WebAuthn Client Capabilities

WebAuthn is the modern standard behind passkeys. Instead of relying on traditional passwords, it leverages public-key cryptography, enabling users to authenticate with passkeys, which can include biometrics (like fingerprints or facial recognition) or physical security keys. This shift not only enhances security but also improves user experience by eliminating the need for password management.

The WebAuthn Level 3 standard introduces new client capabilities (which can be retrieved via the browser API getClientCapabilities()), aimed at providing developers and platforms more control and flexibility when implementing passkeys. These updates bring enhancements that simplify the process of integrating passkeys across devices, browsers, and platforms, ensuring a smoother and more consistent user journey.

In this blog post, we’ll be answering the following questions:

1. What are WebAuthn client capabilities?
2. Which WebAuthn client capabilities do exist?
3. How do WebAuthn client capabilities improve passkey implementations?
4. Why do WebAuthn client capabilities matter to both developers and product managers?

By the end, you'll understand how these features can help you create seamless, secure authentication flows that align with modern user expectations.

WebAuthn client capabilities are a set of features that allow browsers and platforms to communicate what types of WebAuthn functionalities they support. In simple terms, they act like a "feature checklist" that lets websites know which authentication methods and settings are available on a user’s device. This enables developers to tailor authentication flows based on the capabilities of the client, ensuring a smoother and more secure user experience.

For instance, if a browser signals that it supports biometric authentication (like Touch ID), developers can design their login flows to offer users the option to log in with a fingerprint. Conversely, if the browser does not support certain features, developers can provide fallback options, like a password or SMS OTP.

The WebAuthn Level 3 standard introduces several new client capabilities that make passkey implementations more versatile and user-friendly. The first support for the getClientCapabilities() API call was introduced in Safari 17.4.

To detect support in the browser, the following snippet can be useful:

// Check if PublicKeyCredential is supported in the current browser
if (typeof PublicKeyCredential === 'undefined') {
   console.log("PublicKeyCredential is not supported in this browser.");
}
 
// Check if getClientCapabilities method exists on PublicKeyCredential
if (typeof PublicKeyCredential.getClientCapabilities === 'function') {
   try {
      let capabilities = await PublicKeyCredential.getClientCapabilities();
      console.log(capabilities);
   } catch (error) {
      console.error('Error getting client capabilities:', error);
   }
} else {
   console.log('getClientCapabilities is not supported in this browser');
}

getClientCapabilities() allows websites and apps to query the client (e.g., browser or device) to determine which WebAuthn features it supports. By understanding the client's capabilities, developers can optimize authentication flows to leverage available features, like biometric authentication, and provide alternative methods if certain capabilities are absent.

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Here’s a closer look at the WebAuthn client capabilities and how they impact passkey integration:

conditionalCreate enables automatic passkey creation based on specific conditions. An application might use this capability to automatically create passkey during password autofill if the password manager has corresponding support. This feature helps to boost passkey adoption and subsequent usage by automatically transitioning users from passwords to passkeys.

Similar to conditionalCreate, conditionalGet triggers passkey logins automatically. This is useful in scenarios where the best passkey UX should be enabled making the login not only passwordless but also usernameless (users just click on the selected passkey in a modal / dropdown and can authenticate). By using this capability, developers can ensure passkey authentication only occurs when appropriate, minimizing unnecessary prompts and enhancing user experience.

hybridTransport ensures that passkeys can be used across different devices, enabling seamless cross-device authentication (via QR codes and Bluetooth). For instance, a user could use a passkey stored on their smartphone to log in to a service on their desktop. This capability allows users to authenticate securely without the need to manually transfer passkeys or rely on conventional login methods for each device, fostering a unified authentication experience.

Platform authenticators, like Windows Hello, Face ID or Touch ID, are built directly into devices and offer a faster, smoother, and more secure passkey experience by enabling users to authenticate with biometrics or other device-native method (e.g. PIN pattern).

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userVerifyingPlatformAuthenticator ensures that passkey authentication involves user verification, such as an active fingerprint scan or facial recognition, providing an extra layer of security.

The relatedOrigins capability allows for seamless authentication across different domains owned by the same organization (e.g. amazon.com and amazon.de). For instance, if a company manages multiple domains or has different subdomains, users can log in once and access all properties without re-authenticating on each. This capability streamlines the user experience, reducing friction, and is especially valuable for enterprises in international environments or with multi-service platforms.

The signalAllAcceptedCredentials(options) method provides the complete list of WebAuthn credential IDs for a given user. WebAuthn Relying Parties should use this method over signalUnknownCredential() when the user is authenticated, as there is no risk of a privacy leak. This method offers a comprehensive overview of a user’s public key credentials, including any recent changes that may not have been updated on currently connected authenticators.

Let’s have a look at an example. A user (userId: A) has 2 passkeys with Credential IDs that Base64URL encode to X and Y. Then, the user deletes passkey X in the web service’s (example.com) account settings (so the public key is deleted). Now, run the following snippet:

PublicKeyCredential.signalAllAcceptedCredentials({
   rpId: "example.com",
   userId: "A", // WebAuthn User Handle, Base64URL.
   allAcceptedCredentialIds: [
      "Y",
   ]
});

If the authenticator is available at the time of the above code’s execution, then authenticator deletes or hides the passkey X from future authentication ceremonies. However, the authenticator may not be attached at the execution time, so it’s recommended that relying parties should periodically execute this code, e.g. on every sign in.

