CTRAPS: CTAP Client Impersonation and API Confusion on FIDO2

The talk covers the security and privacy of FIDO2, a standard and pervasive authentication technology. We focus on CTAP, an application layer protocol used during second-factor or single-factor authentication by a FIDO2 Authenticator and a FIDO2 Client, like a Yubikey USB/NFC dongle and a laptop. We uncover two new attack classes on CTAP called Client Impersonation (CI) and API Confusion (AC). The attacks exploit eight design issues we discovered in the CTAP specification. They have security and privacy implications on any FIDO2-compliant device, including deleting FIDO2 credentials without user interaction (0-click) or user tracking via FIDO2 credentials. We show eleven attacks taking advantage of CI and AC, and we collectively name them CTRAPS. We implement and evaluate the attacks on popular FIDO2 devices to confirm their large-scale and critical impact. We develop and test effective countermeasures, fixing the attacks and their root causes. We will also present the talk's content at IEEE Euro S&P 2025 and DEF CON 33.