable the module's authprocfilter at a priority level so that it takes place AFTER the first-factor authentication. E.g. at 100 and if standalone registration and name2oid are used together, then the WebAuthn auth proc filter has to run after name2oid.

The authproc filter takes a number of optional parameter that steer which users will be forced into 2FA.

Then you need to copy config-templates/module_webauthn.php to your config directory and adjust settings accordingly. See the file for parameters description.

How to set up (pure) Passwordless authentication

In passwordless mode, the module provides an AuthSource, to be configured as usual in simpleSAMLphp's config/authsources.php

Users' FIDO2 Keys need to be registered with the "Passwordless" checkbox set - this triggers the mandatory registration with a second factor intrinsic to the key (fingerprint, face recognition, transaction PIN, etc. ).

This authsource takes little configuration because authentications happen before the username is known - so no user-specific configuration is possible.

The authsource takes the following parameters in authsources.php:

How to set up simultaneous Passwordless and traditional two-factor

In this mode, the authentication prompt simultaneously allows for either triggering a Passwordless auth, or to enter a username/password as traditional first-factor.

The configuration is almost identical to Passwordless above, but requires one extra required configuration parameter: the authsource that should be used to validate the username/password, if supplied by the user.

The authsource takes the following parameters in authsources.php:

Using storage

The database schema sets itself up on first use automatically. The schema can be found in the sources at src/WebAuthN/Store/Database.php (__construct).

If you want to trim down permissions for the database user, here is the minimal set of required permissions:

The webauthn:Database backend storage has the following options:

class : Must be set to webauthn:Database.

database.dsn : Data Source Name must comply to the syntax for the PHP PDO layer.

database.username : Username for the database user to be used for the connection.

database.password : Password for the database user used for the connection.

timeout : The number of seconds to wait for a connection to the database server. This option is optional. If unset, it uses the default from the database-driver.

Example config using PostgreSQL database:

Example config using MySQL database:

Options

Options of auth proc filter

default_enable : Should WebAuthn be enabled by default for all users? If not, users need to be white-listed in the database - other users simply pass through the filter without being subjected to 2FA. Defaults to "disabled by default" === false

force : This parameter is used only if "use_database" is false. If the value of "force" is true then we trigger WebAuthn only if "attrib_toggle" from the user is not empty. If the value of "force" is false then we switch the value of "default_enable" only if "attrib_toggle" from the user is not empty. Default value is true.

attrib_toggle : This parameter stores the name of the attribute that is sent with user and which determines whether to trigger WebAuthn. Default value is 'toggle'.

use_database : This parameter determines if the database will be used to check whether to trigger second factor authentication or use the "attrib_toggle" instead. Default value of this attribute is true.

Options in module_webauthn.php

scope : FIDO2 is phishing-resistent by binding generated credentials to a scope. Browsers will only invoke the registration/authentication if the scope matches the principal domain name the user is currently visiting. If not specified, the scope will be the hostname of the IdP as per its metadata. It is permissible to widen the scope up to the prinicpal domain though (e.g. authentication service is "saml.example.com" => scope can be extended to "example.com"; but not "examp1e.com". A registered FIDO2 token can then also be used on other servers in the same domain. If configuring this item, be sure that the authentication server name and the desired scope are a suffix match.

registration / use_inflow_registration : Optional parameter which determines whether you will be able to register and manage tokens while authenticating or you want to use the standalone registration page for these purposes. If set to false => standalone registration page, if true => inflow registration. If this parameter is not explicitly set, the value is considered to be true.

registration / auth_source : Optional parameter to define how the user authenticates to the dedicated registration page. Defaults to "default-sp"; ignored if inflow registration was configured.

registration / policy_2fa / minimum_certification_level : Required parameter which specifies the required FIDO certification level for registration, allowed values:

• CERTIFICATION_NOT_REQUIRED - attestation is not checked
• FIDO_CERTIFIED_L1 - FIDO L1 certified
• FIDO_CERTIFIED_L1plus - FIDO L1+ certified
• FIDO_CERTIFIED_L2 - FIDO L2 certified
• FIDO_CERTIFIED_L3 - FIDO L3 certified
• FIDO_CERTIFIED_L3plus - FIDO L3+ certified

registration / policy_2fa / aaguid_whitelist : Optional list of AAGUIDs which are excluded from the minimum certification level check. Defaults to an empty list. Ignored when minimum_certification_level is set to CERTIFICATION_NOT_REQUIRED.

registration / policy_2fa / attestation_format_whitelist : Optional list of attestation formats which are excluded from the minimum certification level check. Defaults to an empty list. Ignored when minimum_certification_level is set to CERTIFICATION_NOT_REQUIRED.

registration / policy_passwordless / minimum_certification_level : Required parameter, same meaning as registration / policy_2fa / minimum_certification_level but for passwordless authentication

registration / policy_passwordless / aaguid_whitelist : Optional parameter, same meaning as registration / policy_2fa / aaguid_whitelist but for passwordless authentication

registration / policy_passwordless / attestation_format_whitelist : Optional parameter, same meaning as registration / policy_2fa / attestation_format_whitelist but for passwordless authentication

User Experience / Workflow

Users for which WebAuthn is enabled cannot continue without a FIDO2 token. The UI is different depending on the number of tokens the user has registered:

• User has 0 tokens: UI requires the user to register a token. The user can choose a convenient name for the token to recognise it later. If request_tokenmodel is set, the name will be appended with the token model and vendor. After successful registration, the authprocfilter is done (user continues to SP)
• User has 1 token: UI requires the user to authenticate. After the authentication, user can optionally enroll another token.
• User has 2+ tokens: UI requires the user to authenticate. After the authentication, user can optionally enroll another token or delete an obsolete one.

