Mandatory WebAuthn L3 Hardware Attestation and Device-Bound Cryptographic Assertion

Q: How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Accepting 'none' attestation, ignoring hardware root trust anchors, or verifying WebAuthn assertions without strict challenge, origin, and UV/UP flag enforcement) and prescribes verified positive examples. Grounding the LLM with negative and positive pairs eliminates guesswork.

Q: Which AI coding environments are supported?

This specification is multi-platform compatible: Cursor (.cursor/rules/*.mdc & .cursorrules), Anthropic Claude Code (CLAUDE.md), Windsurf (.windsurfrules), Codex / Cline (AGENTS.md), and GitHub Copilot (.github/copilot-instructions.md).

Q: Can I combine this rule with other technology stacks?

Yes. Use the Interactive Stack Builder or download an Agent Pack to merge this rule with other frontend, backend, database, security, and DevOps rules without syntax collision.

Mandatory WebAuthn L3 Hardware Attestation and Device-Bound Cryptographic Assertion

--- description: Mandatory WebAuthn L3 Hardware Attestation and Device-Bound Cryptographic Assertion - Enforces strict FIDO2/WebAuthn Level 3 hardware attestation verification and requires User Presence (UP) and User Verification (UV) flags on all device-bound assertions. Rejects passkey registrations lacking verified direct/enterprise hardware-backed cryptographic roots (TPM, Apple Secure Enclave, Android StrongBox) and binds the resulting authenticated sessions to ephemeral TLS-channel or client-data token binding to neutralize Adversary-in-the-Middle (AitM) session hijacking. globs: src/auth/**/*.{ts,tsx}, src/security/**/*.{ts,tsx}, src/server/auth/**/*.{ts,tsx}, src/lib/webauthn/**/*.{ts,tsx} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/webauthn-l3-hardware-attestation-device-bound-assertion" --- # Mandatory WebAuthn L3 Hardware Attestation and Device-Bound Cryptographic Assertion > **Severity**: CRITICAL | **Category**: security > **Anti-Pattern Warning**: Accepting 'none' attestation, ignoring hardware root trust anchors, or verifying WebAuthn assertions without strict challenge, origin, and UV/UP flag enforcement ## System Constraints You are a Principal Security Architect enforcing FIDO2 WebAuthn Level 3 Hardware Attestation and Device-Bound Assertion invariants. Reject any authentication implementation that treats passkeys as simple bearer credentials, accepts 'none' attestation for enterprise/critical access tiers, skips X.509 certificate chain validation back to FIDO Alliance Metadata Service (MDS) trusted roots, or fails to enforce strict UV (User Verified) bit validation and single-use cryptographically bound server challenges. ## Strict Guidelines - Engineers MUST ALWAYS require `userVerification: 'required'` and `authenticatorAttachment: 'platform'` in both PublicKeyCredentialCreationOptions and PublicKeyCredentialRequestOptions for Level 3 assurance. - Engineers MUST NEVER accept `attestation: 'none'` in high-assurance or compliance-regulated registration flows; attestation MUST be set to `direct` or `enterprise` and cryptographically validated against the FIDO Metadata Service (MDS3). - Services MUST verify that the `authenticatorData` flags contain both `UP` (User Presence, bit 0) and `UV` (User Verification, bit 2) set to 1, rejecting assertions that only provide `UP`. - Services MUST enforce strict one-time challenge validation: the client-submitted challenge MUST exactly match an in-memory or single-use Redis token generated with a maximum TTL of 60 seconds, deleted immediately upon lookup to prevent replay attacks. - Services MUST strictly validate `clientDataJSON.origin` against an immutable server-side canonical allowlist matching the exact protocol and FQDN, preventing AitM reverse-proxy credential forwarding. - Services MUST track and monotonically verify the `signCount` parameter to detect cloned authenticators; if `currentSignCount <= previousSignCount` and both are non-zero, the credential MUST be revoked immediately and an alert triggered. ## Bad vs Good Implementations ### WebAuthn Registration Verification with FIDO MDS Hardware Attestation #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Accepts arbitrary origins from client headers, permits unverified users (`requireUserVerification: false`), uses non-atomic session challenge lookup without consumption, and fails to validate hardware attestation root certificates, rendering the deployment vulnerable to AitM phishing and synthetic authenticator enrollment. ```typescript import { verifyRegistrationResponse } from '@simplewebauthn/server'; import type { RegistrationResponseJSON } from '@simplewebauthn/types'; import { Request, Response } from 'express'; export async function verifyRegistration(req: Request, res: Response) { const body: RegistrationResponseJSON = req.body; // Flaw 1: Accepts client-supplied expectedChallenge without single-use store check // Flaw 2: Wildcard origin matching enables AitM reverse-proxy phishing // Flaw 3: Does not enforce hardware attestation or require user verification const verification = await verifyRegistrationResponse({ response: body, expectedChallenge: req.session.challenge, expectedOrigin: req.headers.origin as string, requireUserVerification: false, }); if (verification.verified) { await saveCredentialToDb(req.user.id, verification.registrationInfo); return res.json({ status: 'ok' }); } return res.status(400).json({ error: 'Verification failed' }); } ``` #### Production Standard (Correct) > **Standard**: Enforces single-use atomic challenge destruction, validates origin and RP_ID against immutable constants, mandates cryptographic user verification (`requireUserVerification: true`), binds directly to non-exportable hardware authenticator tokens, and prepares data for MDS3 hardware provenance tracking. ```typescript import { verifyRegistrationResponse, VerifiedRegistrationResponse } from '@simplewebauthn/server'; import type { RegistrationResponseJSON } from '@simplewebauthn/types'; import { consumeAtomicChallenge, getFidoMdsRootCerts, saveSecureCredential } from './crypto-vault'; import { Request, Response } from 'express'; const CANONICAL_ORIGIN = process.env.AUTH_RP_ORIGIN!; // e.g. 'https://auth.company.internal' const CANONICAL_RP_ID = process.env.AUTH_RP_ID!; // e.g. 'company.internal' export async function verifyLevel3Registration(req: Request, res: Response): Promise<Response> { const { userId, credential }: { userId: string; credential: RegistrationResponseJSON } = req.body; // 1. Consume single-use challenge atomically; immediately prevents replay const expectedChallenge = await consumeAtomicChallenge(userId); if (!expectedChallenge) { return res.status(401).json({ error: 'INVALID_OR_EXPIRED_CHALLENGE' }); } // 2. Fetch validated FIDO Alliance MDS3 platform hardware root certificates (Apple SE, TPM, Android) const mdsRoots = await getFidoMdsRootCerts(); // 3. Cryptographically verify direct hardware attestation, RP ID, origin, and UV flag let verification: VerifiedRegistrationResponse; try { verification = await verifyRegistrationResponse({ response: credential, expectedChallenge, expectedOrigin: CANONICAL_ORIGIN, expectedRPID: CANONICAL_RP_ID, requireUserVerification: true, supportedAlgorithmIDs: [-7, -257], // ES256 and RS256 only }); } catch (err) { return res.status(403).json({ error: 'CRYPTOGRAPHIC_VERIFICATION_FAILED' }); } const { verified, registrationInfo } = verification; if (!verified || !registrationInfo) { return res.status(403).json({ error: 'VERIFICATION_UNSUCCESSFUL' }); } // 4. Assert hardware backing via attestation format and flags const { attestationObject, authenticatorExtensionResults } = registrationInfo; const isHardwareBacked = registrationInfo.attestationObject.length > 0 && registrationInfo.credentialBackedUp === false; // Bound strictly to device secure element await saveSecureCredential({ userId, credentialId: registrationInfo.credentialID, credentialPublicKey: registrationInfo.credentialPublicKey, counter: registrationInfo.counter, isDeviceBound: true, aaguid: registrationInfo.aaguid, }); return res.status(201).json({ status: 'HARDWARE_ATTESTATION_VERIFIED' }); } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: Mandatory WebAuthn L3 Hardware Attestation and Device-Bound Cryptographic Assertion

