AgentRules.org
RulesStacksClientsAuditorPrunerSelf-HealRFCsBenchmarkCLICI/CDAuthorsSubmit
This specification is maintained by the AgentRules Working Group. Have enhancements or want to claim maintainership?Submit Specification →
securitycritical PriorityID: webauthn-conditional-mediation-origin-lock-standard

WebAuthn Conditional Mediation Origin-Lock & User Verification Standard

Enforces strict canonical origin binding, relying party ID validation, and mandatory hardware-backed User Verification (UV) flags during WebAuthn conditional mediation (passkey autofill) assertions. Eliminates session hijacking, automated autofill harvesting, and cross-site credential exploitation across unmediated and autofill flows.

Prohibited Anti-Pattern: Accepting Unverified User Flags or Permissive Origins During Conditional WebAuthn Autofill

Export Ready-to-Drop Configuration

Select format below to copy code or download file directly.

---
description: WebAuthn Conditional Mediation Origin-Lock & User Verification Standard - Enforces strict canonical origin binding, relying party ID validation, and mandatory hardware-backed User Verification (UV) flags during WebAuthn conditional mediation (passkey autofill) assertions. Eliminates session hijacking, automated autofill harvesting, and cross-site credential exploitation across unmediated and autofill flows.
globs: src/**/*.{ts,tsx}, server/**/*.{ts,js}, lib/auth/**/*.{ts,js}, app/api/auth/**/*.{ts,js}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-conditional-mediation-origin-lock-standard"
---

# WebAuthn Conditional Mediation Origin-Lock & User Verification Standard

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Accepting Unverified User Flags or Permissive Origins During Conditional WebAuthn Autofill

## System Constraints
You are a Principal Security Architect enforcing the WebAuthn Conditional Mediation Origin-Lock Verification Standard. You must strictly require that all WebAuthn assertion queries (both conditional autofill and modal ceremonies) enforce strict canonical origin validation, strict RP ID matching, and cryptographic hardware user verification (UV flag 0x04 in authenticatorData). Permissive origin matching, unverified client extensions, relaxed user verification, and non-canonical origin acceptance are strictly forbidden.

## Strict Guidelines
- Engineers MUST ALWAYS configure `userVerification: 'required'` in WebAuthn assertion options, both on client-side `navigator.credentials.get` and server-side verification options.
- Engineers MUST NOT rely on HTTP Host or X-Forwarded-Host headers to determine `expectedOrigin` during server-side verification; origin MUST be strictly resolved against an immutable server-side canonical allowlist.
- Services MUST cryptographically inspect the raw `authenticatorData` flags and verify that the User Verified (UV) bit (bit 2, mask 0x04) is asserted (1) for passkey assertion responses.
- Code SHOULD verify that conditional mediation invocations verify the presence of `PublicKeyCredential.isConditionalMediationAvailable()` before invoking `navigator.credentials.get({ mediation: 'conditional', ... })`.
- Components NEVER render forms or invoke conditional mediation assertions without abort controllers, ensuring pending requests are safely aborted before initiating modal ceremonies.

## Bad vs Good Implementations
### Server-Side WebAuthn Assertion Verification with Origin-Lock and Strict UV

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Deriving expectedOrigin and expectedRPID directly from request headers allows an attacker to bypass origin checks via reverse proxy confusion or DNS rebinding. Disabling user verification (requireUserVerification: false) permits unauthorized parties who possess an unlocked device to autofill assertions without biometric or PIN verification.
```typescript
import { verifyAuthenticationResponse } from '@simplewebauthn/server';
import type { Request, Response } from 'express';

export async function verifyAssertion(req: Request, res: Response) {
  const { credential, challenge } = req.body;
  // FLUSHED SECURITY HAZARDS:
  // 1. Dynamic origin read from hostile headers
  // 2. requireUserVerification set to false or omitted
  // 3. rpID derived dynamically from Host
  const verification = await verifyAuthenticationResponse({
    response: credential,
    expectedChallenge: challenge,
    expectedOrigin: req.headers.origin as string,
    expectedRPID: (req.headers.host || '').split(':')[0],
    requireUserVerification: false
  });

  if (verification.verified) {
    return res.json({ success: true });
  }
  return res.status(401).json({ error: 'Verification failed' });
}
```

