WebAuthn API Hijacking: A CISO’s Guide to Nullifying Passkey Phishing

In Sovereign Cybersecurity ↑ This article is part of our Digital Security section, continuing our research on zero-trust hardware exploits and countermeasures.

What is a WebAuthn API Hijacking Attack?

A WebAuthn interception attack via a spoofable authentication prompt (WebAuthn API Hijacking) consists of imitating in real time the authentication window displayed by a system or browser. Consequently, the attacker does not seek to break the cryptographic algorithm; instead, they reproduce the user interface (UI) at the exact moment the victim expects to see a legitimate prompt. Visual lures, precise timing, and perfect synchronization make the deception indistinguishable to the user.

The DEF CON 33 Demonstration – WebAuthn API Hijacking in Action

In Las Vegas, at the heart of DEF CON 33 (August 8–11, 2025), the world’s most respected hacker community witnessed a demonstration that made many squirm. In fact, researchers at Allthenticate showed live that a vulnerable synced passkey – despite being labeled “phishing-resistant” – could be tricked. So how did they do it? Through a WebAuthn interception attack of the spoofable authentication prompt type (real-time prompt spoofing). They created a fake authentication dialog box, perfectly timed and visually identical to the legitimate UI. Ultimately, the user believed they were validating a legitimate authentication, but the adversary hijacked the session in real time.
This proof of concept makes the “Passkeys WebAuthn Interception Flaw” tangible through a real-time spoofable prompt.

Technical Context – The WebAuthn Interception Flaw

To fully understand the scope of this vulnerability and learn how to protect against WebAuthn interception, you must consider the two main families of passkeys:

• Vulnerable synced passkeys: Stored in an Apple, Google, or Microsoft cloud, they are accessible on all your devices. While convenient, their authentication relies on a spoofable authentication prompt—an exploitable anchoring point.
• Device-bound passkeys: The private key remains locked within the device (Secure Enclave, TPM, YubiKey). This model has no cloud, and therefore a smaller attack surface.

In this context, the “Passkeys WebAuthn Interception Flaw” results from a sequence where the validated UI becomes the attack’s anchoring point.

The problem is straightforward: attackers can imitate any mechanism that depends on a system prompt. If the attacker reproduces the UI and captures the timing, they can execute a WebAuthn interception attack and hijack the authentication act. In other words, the weak link is not cryptography but the user interface.

Comparison – WebAuthn Interception Flaw: Prompt Spoofing vs. DOM Clickjacking

At DEF CON 33, two major research findings shook confidence in modern authentication mechanisms. Indeed, both exploit flaws related to the user interface (UX) rather than cryptography, but their vectors and targets differ radically.

▸ Key takeaway: This article focuses exclusively on prompt spoofing, which illustrates a major WebAuthn interception flaw and challenges the promise of “phishing-resistant passkeys.” For a complete study on DOM clickjacking, please see the related article.

Regulations & Compliance – MFA and WebAuthn Interception

Official documents such as the CISA guide on phishing-resistant MFA or the OMB M-22-09 directive insist on this point: authentication is “phishing-resistant” only if no intermediary can intercept or hijack the WebAuthn flow.
In theory, WebAuthn passkeys respect this rule. In practice, however, the implementation of vulnerable synced passkeys opens an interception flaw that attackers can exploit via a spoofable authentication prompt.

In Europe, both the NIS2 directive and the SecNumCloud certification reiterate the same requirement: no dependence on un-mastered third-party services.

As such, the “Passkeys WebAuthn Interception Flaw” contradicts the spirit of a so-called phishing-resistant MFA, because synchronization introduces an intermediary.

In other words, a US cloud managing your passkeys falls outside the scope of strict digital sovereignty.

European & Francophone Statistics – Real-time Phishing and WebAuthn Interception

Public reports confirm that advanced phishing attacks — including real-time techniques — represent a major threat in the European Union and the Francophone area.

• European Union — ENISA: According to the Threat Landscape 2024 report, phishing and social engineering account for 38% of reported incidents in the EU, with a notable increase in Adversary-in-the-Middle methods and real-time prompt spoofing, associated with WebAuthn interception. Source: ENISA Threat Landscape 2024
• France — Cybermalveillance.gouv.fr: In 2023, phishing generated 38% of assistance requests, with over 1.5M consultations related to this type of attack. Fake bank advisor scams jumped by +78% vs. 2022, often via spoofable authentication prompts. Source: 2023 Activity Report
• Canada (Francophone) — Canadian Centre for Cyber Security: The National Cyber Threat Assessment 2023-2024 indicates that 65% of businesses expect to experience a phishing or ransomware attack. Phishing remains a preferred vector for bypassing MFA, including via WebAuthn flow interception. Source: Official Assessment

