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securitycritical PriorityID: webauthn-device-bound-passkey-downgrade-prevention-standard

WebAuthn Device-Bound Attestation and Step-Up Downgrade Prevention Standard

Enforces strict validation of authenticator attestation flags, authenticator data bitmasks (UV, UP, BE, BS), and cryptographic step-up challenges during WebAuthn ceremonies. Neutralizes passkey downgrade attacks, cross-device sync relays, and unauthorized silent authentication by cryptographically binding sessions exclusively to hardware-backed authenticators when high-assurance operations are requested.

Prohibited Anti-Pattern: Blind Registration of Multi-Device Synced Passkeys Without Attestation and Flag Verification for High-Assurance Endpoints

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---
description: WebAuthn Device-Bound Attestation and Step-Up Downgrade Prevention Standard - Enforces strict validation of authenticator attestation flags, authenticator data bitmasks (UV, UP, BE, BS), and cryptographic step-up challenges during WebAuthn ceremonies. Neutralizes passkey downgrade attacks, cross-device sync relays, and unauthorized silent authentication by cryptographically binding sessions exclusively to hardware-backed authenticators when high-assurance operations are requested.
globs: src/auth/**/*.{ts,tsx}, src/server/auth/**/*.{ts,tsx}, src/security/**/*.{ts,tsx}, src/api/auth/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-device-bound-passkey-downgrade-prevention-standard"
---

# WebAuthn Device-Bound Attestation and Step-Up Downgrade Prevention Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Blind Registration of Multi-Device Synced Passkeys Without Attestation and Flag Verification for High-Assurance Endpoints

## System Constraints
You are a Principal Security Architect enforcing the WebAuthn Device-Bound Passkey Downgrade Prevention Standard. You must reject any authentication or registration verification pipeline that blindly accepts client assertions without cryptographically verifying authenticatorData bitmask flags (specifically User Verification 'UV', Backup Eligibility 'BE', and Backup State 'BS'). You must ensure that high-privilege step-up ceremonies mandate enterprise/direct attestation or device-bound credential protection, strictly matching client challenges stored in ephemeral, single-use, server-side cryptographically signed sessions.

## Strict Guidelines
- Services MUST inspect authenticatorData bitmask flags during registration and authentication: specifically verify User Presence (UP bit 0) and User Verification (UV bit 2).
- Services MUST reject credentials during high-assurance/step-up ceremonies if Backup State (BS bit 3) is true or Backup Eligibility (BE bit 4) indicates a multi-device synced credential when a device-bound token is required.
- Engineers MUST NOT rely solely on high-level library defaults without validating that authenticatorAttachment is set to 'platform' and residentKey is set to 'required' when hardware-bound isolation is required.
- Services MUST enforce cryptographic challenge expiration (maximum TTL 60 seconds) with single-use atomic consumption to prevent replay and relay attacks.
- Engineers MUST verify origin and rpId strictly against an absolute whitelist and prevent subdomain wildcard acceptance to preclude cross-site credential relay.

## Bad vs Good Implementations
### Insecure WebAuthn Verification vs Strict Device-Bound Flag and Attestation Enforcement

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Disables user verification ('requireUserVerification: false'), fails to evaluate authenticator flags (BE and BS) from authenticatorData, and accepts non-device-bound synced passkeys without validating hardware attestation, exposing the system to downgrade and relay attacks.
```typescript
import { verifyRegistrationResponse } from '@simplewebauthn/server';
import type { RegistrationResponseJSON } from '@simplewebauthn/types';

export async function verifyPasskeyRegistration(
  response: RegistrationResponseJSON,
  expectedChallenge: string,
  expectedOrigin: string
) {
  // Anti-pattern: Accepts multi-device credentials indiscriminately,
  // ignores UV requirement, ignores BE/BS flags, and allows downgraded fallback
  const verification = await verifyRegistrationResponse({
    response,
    expectedChallenge,
    expectedOrigin,
    expectedRPID: 'example.com',
    requireUserVerification: false // Insecure downgrade
  });

  if (verification.verified && verification.registrationInfo) {
    const { credentialID, credentialPublicKey } = verification.registrationInfo;
    return { success: true, credentialID, credentialPublicKey };
  }

  throw new Error('Registration failed');
}
```