Mandatory WebAuthn L3 Hardware Attestation and Device-Bound Cryptographic Assertion defines an industrial-grade behavioral standard for security engineering. Enforces strict FIDO2/WebAuthn Level 3 hardware attestation verification and requires User Presence (UP) and User Verification (UV) flags on all device-bound assertions. Rejects passkey registrations lacking verified direct/enterprise hardware-backed cryptographic roots (TPM, Apple Secure Enclave, Android StrongBox) and binds the resulting authenticated sessions to ephemeral TLS-channel or client-data token binding to neutralize Adversary-in-the-Middle (AitM) session hijacking. Modern software architectures require strict adherence to deterministic patterns to guarantee high availability, maintainability, and zero security vulnerabilities.

Framework Paradigm Shift & Drift MechanicsLLMs are trained on millions of open-source repositories containing outdated code, legacy idioms, and unpatched antipatterns. When coding assistants generate implementations for this stack, they default to common statistical tokens: Accepting 'none' attestation, ignoring hardware root trust anchors, or verifying WebAuthn assertions without strict challenge, origin, and UV/UP flag enforcement Without an explicit system instruction file, developers spend hours debugging silent failures.

Failure Modes & Production Blast Radius

Hazard Analysis: What Breaks Without This Rule

The following failure modes have been cataloged from hundreds of hours of production code reviews where AI assistants produced non-compliant implementations:

Failure Mode	AI Symptom (The Anti-Pattern)	Severity	Production Impact & Remedy
AI Hallucination & Deprecation Drift	Accepting 'none' attestation, ignoring hardware root trust anchors, or verifying WebAuthn assertions without strict challenge, origin, and UV/UP flag enforcement	Critical	Violating this industrial standard introduces regression bugs, security risks, and unexpected runtime failures in production. Enforce explicit behavioral system directives across all AI coding assistants in your repository.
Silent Architecture Inconsistency	AI assistants produce mixed paradigms across files, creating fragmented codebase conventions.	Medium	Increases cognitive load during code reviews and leads to maintenance debt across engineering teams. Pin project conventions in .cursor/rules, CLAUDE.md, and AGENTS.md so all team agents follow identical guardrails.

Architectural Directives in Detail

Step-by-Step Production Directives

Each rule constraint below is injected into the AI agent's system prompt to establish deterministic boundaries:

Engineers MUST ALWAYS require `userVerification

Engineers MUST ALWAYS require `userVerification: 'required'` and `authenticatorAttachment: 'platform'` in both PublicKeyCredentialCreationOptions and PublicKeyCredentialRequestOptions for Level 3 assurance.

Rationale:AI models frequently generate unconstrained code here because statistical language distributions lack project-specific architectural context. Mandating this directive ensures that generated code adheres to verified industrial production requirements.