Passkey Assertion Replay (Sign-Count Not Enforced)

← Insecure Authentication / Authorization

passkey_assertion_replay_signcount   HARD

Description

The authenticator sign-count / credential counter is never persisted or compared, so a captured WebAuthn assertion replays to create additional authenticated sessions (Craft CMS CVE-2026-72780 class).

How it works

The relying party never persists or compares the authenticator signature counter. A single captured assertion (sign-count = 5) is replayed to mint several authenticated sessions. A correct verifier stores the last counter and rejects any assertion that does not strictly advance it. Scope: the FIDO2 signature is assumed valid (signature verification is out of scope); the flaw demonstrated is in the relying-party authorization logic on top of it.

How to exercise it. DVMA is the harness - open this module from the home index and tap the demo action. The screen ships the malicious input and simulates the attacker (e.g. the companion app, crafted intent, or scanned payload) in-process, and the evidence panel prints the proof. The Tools (optional) and Attack inputs below are only needed to reproduce the exploit end-to-end on a real device.

Exploit steps

1. Set up. Build DVMA with a flavor that enables the Insecure Authentication / Authorization category (e.g. --dart-define-from-file=config/flavors/dev.json) and run on an emulator/simulator you control. The demo needs no external tooling; for the optional on-device reproduction the relevant tools are: webauthn test harness, mitmproxy, Burp Suite.
2. Locate the target. From the home index, open Passkey Assertion Replay (Sign-Count Not Enforced) (passkey_assertion_replay_signcount). The How it works section above describes this module’s specific weakness; the screen states the intended-secure behavior and exposes the vulnerable action.
3. Exploit. Drive the authentication/authorization path, capture the token / assertion / decision, then replay or tamper with it (mitmproxy, a WebAuthn test harness, or by editing the value) and confirm the server-side check accepts the manipulated request.
4. Observe the evidence. Trigger the vulnerable action and read the evidence panel - it prints the concrete proof (leaked value, accepted replay, executed payload, or unauthorized result).
5. Contrast with the secure path. Run the module’s secure/hardened action (where provided) and confirm the same attack is rejected - this is what a correct implementation should do.

Tools (optional)

• webauthn test harness
• mitmproxy
• Burp Suite

Real-world references

Concrete public disclosures that match this vulnerability class:

• CVE-2026-72780 (Craft CMS WebAuthn counter replay)
• W3C WebAuthn: signature counter