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securitycritical PriorityID: webauthn-conditional-ui-rp-isolation

WebAuthn Conditional UI RP Isolation & Tenant Boundary Invariant

Mandates strict Relying Party ID (rpId) and clientDataJSON origin isolation for WebAuthn Conditional Mediation autofill across federated multi-tenant architectures. Prevents cross-subdomain passkey harvesting, ambient tenant credential leakage, and origin spoofing by cryptographically pinning tenant context to WebAuthn challenge generation and assertion verification.

Prohibited Anti-Pattern: Permissive Root Domain RP ID Assignment and Unvalidated Dynamic Origin Trust in Passkey Autofill

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---
description: WebAuthn Conditional UI RP Isolation & Tenant Boundary Invariant - Mandates strict Relying Party ID (rpId) and clientDataJSON origin isolation for WebAuthn Conditional Mediation autofill across federated multi-tenant architectures. Prevents cross-subdomain passkey harvesting, ambient tenant credential leakage, and origin spoofing by cryptographically pinning tenant context to WebAuthn challenge generation and assertion verification.
globs: src/auth/**/*.{ts,tsx}, src/server/auth/**/*.{ts,tsx}, packages/auth/src/**/*.{ts,tsx}, apps/**/api/auth/**/*.{ts,tsx}
alwaysApply: true
verified_by: "agentrules.org"
license: "CC-BY-4.0"
source: "https://agentrules.org/rules/webauthn-conditional-ui-rp-isolation"
---

# WebAuthn Conditional UI RP Isolation & Tenant Boundary Invariant

> **Severity**: CRITICAL | **Category**: security
> **Anti-Pattern Warning**: Permissive Root Domain RP ID Assignment and Unvalidated Dynamic Origin Trust in Passkey Autofill

## System Constraints
You are a Principal Security Architect enforcing the WebAuthn Conditional UI Relying Party Isolation Invariant. Mandate that all passkey autofill workflows strictly bind challenge generation, RP ID resolution, and assertion verification to isolated, cryptographically verified tenant boundaries. Reject any code that derives RP IDs directly from unsanitized Host/Origin headers or pools credentials across shared root domains in untrusted multi-tenant environments.

## Strict Guidelines
- Engineers MUST ALWAYS resolve Relying Party ID (rpId) against an explicit, immutable allowlist derived from verified tenant metadata rather than client-controlled Host or Origin headers.
- Engineers MUST NOT configure a shared root domain as the rpId (e.g., 'example.com') across untrusted or dynamically provisioned tenant subdomains (e.g., 'tenant-a.example.com' vs 'tenant-b.example.com').
- Services MUST enforce that the parsed 'clientDataJSON.origin' exactly matches the canonical tenant origin using strict URL parsing, rejecting wildcard subdomains and protocol-relative variations.
- Components MUST bind WebAuthn Conditional UI ('mediation: conditional') lifecycle to an AbortController and immediately abort active credential mediation before executing non-conditional requests or cross-tenant navigation.
- Services MUST verify in the credential store that the asserted credential ID is explicitly registered under the target tenant partition prior to verifying the cryptographic signature.
- Components NEVER invoke 'navigator.credentials.get()' with conditional mediation without first checking 'PublicKeyCredential.isConditionalMediationAvailable()' returns true.

## Bad vs Good Implementations
### Multi-Tenant RP ID Resolution and Conditional UI Invocation

#### Bad Antipattern (AI Common Hallucination)
> **Why it fails**: Deriving rpId dynamically from host headers or stripping subdomains down to the apex domain allows malicious or compromised subtenants to request and capture passkeys across tenant boundaries. Additionally, invoking conditional mediation without feature detection and abort lifecycle management risks unhandled runtime rejections and credential collision.
```typescript
import { generateAuthenticationOptions, verifyAuthenticationResponse } from '@simplewebauthn/server';

// ANTI-PATTERN: Dynamically inferring rpId from raw host header without tenant boundary isolation,
// and using shared apex domain that exposes all subtenants to cross-origin credential harvesting.
export async function getAuthOptions(req: Request) {
  const host = req.headers.get('host') || 'enterprise.com';
  return generateAuthenticationOptions({
    rpID: host.includes('.') ? host.split('.').slice(-2).join('.') : host,
    userVerification: 'preferred',
  });
}

export async function handleClientAutofill() {
  // ANTI-PATTERN: Missing feature detection, unmanaged AbortController, unbounded autofill
  const credential = await navigator.credentials.get({
    mediation: 'conditional',
    publicKey: { challenge: new Uint8Array([1, 2, 3]) } as any,
  });
  return credential;
}
```