Passkeys not present in allAcceptedCredentialIdswill be removed or hidden, potentially irreversibly. So, it’s important for relying parties to pay attention that valid WebAuthn credential IDs are never removed from the list. If a valid credential ID was accidentally remove, then the relying party should immediately include it in another signalAllAcceptedCredentials(options) call as soon as possible to “unhide” the passkey. If the passkey is not hidden but removed, then there’s nothing much to fix things.

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The signalCurrentUserDetails(options) method signlas the user’s current name and WebAuthn Display Name. When signalCurrentUserDetails(options) is called, the client follows a set of defined steps to execute this action.

Let’s see an example. A user with WebAuthn User ID A updates their name in the account settings of a website (example.com). Then, the relying party can run the following code:

PublicKeyCredential.signalCurrentUserDetails({
   rpId: "example.com",
   userId: "A", // user ID, Base64URL.
   name: "New user name",
   displayName: "New display name"
});

The authenticator would then update the locally saved passkey’s metadata. The big benefit is that in future Conditional UI / passkey autofill requests, the Conditional UI selection / dropdown menu shows the updated name and WebAuthn Display Name.

The signalUnknownCredential(options) method signals that a WebAuthn Credential ID is not recognized by the WebAuthn Relying Party, for instance, if the passkey was deleted by the user. Unlike signalAllAcceptedCredentials(options), this method does not require providing the full list of accepted credential IDs and the WebAuthn User Handle, thereby preventing potential privacy leaks to unauthenticated callers.

Let’s see an example. A user deletes a passkey with credential ID X on a website’s (example.com) account settings (so the public key is deleted). However, the private key is still available on the user’s device. This means that in future Conditional UI / passkey autofill login requests (with an empty allowCredentials list), the passkey can be still selected. The login attempt will fail though, as the public key is deleted already, so the relying party should run:

 
PublicKeyCredential.signalUnknownCredential({
    rpId: "example.com",
    credentialId: "X" // credential ID the user just tried, Base64URL
});

The authenticator would then delete or hide passkey with credential ID X from future authentication ceremonies.

As the WebAuthn Level 3 standard is still in draft status, the adoption of the new client capabilities is not yet widespread. Different browsers have been gradually implementing these features, but support varies. Currently, only Safari 17.4+ and Chrome 131 (behind flag and will ship with Chrome 133) has support for WebAuthn client capabilities. Below is an overview of the availability across major browsers:

The adoption of these capabilities will likely accelerate as the WebAuthn Level 3 standard matures and more browsers begin integrating the new features. Developers should stay updated with browser release notes and support documentation to plan for broader compatibility in the future.

If you want to know how many of your users have client devices and browsers ready to use client capabilities, you can use the free Passkeys Analyzer that provides specific data.

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As a developer, you might ask yourself what these new WebAuthn client capability detection means for you and how you should use them in your app. In the following, you find recommendations to use them.

However, be aware that not all browsers yet support the getClientCapabilities() API call (as of November 2024). There is a polyfill available here, that can be used until all browsers catch-up.

Use getClientCapabilities() early in your code to detect the client’s supported features at the start of the page load / authentication flow. This will allow you to customize the experience dynamically, and providing the passkey features that work on the device / browser, e.g. pushing for platform authentication when supported or offering alternative methods (e.g., SMS OTPs or hardware security keys) if not.

If you add passkeys to a website / app that currently uses passwords, the conditionalCreate feature can be a real booster for your passkey adoption. In the background, during a password autofill with a suitable credential manager ( only Apple Passwords as of October 2024), a passkey is automatically generated and will be preferred in future autofills.

To not only have a high passkey adoption, but also a high passkey login usage, try to check if the device / browser can use Conditional UI / Passkey Autofill by checking for conditionalGet. This way you will nudge users to use the created passkey for logins, as it’s proactively suggested by the operating system / browser and requires even less effort than autofilling a password.

Utilize hybridTransport to enable cross-device authentication (via QR code and Bluetooth), allowing users to log in seamlessly from their smartphone, even if they’re accessing your service on a desktop.

WebAuthn client capabilities represent a significant step forward in addressing currently existing passkey gaps. In this blog post, we addressed key questions about WebAuthn client capabilities:

1. What are WebAuthn client capabilities? We explained how these features allow browsers and platforms to signal their support for specific functionalities, giving developers more control over authentication flows.
2. Which WebAuthn client capabilities exist? We provided an overview of the new capabilities introduced in the Level 3 standard, including getClientCapabilities, conditionalCreate, hybridTransport, and more.
3. How do WebAuthn client capabilities improve passkey implementations? We discussed how these capabilities streamline integration, enhance cross-device usage, and improve security.
4. Why do WebAuthn client capabilities matter to developers? These features help create seamless, secure authentication experiences that align with modern user expectations, making implementation easier and more effective.

We encourage you to explore the new WebAuthn Level 3 features and stay updated on their adoption across browsers. If you’re looking to implement passkeys and take advantage of these advanced capabilities, reach out to us for expert guidance and support.