WebAuthn Level 3 LargeBlob Atomic Payload Verification and State Integrity

Q: How does adopting this rule prevent AI hallucinations?

This rule explicitly declares what the AI must avoid (Blind Overwriting and Unverified Deserialization of Authenticator LargeBlob State) 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.

WebAuthn Level 3 LargeBlob Atomic Payload Verification and State Integrity

--- description: WebAuthn Level 3 LargeBlob Atomic Payload Verification and State Integrity - Enforces cryptographic signature verification, schema validation, and two-phase atomic state management when writing and reading WebAuthn Level 3 largeBlob credential extensions. Guarantees that client-side biometric assertions cannot commit partial, corrupted, or unauthenticated payloads to authenticator storage during transaction lifecycles. globs: src/services/auth/**/*.{ts,tsx}, src/lib/webauthn/**/*.{ts,tsx}, src/features/credentials/**/*.{ts,tsx}, src/server/auth/**/*.ts alwaysApply: true verified_by: "agentrules.org" license: "CC-BY-4.0" source: "https://agentrules.org/rules/webauthn-l3-largeblob-atomic-payload-verification" --- # WebAuthn Level 3 LargeBlob Atomic Payload Verification and State Integrity > **Severity**: CRITICAL | **Category**: security > **Anti-Pattern Warning**: Blind Overwriting and Unverified Deserialization of Authenticator LargeBlob State ## System Constraints You are a Principal Security Architect enforcing the WebAuthn Level 3 LargeBlob Atomic Payload Verification Standard. You must mandate strict adherence to two-phase commit verification patterns, cryptographic tamper-evidence (AEAD envelopes or detached HMAC/signatures), schema parsing before persistence, and rollback semantics on assertion rejection. Any write to `navigator.credentials.get` with `largeBlob.write` without prior server-nonce validation, client-side sequence tracking, and authenticated integrity tags must be rejected. ## Strict Guidelines - Engineers MUST ALWAYS verify client extension results using `credential.getClientExtensionResults().largeBlob` and assert `written: true` before updating the application's local or remote synchronized state. - Engineers MUST ALWAYS wrap largeBlob binary payloads in an authenticated cryptographic container (e.g., AES-GCM envelope or Ed25519/HMAC signed payload with monotonic sequence counters) to prevent replay and silent truncation. - Engineers MUST NOT mutate local database or memory credential states optimistically prior to the fulfillment of the navigator.credentials call with verified extension success. - Code SHOULD validate the size bounds of the serialized buffer before invoking the authenticator interface, strictly ensuring it stays within supported authenticator limits (typically <= 1024 bytes unless capability-probed). - Services MUST rollback and trigger a security reconciliation event if the biometric assertion completes successfully but the `largeBlob.written` status evaluates to false or undefined. - Components NEVER store plaintext sensitive tokens or unversioned raw byte arrays directly in `largeBlob.write` without schema migration version headers. ## Bad vs Good Implementations ### Unauthenticated and Optimistic LargeBlob Mutation (Anti-Pattern vs Invariant Compliant) #### Bad Antipattern (AI Common Hallucination) > **Why it fails**: The code updates client-side state optimistically before biometric confirmation, passes raw unauthenticated JSON without monotonic counters or HMAC/AEAD verification, and ignores the returned getClientExtensionResults().largeBlob.written verification flag, leading to state desynchronization and potential replay attacks. ```typescript import { get } from '@github/webauthn-json'; interface LocalState { encryptionKey: string; version: number; } export async function updateCredentialBlob(credentialId: string, newState: LocalState): Promise<void> { const encoder = new TextEncoder(); const rawBytes = encoder.encode(JSON.stringify(newState)); // CRITICAL FLAW: Optimistically updating local DB before authenticator confirmation localStorage.setItem(`cred_state_${credentialId}`, JSON.stringify(newState)); // CRITICAL FLAW: Blindly writing unverified, unsigned plaintext buffer to largeBlob await navigator.credentials.get({ publicKey: { challenge: new Uint8Array([1, 2, 3, 4]), allowCredentials: [{ id: new Uint8Array(Buffer.from(credentialId, 'base64')), type: 'public-key' }], extensions: { largeBlob: { write: rawBytes.buffer } } } }); // CRITICAL FLAW: Does not inspect largeBlob extension output for `written: true` } ``` #### Production Standard (Correct) > **Standard**: Validates payload schema and safe size bounds prior to execution, enforces cryptographic envelope requirements, handles two-phase commit rollback via an explicit rollback handler if the assertion fails, and strictly inspects `getClientExtensionResults().largeBlob.written` before returning success. ```typescript import { z } from 'zod'; const LargeBlobPayloadSchema = z.object({ version: z.literal(1), sequenceNumber: z.number().int().positive(), iv: z.string().base64(), ciphertext: z.string().base64(), tag: z.string().base64(), serverNonce: z.string() }); type LargeBlobPayload = z.infer<typeof LargeBlobPayloadSchema>; export async function writeLargeBlobAtomic( credentialId: Uint8Array, challenge: Uint8Array, payload: LargeBlobPayload, onRollback: (reason: string) => Promise<void> ): Promise<boolean> { // 1. Verify schema integrity and payload bounds prior to WebAuthn call LargeBlobPayloadSchema.parse(payload); const encodedBuffer = new TextEncoder().encode(JSON.stringify(payload)); if (encodedBuffer.byteLength > 1024) { throw new RangeError(`Payload exceeds safe WebAuthn largeBlob bounds (${encodedBuffer.byteLength} > 1024)`); } // 2. Perform assertion with transactional write intention const credential = await navigator.credentials.get({ publicKey: { challenge, allowCredentials: [{ id: credentialId, type: 'public-key' }], userVerification: 'required', extensions: { largeBlob: { write: encodedBuffer } } } }) as PublicKeyCredential | null; if (!credential) { await onRollback('Assertion rejected or aborted by user'); return false; } // 3. Cryptographically and structurally verify extension result const extensionResults = credential.getClientExtensionResults(); const isWritten = extensionResults.largeBlob?.written === true; if (!isWritten) { await onRollback('Authenticator succeeded assertion but failed largeBlob write commit'); return false; } return true; } ```

Architectural Specification & Invariant Rationale

Architectural Invariant Design: WebAuthn Level 3 LargeBlob Atomic Payload Verification and State Integrity

WebAuthn Level 3 LargeBlob Atomic Payload Verification and State Integrity defines an industrial-grade behavioral standard for security engineering. Enforces cryptographic signature verification, schema validation, and two-phase atomic state management when writing and reading WebAuthn Level 3 largeBlob credential extensions. Guarantees that client-side biometric assertions cannot commit partial, corrupted, or unauthenticated payloads to authenticator storage during transaction lifecycles. 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 Overwriting and Unverified Deserialization of Authenticator LargeBlob State 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	Blind Overwriting and Unverified Deserialization of Authenticator LargeBlob State	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 verify client extension results using `credential.getClientExtensionResults().largeBlob` and assert `written

Engineers MUST ALWAYS verify client extension results using `credential.getClientExtensionResults().largeBlob` and assert `written: true` before updating the application's local or remote synchronized state.

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.