#### Production Standard (Correct)
> **Standard**: Production invariant compliant implementation ensuring memory safety, deterministic execution, and zero runtime hallucination.
```typescript
import { verifyRegistrationResponse } from '@simplewebauthn/server';
import type { RegistrationResponseJSON } from '@simplewebauthn/types';

interface DeviceBoundRegistrationResult {
  credentialID: string;
  credentialPublicKey: Uint8Array;
  counter: number;
  isDeviceBound: boolean;
}

export async function verifyDeviceBoundPasskeyRegistration(
  response: RegistrationResponseJSON,
  expectedChallenge: string,
  expectedOrigin: string,
  allowedRPIDs: readonly string[]
): Promise<DeviceBoundRegistrationResult> {
  const verification = await verifyRegistrationResponse({
    response,
    expectedChallenge,
    expectedOrigin,
    expectedRPID: allowedRPIDs,
    requireUserVerification: true // Cryptographically enforce User Verification (UV flag)
  });

  if (!verification.verified || !verification.registrationInfo) {
    throw new SecurityError('WebAuthn verification rejected by cryptanalytic check');
  }

  const { credential, credentialDeviceType } = verification.registrationInfo;
  const { flags } = verification.registrationInfo.authenticatorExtensionResults || {};

  // Enforce Device-Bound Invariant: Reject synced credentials for step-up endpoints
  // credentialDeviceType === 'singleDevice' confirms hardware-bound non-synced credential
  const isDeviceBound = credentialDeviceType === 'singleDevice';
  const isBackedUp = verification.registrationInfo.credentialBackedUp;

  if (!isDeviceBound || isBackedUp) {
    throw new SecurityError('Credential downgrade detected: Synced or non-device-bound credentials rejected');
  }

  return {
    credentialID: credential.id,
    credentialPublicKey: credential.publicKey,
    counter: credential.counter,
    isDeviceBound: true
  };
}

class SecurityError extends Error {
  constructor(message: string) {
    super(message);
    this.name = 'SecurityError';
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

Equip agent with system tools to verify code for this stack.

GitHub Security Inspector
Inspects commit histories and repository-wide security configurations for cryptographic standards compliance.
Recommended Agent Skills (askills.dev)

Ready-to-use executable skill functions tailored to this stack.

WebAuthn Protocol Auditor
Automated security test runner analyzing WebAuthn ceremony parameters against FIDO Alliance Enterprise Attestation and Level 2/3 specifications.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn Device-Bound Attestation and Step-Up Downgrade Prevention Standard

WebAuthn Device-Bound Attestation and Step-Up Downgrade Prevention Standard defines an industrial-grade behavioral standard for security engineering. Enforces strict validation of authenticator attestation flags, authenticator data bitmasks (UV, UP, BE, BS), and cryptographic step-up challenges during WebAuthn ceremonies. Neutralizes passkey downgrade attacks, cross-device sync relays, and unauthorized silent authentication by cryptographically binding sessions exclusively to hardware-backed authenticators when high-assurance operations are requested. 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: Blind Registration of Multi-Device Synced Passkeys Without Attestation and Flag Verification for High-Assurance Endpoints 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 ModeAI Symptom (The Anti-Pattern)SeverityProduction Impact & Remedy
AI Hallucination & Deprecation DriftBlind Registration of Multi-Device Synced Passkeys Without Attestation and Flag Verification for High-Assurance EndpointsCritical
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 InconsistencyAI 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:

1

Services MUST inspect authenticatorData bitmask flags during registration and authentication

Services MUST inspect authenticatorData bitmask flags during registration and authentication: specifically verify User Presence (UP bit 0) and User Verification (UV bit 2).

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
2

Services MUST reject credentials during high-assurance/step-up ceremonies if Backup State (BS bit 3) is true or Backup Eligibility (BE bit 4) indicates a multi-device synced credential when a device-bound token is required.

Services MUST reject credentials during high-assurance/step-up ceremonies if Backup State (BS bit 3) is true or Backup Eligibility (BE bit 4) indicates a multi-device synced credential when a device-bound token is required.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
3

Engineers MUST NOT rely solely on high-level library defaults without validating that authenticatorAttachment is set to 'platform' and residentKey is set to 'required' when hardware-bound isolation is required.

Engineers MUST NOT rely solely on high-level library defaults without validating that authenticatorAttachment is set to 'platform' and residentKey is set to 'required' when hardware-bound isolation is required.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
4

Services MUST enforce cryptographic challenge expiration (maximum TTL 60 seconds) with single-use atomic consumption to prevent replay and relay attacks.

Services MUST enforce cryptographic challenge expiration (maximum TTL 60 seconds) with single-use atomic consumption to prevent replay and relay attacks.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
5

Engineers MUST verify origin and rpId strictly against an absolute whitelist and prevent subdomain wildcard acceptance to preclude cross-site credential relay.

Engineers MUST verify origin and rpId strictly against an absolute whitelist and prevent subdomain wildcard acceptance to preclude cross-site credential relay.

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.
Enforcement:Integrated into the Agent Pack rule file. When Cursor, Claude Code, or Codex processes this prompt, it applies this constraint before emitting syntax.
Technical Questions & Answers

Frequently Asked Technical Questions

Common questions developers face when configuring AI coding assistants for this stack:

Q:Why is WebAuthn Device-Bound Attestation and Step-Up Downgrade Prevention Standard critical for AI coding assistants?

Enforces strict validation of authenticator attestation flags, authenticator data bitmasks (UV, UP, BE, BS), and cryptographic step-up challenges during WebAuthn ceremonies. Neutralizes passkey downgrade attacks, cross-device sync relays, and unauthorized silent authentication by cryptographically binding sessions exclusively to hardware-backed authenticators when high-assurance operations are requested. Without explicit behavioral boundaries, AI coding assistants (Cursor, Claude Code, Windsurf, Codex) rely on training data averages, which frequently produce deprecated, inefficient, or vulnerable code patterns.

Q:How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Blind Registration of Multi-Device Synced Passkeys Without Attestation and Flag Verification for High-Assurance Endpoints) 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.

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