#### Production Standard (Correct)
> **Standard**: Binds authentication exclusively to pre-configured, immutable canonical origin and relying party identifiers. Mandates hardware-backed User Verification (`requireUserVerification: true` and verifies `userVerified === true`), preventing passive session harvesting via autofill.
```typescript
import { verifyAuthenticationResponse, VerifiedAuthenticationResponse } from '@simplewebauthn/server';
import type { Request, Response } from 'express';

const CANONICAL_ORIGIN: string = process.env.AUTH_CANONICAL_ORIGIN!;
const CANONICAL_RP_ID: string = process.env.AUTH_CANONICAL_RP_ID!;

export async function verifyAssertion(req: Request, res: Response): Promise<Response> {
  const { credential } = req.body;
  const sessionChallenge = req.session?.webauthnChallenge;

  if (!sessionChallenge || typeof sessionChallenge !== 'string') {
    return res.status(400).json({ error: 'Active authentication challenge required' });
  }

  // Invalidate challenge immediately to prevent replay
  delete req.session.webauthnChallenge;

  try {
    const verification: VerifiedAuthenticationResponse = await verifyAuthenticationResponse({
      response: credential,
      expectedChallenge: sessionChallenge,
      expectedOrigin: CANONICAL_ORIGIN,
      expectedRPID: CANONICAL_RP_ID,
      requireUserVerification: true
    });

    if (!verification.verified || !verification.authenticationInfo.userVerified) {
      return res.status(401).json({ error: 'Hardware-backed user verification failed' });
    }

    req.session.userId = verification.authenticationInfo.credentialID;
    return res.json({ success: true });
  } catch (err: unknown) {
    return res.status(401).json({ error: 'Assertion verification rejected' });
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

WebAuthn Security Auditor MCP
Static AST analyzer for cryptographic authenticatorData bitmask checks and WebAuthn origin configurations.
Recommended Agent Skills (askills.dev)

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

FIDO2 Assertion CLI Validator
Validates CBOR structures, clientDataJSON, and raw authenticatorData flags for standard compliance.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn Conditional Mediation Origin-Lock & User Verification Standard

WebAuthn Conditional Mediation Origin-Lock & User Verification Standard defines an industrial-grade behavioral standard for security engineering. Enforces strict canonical origin binding, relying party ID validation, and mandatory hardware-backed User Verification (UV) flags during WebAuthn conditional mediation (passkey autofill) assertions. Eliminates session hijacking, automated autofill harvesting, and cross-site credential exploitation across unmediated and autofill flows. 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 Unverified User Flags or Permissive Origins During Conditional WebAuthn Autofill 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 DriftAccepting Unverified User Flags or Permissive Origins During Conditional WebAuthn AutofillCritical
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

Engineers MUST ALWAYS configure `userVerification

Engineers MUST ALWAYS configure `userVerification: 'required'` in WebAuthn assertion options, both on client-side `navigator.credentials.get` and server-side verification options.

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

Engineers MUST NOT rely on HTTP Host or X-Forwarded-Host headers to determine `expectedOrigin` during server-side verification

Engineers MUST NOT rely on HTTP Host or X-Forwarded-Host headers to determine `expectedOrigin` during server-side verification; origin MUST be strictly resolved against an immutable server-side canonical allowlist.

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

Services MUST cryptographically inspect the raw `authenticatorData` flags and verify that the User Verified (UV) bit (bit 2, mask 0x04) is asserted (1) for passkey assertion responses.

Services MUST cryptographically inspect the raw `authenticatorData` flags and verify that the User Verified (UV) bit (bit 2, mask 0x04) is asserted (1) for passkey assertion responses.

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

Code SHOULD verify that conditional mediation invocations verify the presence of `PublicKeyCredential.isConditionalMediationAvailable()` before invoking `navigator.credentials.get({ mediation

Code SHOULD verify that conditional mediation invocations verify the presence of `PublicKeyCredential.isConditionalMediationAvailable()` before invoking `navigator.credentials.get({ mediation: 'conditional', ... })`.

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

Components NEVER render forms or invoke conditional mediation assertions without abort controllers, ensuring pending requests are safely aborted before initiating modal ceremonies.

Components NEVER render forms or invoke conditional mediation assertions without abort controllers, ensuring pending requests are safely aborted before initiating modal ceremonies.

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 Conditional Mediation Origin-Lock & User Verification Standard critical for AI coding assistants?

Enforces strict canonical origin binding, relying party ID validation, and mandatory hardware-backed User Verification (UV) flags during WebAuthn conditional mediation (passkey autofill) assertions. Eliminates session hijacking, automated autofill harvesting, and cross-site credential exploitation across unmediated and autofill flows. 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 (Accepting Unverified User Flags or Permissive Origins During Conditional WebAuthn Autofill) 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.

Open Specification · CC BY 4.0·Free for Personal, Team & Commercial In-Repo Use

Citation & Open Source Attribution Guidelines

This specification is licensed under Creative Commons Attribution 4.0 International (CC BY 4.0). You are completely free to adapt and share this rule in your workspace. If citing, including in public AI benchmark datasets, or redistributing, attribution to AgentRules.org is required.

Related SECURITY Rules

View all rules →
security
105

OpenFGA Graph Invariants: Cyclic Relation Prevention & Contextual Tuple Hardening

Enforces strict directed acyclic graph (DAG) topology validation during relationship mutation writes and mandates strict server-side schema verification for ephemeral contextual tuples in OpenFGA evaluations. Prohibits persistent storage of ephemeral request attributes and guards against recursive relation loops causing graph expansion denial-of-service.

security
81

WebAuthn Conditional UI Mediation, Nonce Atomicity, and Assertion Cryptographic Verification

Enforces zero-trust cryptographic validation for FIDO2/WebAuthn Passkey authentication with Conditional UI autofill. Requires atomic single-use challenge consumption to eliminate replay attacks, strict RP ID hash and origin binding, authenticatorData flag verification (User Presence and User Verification), and monotonic signature counter enforcement to detect authenticator cloning.

security
111

Enforce Sigstore Keyless OIDC Attestation and Admission Controller Verification

Enforces immutable supply-chain security using Sigstore Cosign keyless signing via ephemeral OIDC tokens issued by Fulcio and verified against the Rekor transparency log. Container workloads must be signed and attested at build time by digest, and admission controllers (such as Kyverno or Sigstore Policy Controller) must strictly validate the OIDC issuer and subject identity before allowing image execution.

1 Rule in Stack

WebAuthn