WebAuthn Conditional Mediation AbortController Lifecycle Standard

--- description: WebAuthn Conditional Mediation AbortController Lifecycle Standard - Enforces deterministic cancellation and resource cleanup for ambient WebAuthn Conditional UI (autofill) credential requests. Prevents DOMException InvalidStateError collisions, UI thread deadlocks, and race conditions when users switch from autofill to explicit modal authentication flows or navigate away. globs: src/**/*.{ts,tsx}, app/**/*.{ts,tsx}, lib/auth/**/*.{ts,tsx} alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/webauthn-conditional-ui-abortcontroller-standard" --- # WebAuthn Conditional Mediation AbortController Lifecycle Standard > **Severity**: CRITICAL | **Category**: security > **Anti-Pattern Warning**: Uncontrolled Ambient WebAuthn Autofill Blocking Explicit Modal Authentication Invocations ## System Constraints You are a Principal Software Architect enforcing production security standards for WebAuthn Conditional UI (Passkey autofill) and AbortController lifecycle management. Only one WebAuthn credential ceremony may execute in a browser browsing context at any given time. Conditional UI requests occupy the browser's credential mediation pipeline continuously until aborted or fulfilled. You must strictly mandate that all conditional credential requests pass an AbortSignal, abort cleanly before triggering modal assertions, and catch AbortError exceptions without polluting error-monitoring pipelines or failing authentication state. ## Strict Guidelines - Engineers MUST ALWAYS bind an explicit `AbortController.signal` to every `navigator.credentials.get({ mediation: 'conditional', ... })` invocation. - Engineers MUST abort any in-flight conditional authentication request immediately before initiating an explicit modal credential call (`navigator.credentials.get()` with `mediation: 'optional'` | `'required'`, or `navigator.credentials.create()`). - Engineers MUST specifically filter and swallow `DOMException` instances where `name === 'AbortError'` in conditional flows, preventing telemetry pollution and false-positive user authentication errors. - Engineers MUST NEVER reuse an `AbortController` instance once aborted; instantiate a fresh `AbortController` for each authentication invocation. - Components and page controllers MUST invoke `abortController.abort()` in their lifecycle teardown or unmount callbacks (e.g., `useEffect` cleanup, routing transitions). - Services MUST guard WebAuthn invocations against concurrency using an atomic state transition or a dedicated credential mediation manager to prevent concurrent ceremony attempts. ## Bad vs Good Implementations ### Conditional UI to Modal Authentication Transition #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: Failing to pass an AbortSignal and aborting the conditional autofill request causes the modal authentication ceremony to fail immediately with an InvalidStateError DOMException, locking the user out of manual authentication. ```typescript class PasskeyAuthService { // Anti-Pattern: In-flight conditional UI lacks lifecycle cancellation, // causing explicit modal calls to reject with InvalidStateError. async startConditionalAutofill(options: CredentialRequestOptions): Promise<Credential | null> { return await navigator.credentials.get({ ...options, mediation: 'conditional' }); } async triggerModalLogin(options: CredentialRequestOptions): Promise<Credential | null> { // Throws DOMException: A request is already pending or not allowed in this state return await navigator.credentials.get(options); } } ``` #### Production Standard (Correct) > **Standard**: The service maintains an AbortController instance for the ambient conditional UI call, aborts it prior to triggering modal authentication, and gracefully absorbs AbortError DOMExceptions without corrupting application state. ```typescript class PasskeyAuthService { private conditionalAbortController: AbortController | null = null; async startConditionalAutofill(options: CredentialRequestOptions): Promise<Credential | null> { this.cancelConditionalAutofill(); this.conditionalAbortController = new AbortController(); try { return await navigator.credentials.get({ ...options, mediation: 'conditional', signal: this.conditionalAbortController.signal }); } catch (err: unknown) { if (err instanceof DOMException && err.name === 'AbortError') { // Expected graceful cancellation; return null without throwing or alerting return null; } throw err; } finally { this.conditionalAbortController = null; } } cancelConditionalAutofill(): void { if (this.conditionalAbortController && !this.conditionalAbortController.signal.aborted) { this.conditionalAbortController.abort('New authentication ceremony initiated'); this.conditionalAbortController = null; } } async triggerModalLogin(options: CredentialRequestOptions): Promise<Credential | null> { // Deterministically abort pending autofill before entering modal prompt this.cancelConditionalAutofill(); return await navigator.credentials.get(options); } } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn Conditional Mediation AbortController Lifecycle Standard

WebAuthn Conditional Mediation AbortController Lifecycle Standard defines an industrial-grade behavioral standard for security engineering. Enforces deterministic cancellation and resource cleanup for ambient WebAuthn Conditional UI (autofill) credential requests. Prevents DOMException InvalidStateError collisions, UI thread deadlocks, and race conditions when users switch from autofill to explicit modal authentication flows or navigate away. 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: Uncontrolled Ambient WebAuthn Autofill Blocking Explicit Modal Authentication Invocations 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	Uncontrolled Ambient WebAuthn Autofill Blocking Explicit Modal Authentication Invocations	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 bind an explicit `AbortController.signal` to every `navigator.credentials.get({ mediation

Engineers MUST ALWAYS bind an explicit `AbortController.signal` to every `navigator.credentials.get({ mediation: 'conditional', ... })` invocation.

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.