If a user is enabled but has forgotten all of his tokens, the person would need to contact his administrator and have his account temporarily disabled for two- factor authentication.

As long as a user account has 0 tokens there is no benefit yet; it's effectively still single factor authentication because anyone with the user's password can register any token. That is in the nature of things. It could be avoided with an out-of-band registration process (in the same scope).

If the standalone registration page is used, the user can't optionally enroll and manage tokens while logging in. The standalone registration page can be found under <basedir>module.php/webauthn/registration, it requires authentication (with the auth source registration_auth_source) and after that you are redirected to a page where you can manage tokens.

Device model detection

The option registration / minimum_certification_level can be used to get a token's so-called AAGUID which uniquely identifies the model and manufacturer (it is not a serial number). You can then make decisions about the acceptability of a certain authenticator model.

Mapping the AAGUID to a cleartext model and manufacturer name is done by having (or not) meta-information about the AAGUID. The FIDO Alliance operates a Metadata Service (MDS) which has a good number of AAGUIDs registered. However, manufacturers are not required to submit their AAGUIDs and metadata to that MDS, and indeed, some manufacturers are missing.

The module contains a full list of AAGUIDs and relevant metadata as pulled from the FIDO MDS. This list is in the config/webauthn-aaguid.json file, and this file needs to be moved to your SimpleSAMLphp configuration directory.

If you want, you can also manually update this file, if you believe there might be new models listed. In order to do that, run the bin/updateMetadata.php script like this:

where <blob file> is the metadata JWT blob you get from here.

As a consequence, depending on the token model the user uses, even if the AAGUID is being sent as part of the registration process, it may be that the device is still stored as unknown model/unknown vendor.

I contacted FIDO Alliance to ask about the lack of complete information in their MDS. Purportedly, listing in the MDS has chances of becoming mandatory in a future FIDO Certification. Until then, there is no good solution to the problem.

Disabling WebAuthn

You can disable the module entirely by not listing it as an authprocfilter.

You can disable the module by default by setting default_enable = false. You can then enable WebAuthn second-factor authentication for individual users by adding them with status "FIDO2Enabled" to the userstatus table or if you don't want to use the userstatus table, you can send an attribute whose name is stored in attrib_toggle for this.

If the module is enabled by default, you can selectively disable WebAuthn second-factor authentication by adding the username with status FIDO2Disabled to the userstatus table or if you don't want to use the userstatus table, you can send an attribute whose name is stored in attrib_toggle for this.

Limitations / Design Decisions

This implementation does not validate token bindings, if sent by the authenticator (§7.1 Step 7 / §7.2 Step 11 skip token binding information validation if present). That is because Yubikeys do not support token binding and the corresponding functionality thus has no test case.

Both User Present and User Verified variants are considered sufficient to authenticate successfully in second-factor scenarios (§7.1 steps 11 and 12 are joined into one condition). The module logs into the credential database which of the two was used during registration time and does not allow downgrades during authentication time. Passwordless authentication always requires User Verified during registration and authentication transactions.

The implementation requests and supports ECDSA and RSA keys (algorithms -7, -257).

The implementation does not request any client extensions. The specification gives implementations a policy choice on what to do if a client sends extensions anyway: this implementation chose to then fail the registration/authentication.

The implementation supports the attestation formats

• "none" (No Attestation)
• "packed / x5c" (Packed Attestation, X.509 certificate)
• "packed / self" (Packed Attestation, Self-Attestation)
• "fido-u2f" (FIDO U2F Attestation)
• "apple" (Apple Anonymous Attestation)
• "android-key" (Android key attestation with Keymaster 4, for Android 9 and up) Other attestation formats lead to a registration failure.

For the attation type "packed / x5c",

• the optional OCSP checks are not performed (this is explicitly permitted in the spec due to other means of revocation checking in the FIDO MDS).

For both "packed / x5c" and "fido-u2f":

• all attestations are classified as "Basic" (i.e. no "AttCA" level); i.e. validation as per §7.1 Step 18 is not executed.
• Regarding §7.1 Step 21: When minimum certification levels are configured, "Self" and "Basic" attestation levels are considered acceptable; "None" is not acceptable.

If the implementation detects signs of physical object cloning (not incremented signature counter), it follows the policy of failing authentication.