Pass-the-Passkey: a Black Hat researcher found the WebAuthn implementation bugs, not the standard

At Black Hat USA 2026 this week, DSInternals researcher Michael Grafnetter presented "Pass-the-Passkey," a family of attacks he describes as fundamentally similar to Pass-the-Hash and NTLM Relay, aimed not at the WebAuthn standard but at how real deployments implement it. The findings include an unnamed major cloud service whose passkey implementation is vulnerable to the very attack class passkeys were designed to prevent, and YubiKey signatures stored in cleartext and readable by any authenticated unprivileged user, including remotely. Grafnetter attributes the pattern to WebAuthn's 22-step validation process, cryptography intricate enough that even a co-author of the standard missed validation steps in its own implementation. The demonstrated techniques, phishing, tampering, spoofing, fuzzing, and prompt-flooding, can impersonate privileged identities while evading both phishing-resistant MFA enforcement and popular XDR detection.

Why it matters

Passkeys are the recommended fix for phishing-resistant authentication, so a talk that breaks specific implementations without breaking WebAuthn/FIDO2 itself is a more useful signal than it sounds: the standard held, the shortcuts vendors took building on top of it didn't. A signature readable in cleartext by any logged-in user turns "phishing-resistant" into a false sense of security the moment an attacker has any foothold on the box.

The practical takeaway isn't to distrust passkeys, it's to stop treating "we deployed passkeys" as a finished project. Ask your identity provider and hardware key vendor directly whether their specific implementation was in scope for this research, and don't assume compliance with the standard means immunity from implementation bugs. A related but independently discovered attack chain against Google's synced passkeys, published the same week, is covered separately.

Source: DSInternals