the WebAuthn surface is as complete as the claimed platform's browser ships it
Check id webauthn-surface-vs-platform
What an ordinary browser yields
On a secure context claiming desktop Chrome, PublicKeyCredential exists and exposes isUserVerifyingPlatformAuthenticatorAvailable — the shape every current Chrome ships.
What a detector infers
WebAuthn reaches further down the stack than most web APIs: PublicKeyCredential is not a self-contained JavaScript object but the front door to the platform's authenticator plumbing, which is why isUserVerifyingPlatformAuthenticatorAvailable exists at all — it asks the operating system a question. That gives the surface a shape that is hard to counterfeit and easy to check. Every current Chrome on a secure context defines the constructor and that query; recent versions add getClientCapabilities alongside. The failure this catches is a build where the constructor survives but the plumbing behind it does not, producing a combination — the front door present, the thing it opens onto missing — that no shipping browser has. It is scored against the claimed desktop OS and gated on a secure context, because WebAuthn is correctly absent on an insecure origin and that absence says nothing about the build. Warn rather than critical: the surface has been moving as the specification lands, and a row that reads a young API should not convict on a version difference.
How to resolve it
The authenticator layer is compiled into the browser and talks to the operating system, so it cannot be restored from page script. If a persona claims a desktop Chrome, run a build that has the layer rather than defining the constructor on window — the first capability call made against a stand-in gives it away immediately.
Read in the wild by
CloakBrowser issues
A stealth-browser tracker records the WebAuthn client-capability surface as one that goes missing on patched builds while the identity still claims a platform whose browser ships it.
CloakHQ/CloakBrowser #147, #176 (WebAuthn / client capabilities surface)
CloakHQ/CloakBrowser#147Every attribution traces to a published artifact. See the sources and their limits.
Nearby checks in Capability surfaces
- emoji advance widths are identical in a fresh realm
emoji-surface-vs-realmEmoji do not render from the page's fonts but from a platform font supplied by the operating system — Segoe UI Emoji on Windows, Apple Color… - the Web Animations surface is identical in a fresh realm
animation-surface-vs-realmThe Web Animations API exposes a small, precisely specified surface: whether Element.animate exists, whether the timeline and KeyframeEffect… - the platform theme resolves the CSS system colours coherently
system-colors-vs-platformThe CSS system-colour keywords — Highlight, HighlightText, Canvas, CanvasText, ButtonFace, ButtonText, GrayText and their newer siblings… - the user-agent stylesheet resolves identically in a fresh realm
default-style-values-vs-realmEvery browser ships a user-agent stylesheet: the rules that make a heading bold and a button look like a button before any site CSS exists. - the permission descriptors this build accepts are the platform's, not a lookup table's
permission-name-support-setThe Permissions specification deliberately fixes no list of names. - form-factor-gated APIs match the form factor the client hints claim
platform-gated-apis-vs-form-factorSome Web APIs are not compiled for every platform, so their presence is a fact about the binary rather than about the user agent. - Web Bluetooth is present or absent as the claimed desktop platform requires
bluetooth-availability-vs-platformnavigator.bluetooth is part of Chrome's desktop surface on Windows, macOS and Linux, and it is provided by the platform layer rather than by… - the deprecated and modern storage-quota APIs report the same origin quota
legacy-quota-surface-agreementnavigator.webkitTemporaryStorage.queryUsageAndQuota and navigator.storage.estimate() describe the same thing — this origin's storage quota…
Clearcote is a browser built for fingerprint coherence
It is a Chromium fork, maintained by the same people who wrote this reference. It ships as a compiled browser rather than as a stealth script injected into someone else's — which is a description of how it is built, and is not an argument about how it behaves on this check.
This audit takes no position on how Clearcote scores on Capability surfaces checks, on this one, or anywhere else. It has no baseline corpus of other people's fingerprints to rank you against and no vendor scoreboard — nearly every check is self-referential, asking one browser the same question through two independent APIs and reporting whether both answers can be true at once. It runs identically on any browser, including ours. Run it on yours and read the result yourself.
See the other checks in Capability surfaces — the family webauthn-surface-vs-platform belongs to.