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

interface TenantConfig {
  readonly tenantId: string;
  readonly canonicalOrigin: string;
  readonly rpId: string;
}

export async function getAuthOptions(
  tenant: TenantConfig,
  sessionChallengeStore: Map<string, string>
) {
  // INVARIANT: rpId is pinned to the verified tenant configuration
  const options = await generateAuthenticationOptions({
    rpID: tenant.rpId,
    userVerification: 'required',
  });

  sessionChallengeStore.set(tenant.tenantId, options.challenge);
  return options;
}

export async function verifyTenantAssertion(
  response: any,
  tenant: TenantConfig,
  expectedChallenge: string,
  tenantCredentialStore: { getCredential(id: string, tenantId: string): Promise<any> }
) {
  const credential = await tenantCredentialStore.getCredential(response.id, tenant.tenantId);
  if (!credential) {
    throw new SecurityError(`Credential not bound to tenant: ${tenant.tenantId}`);
  }

  return verifyAuthenticationResponse({
    response,
    expectedChallenge,
    expectedOrigin: tenant.canonicalOrigin,
    expectedRPID: tenant.rpId,
    credential: {
      id: credential.id,
      publicKey: credential.publicKey,
      counter: credential.counter,
    },
  });
}

export class PasskeyAutofillController {
  private abortController: AbortController | null = null;

  async initialize(options: PublicKeyCredentialRequestOptions): Promise<Credential | null> {
    if (typeof window === 'undefined' || !window.PublicKeyCredential) return null;
    
    const available = await window.PublicKeyCredential.isConditionalMediationAvailable?.();
    if (!available) return null;

    this.abortController?.abort('Re-initializing credential mediation');
    this.abortController = new AbortController();

    try {
      return await navigator.credentials.get({
        mediation: 'conditional',
        publicKey: options,
        signal: this.abortController.signal,
      });
    } catch (err: unknown) {
      if (err instanceof DOMException && err.name === 'AbortError') return null;
      throw err;
    }
  }

  abort(): void {
    this.abortController?.abort('Component unmounted or navigation initiated');
    this.abortController = null;
  }
}
```
Recommended MCP Servers (mcp-serverlist.com)

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

WebAuthn Protocol Inspector MCP
Validates CBOR decoding, clientDataJSON structures, and RP ID scoping against FIDO2 specifications.
Recommended Agent Skills (askills.dev)

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

FIDO2/WebAuthn Assertion Verifier CLI
Validates cryptographic signatures, AAGUIDs, and RP ID integrity in assertion payloads.
Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn Conditional UI RP Isolation & Tenant Boundary Invariant

WebAuthn Conditional UI RP Isolation & Tenant Boundary Invariant defines an industrial-grade behavioral standard for security engineering. Mandates strict Relying Party ID (rpId) and clientDataJSON origin isolation for WebAuthn Conditional Mediation autofill across federated multi-tenant architectures. Prevents cross-subdomain passkey harvesting, ambient tenant credential leakage, and origin spoofing by cryptographically pinning tenant context to WebAuthn challenge generation and assertion verification. 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: Permissive Root Domain RP ID Assignment and Unvalidated Dynamic Origin Trust in Passkey 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 DriftPermissive Root Domain RP ID Assignment and Unvalidated Dynamic Origin Trust in Passkey 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 resolve Relying Party ID (rpId) against an explicit, immutable allowlist derived from verified tenant metadata rather than client-controlled Host or Origin headers.

Engineers MUST ALWAYS resolve Relying Party ID (rpId) against an explicit, immutable allowlist derived from verified tenant metadata rather than client-controlled Host or Origin headers.

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 configure a shared root domain as the rpId (e.g., 'example.com') across untrusted or dynamically provisioned tenant subdomains (e.g., 'tenant-a.example.com' vs 'tenant-b.example.com').

Engineers MUST NOT configure a shared root domain as the rpId (e.g., 'example.com') across untrusted or dynamically provisioned tenant subdomains (e.g., 'tenant-a.example.com' vs 'tenant-b.example.com').

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 enforce that the parsed 'clientDataJSON.origin' exactly matches the canonical tenant origin using strict URL parsing, rejecting wildcard subdomains and protocol-relative variations.

Services MUST enforce that the parsed 'clientDataJSON.origin' exactly matches the canonical tenant origin using strict URL parsing, rejecting wildcard subdomains and protocol-relative variations.

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

Components MUST bind WebAuthn Conditional UI ('mediation

Components MUST bind WebAuthn Conditional UI ('mediation: conditional') lifecycle to an AbortController and immediately abort active credential mediation before executing non-conditional requests or cross-tenant navigation.

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

Services MUST verify in the credential store that the asserted credential ID is explicitly registered under the target tenant partition prior to verifying the cryptographic signature.

Services MUST verify in the credential store that the asserted credential ID is explicitly registered under the target tenant partition prior to verifying the cryptographic signature.

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.
6

Components NEVER invoke 'navigator.credentials.get()' with conditional mediation without first checking 'PublicKeyCredential.isConditionalMediationAvailable()' returns true.

Components NEVER invoke 'navigator.credentials.get()' with conditional mediation without first checking 'PublicKeyCredential.isConditionalMediationAvailable()' returns true.

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 UI RP Isolation & Tenant Boundary Invariant critical for AI coding assistants?

Mandates strict Relying Party ID (rpId) and clientDataJSON origin isolation for WebAuthn Conditional Mediation autofill across federated multi-tenant architectures. Prevents cross-subdomain passkey harvesting, ambient tenant credential leakage, and origin spoofing by cryptographically pinning tenant context to WebAuthn challenge generation and assertion verification. 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 (Permissive Root Domain RP ID Assignment and Unvalidated Dynamic Origin Trust in Passkey 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.

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