A PHP implementation of a FIDO2 / WebAuthn authentication agent

v1.0.3 2021-11-19 15:38 UTC


Build Status Coverage Status Scrutinizer Code Quality

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.


You can install this module with composer:

% composer require simplesamlphp/simplesamlphp-module-webauthn

How to setup the webauthn module

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.

100 => [
        'class' => 'webauthn:WebAuthn',

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

Using storage

You first need to setup the database.

Here is the initialization SQL script:

CREATE TABLE credentials (
    user_id VARCHAR(80) NOT NULL,
    credentialId VARCHAR(500) NOT NULL,
    credential MEDIUMBLOB NOT NULL,
    signCounter INT NOT NULL,
    friendlyName VARCHAR(100) DEFAULT "Unnamed Token",
    UNIQUE (user_id,credentialId)

GRANT SELECT,INSERT,UPDATE,DELETE ON ...credentials TO '...dbuser'@'' IDENTIFIED BY '...dbpass';

CREATE TABLE userstatus (
    user_id VARCHAR(80) NOT NULL,
    fido2Status ENUM("FIDO2Disabled","FIDO2Enabled") NOT NULL DEFAULT "FIDO2Disabled",
    UNIQUE (user_id)

GRANT SELECT ON ...userstatus TO '...dbuser'@'' IDENTIFIED BY '...dbpass';

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:

    100 => array(
        'class'	=> 'webauthn:WebAuthn', 
        'store'	=> array(
            'database.dsn' => ';dbname=fido2',
            'database.username' => 'simplesaml',
            'database.password' => 'sdfsdf',

Example config using MySQL database:

    100 => array(
        'class'	=> 'webauthn:WebAuthn', 
        'store'	=> array(
            'database.dsn' => ';dbname=fido2',
            'database.username' => 'simplesaml',
            'database.password' => 'sdfsdf',


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 "" => scope can be extended to ""; but not "". 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.

request_tokenmodel : The following will interactively ask the user if he is willing to share manufacturer and model information during credential registration. The user can decline, in which case registration will still succeed but vendor and model will be logged as "unknown model [unknown vendor]". When not requesting this, there is one less user interaction during the registration process; and no model information will be saved. Defaults to "false".

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.

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.

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 webauthn/registration.php, it requires authentication and after that you are redirected to a page where you can manage tokens.

Device model detection

The option request_tokenmodel can be used to get a token's so-called AAGUID which uniquely identifies the model and manufacturer (it is not a serial number).

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. It also has a limited amount of manually curated information of some AAGUIDs which are not in the FIDO MDS, namely for Yubico products and Microsoft. 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:

% php bin/updateMetadata.php <MDS_TOKEN>

where MDS_TOKEN is the API token you get after registering 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.

This implementation does not distinguish between User Presence (user has proven to be near the authenticator) and User Verification (user has proven to be near the authenticator AND to have unlocked the authenticator with a personal asset such as PIN or fingerprint). Both variants are considered sufficient to authenticate successfully (§7.1 steps 11 and 12 are joined into one condition)

The implementation requests ECDSA keys (algorithm -7) because all Yubikeys support that. It is trivial to add RSA support if there are keys which don't.

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", "packed / x5c" and "packed / self", and "fido-u2f". 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 bith "packed / x5c" and "fido-u2f":

  • due to the lack of any externally provided knowledge about CAs(???) all attestations are classified as "Basic" (i.e. no "AttCA" level)

Given the sorry state of completeness of the FIDO MDS, only very few attestation root CAs are known and validation as per §7.1 Step 18 would often fail. That step is therefore ignored. All the "None", "Self" and "Basic" attestation levels are considered acceptable; meaning §7.1 Step 21 does not apply.

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