Sovereign Use Case – Neutralizing WebAuthn Interception

In a practical scenario, a regulatory authority reserves synced passkeys for low-risk public portals. Conversely, the PassCypher choice eliminates the root cause of the “Passkeys WebAuthn Interception Flaw” by removing the prompt, the cloud, and any DOM exposure.
For critical systems (government, sensitive operations, vital infrastructure), it deploys PassCypher in two forms:

• PassCypher NFC HSM — offline hardware authentication, with no server and BLE AES-128-CBC keyboard emulation. Consequently, no spoofable authentication prompt can exist.
• PassCypher HSM PGP — sovereign management of inexportable segmented keys, with cryptographic validation that is cloud-free and synchronization-free.

Why PassCypher Eliminates the WebAuthn Interception Risk

PassCypher solutions stand in radical contrast to FIDO passkeys that are vulnerable to the WebAuthn interception attack:

• No OS/browser prompt — thus no spoofable authentication prompt.
• No cloud — no vulnerable synchronization or third-party dependency.
• No DOM — no exposure to scripts, extensions, or iframes.

PassCypher NFC HSM — Hardware Neutralization of Interception

Allthenticate’s attack at DEF CON 33 proves that attackers can spoof any system that depends on an OS/browser prompt.
PassCypher NFC HSM removes this vector: there is no prompt, no cloud sync, secrets are encrypted for life in a nano-HSM NFC, and validated by a physical tap.

User operation:

• Mandatory NFC tap — physical validation with no software interface.
• HID BLE AES-128-CBC Mode — out-of-DOM transmission, resistant to keyloggers.
• Zero-DOM Ecosystem — no secret ever appears in the browser.

🛡️ Attacks Neutralized by PassCypher NFC HSM

PassCypher HSM PGP — Segmented Keys Against Phishing

The other pillar, PassCypher HSM PGP, applies the same philosophy: no exploitable prompt.
Secrets (credentials, passkeys, SSH/PGP keys, TOTP/HOTP) reside in AES-256 CBC PGP encrypted containers, protected by a patented system of segmented keys.

• No prompt — so there is no window to spoof.
• Segmented keys — they are inexportable and assembled only in RAM.
• Ephemeral decryption — the secret disappears immediately after use.
• Zero cloud — there is no vulnerable synchronization.

Attack Surface Comparison

Weak Signals – Trends Related to WebAuthn Interception

Strategic Glossary

A review of the key concepts used in this article, for both beginners and advanced readers.

• Passkey: A passwordless digital credential based on the FIDO/WebAuthn standard, intended to be “phishing-resistant.”
• Vulnerable synced passkeys: Stored in a cloud (Apple, Google, Microsoft) and usable across multiple devices. They offer a UX advantage but a strategic weakness: dependence on a spoofable authentication prompt and the cloud.
• Device-bound passkeys: Linked to a single device (TPM, Secure Enclave, YubiKey). More secure because they lack cloud synchronization.
• Prompt: A system or browser dialog box that requests a user’s validation (Face ID, fingerprint, FIDO key). This is the primary target for spoofing.
• WebAuthn Interception Attack: The manipulation of the authentication flow by imitating the UI.
• Real-time prompt spoofing: The live spoofing of an authentication window, which is indistinguishable to the user.
• DOM Clickjacking: An attack using invisible iframes and Shadow DOM to hijack autofill and steal credentials.
• Zero-DOM: A sovereign architecture where no secret is exposed to the browser or the DOM.
• NFC HSM: A secure hardware module that is offline and compatible with HID BLE AES-128-CBC.
• Segmented keys: Cryptographic keys that are split into segments and only reassembled in volatile memory.
• Device-bound credential: A credential attached to a physical device that is non-transferable and non-clonable.

Technical FAQ (Integration & Use Cases)

CISO/CSO Advice – Universal & Sovereign Protection

Frequently Asked Questions for CISOs/CSOs

CISO/CSO Action Plan

Strategic Outlook

The message from DEF CON 33 is clear: authentication security is won or lost at the interface. In other words, as long as the user validates graphical authentication prompts synchronized with a network flow, real-time phishing and WebAuthn interception will remain possible.

Thus, prompt-free and cloud-free models — embodied by sovereign HSMs like PassCypher — radically reduce the attack surface.

In the short term, generalize the use of device-bound solutions for sensitive applications. In the medium term, the goal is to eliminate the spoofable UI from critical pathways. Ultimately, the recommended trajectory will permanently eliminate the “Passkeys WebAuthn Interception Flaw” from critical pathways through a gradual transition to a full-PassCypher model, providing a definitive solution for vulnerable passkeys in a professional context.