webauthn-attestation-review

name	webauthn-attestation-review
description	Review WebAuthn/passkey attestation conveyance, statement formats, trust anchors, and AAGUID policy for registration. Use when choosing or auditing attestationPreference (none/indirect/direct/enterprise), MDS/trust-store verification, enterprise device allowlists, or privacy risks of storing full attestation objects on consumer vs high-assurance RPs.

WebAuthn Attestation Review

Deep review of attestation conveyance and verification at registration—not full ceremony design. Hand ceremony/assert to passkeys-webauthn-basics; hand OTP second factors to totp-mfa-implementation.

When To Use

• Setting or reviewing attestation / attestationPreference on create options.
• RP enforces hardware/device class via packed/tpm/android-key/apple statements, FIDO MDS, or enterprise vendor certs.
• Need AAGUID allow/deny, trust-store pinning, or enterprise attestation (EA).
• Privacy/compliance: storing attestation certs, serials, or batch keys.
• Keywords: none/indirect/direct/enterprise, attStmt, AAGUID, MDS, trust anchor, conveyance, self attestation, none fmt.

Not primary: full passkey register/assert → passkeys-webauthn-basics; TOTP → totp-mfa-implementation; MFA skip testing → mfa-bypass-methodology.

Scope And Authorization

• Owned apps, labs, CTFs, or written engagement scope only.
• Do not harvest production attestation certs, serials, or EA material from third-party users without authorization.
• Prefer staging authenticators and test MDS/trust packs; keep originals immutable; treat device-identifying attestation as sensitive in logs and reports.

Workflow

1. Inventory conveyance and policy intent

Field	Capture
Requested conveyance	none / indirect / direct / enterprise
RP assurance goal	Phishing-resistant login only vs device-class gate
Server verify path	Library options; custom CBOR/attStmt parsing
Trust material	MDS cache, vendor roots, enterprise CA, pin set
AAGUID policy	Allowlist, denylist, log-only, unused
Storage	Full attestationObject vs stripped after verify
Failure mode	Reject enroll vs soft-fail to none

create() options.attestation → authenticator may return attStmt
  → RP verifies fmt + trust path + AAGUID policy
  → store credential public key (+ optional metadata)

2. Conveyance types (risks and fit)

Preference	Typical result	When appropriate	Main risks
none	fmt=none / empty trust path	Consumer passkeys; privacy-first	No device proof; do not claim hardware assurance
indirect	Anonymized/batch or proxy attestation	Some signal without unique device ID	Still needs correct verify; proxy quality varies
direct	Full statement + chain when supported	High-assurance / regulated device binding	Privacy; broken verify = false confidence
enterprise	Platform EA when RP is enterprise-approved	Managed fleet, MDM-aligned RPs	Mis-issued EA; logging blobs; non-EA clients fail

Requesting direct/enterprise without a working trust store and reject path is worse than none (false hardware claims). Authenticators may downgrade despite direct—define accept vs reject for missing/untrusted statements.

3. Statement verification (when not none)

1. Parse attestationObject (authData + fmt + attStmt) via maintained lib.
2. Bind ceremony: challenge, origin, rpIdHash, AT flag, credential public key (same create checks as passkeys-webauthn-basics).
3. Per-fmt (packed, tpm, android-key, apple, fido-u2f, none, …): signature over expected bytes; cert chain when present.
4. Trust store: pin roots / current MDS; reject unknown roots and expired/revoked paths; never trust “any chain in attStmt.”
5. Self / none: no hardware proof—only if policy is explicitly none-equivalent.
6. Persist COSE key, credential id, signCount, AAGUID/transports as needed; drop or encrypt raw certs unless retention is required.

4. AAGUID and metadata policy

Control	Secure behavior
Extract AAGUID	From authenticator data (16 bytes); stable policy key
Allowlist	High-assurance RPs: only approved models/vendors
Denylist	Known-bad or retired authenticators
MDS fields	Status (revoked, update available)—not sole trust
Missing AAGUID	Define reject vs accept (platform passkeys vary)

Document enforced vs telemetry-only. Do not equate AAGUID marketing names with security without a verified trust path.

5. Conveyance and storage risks

• False assurance: accept enroll when verify fails or is skipped.
• Privacy: long-lived unique certs/serials enable tracking; prefer summary (AAGUID, type, verify result).
• Supply chain: stale MDS / unpinned vendor CA accepts untrusted authenticators.
• Enterprise EA: only for EA-registered RPs and managed populations; BYOD fallback must not silently lower the bar without UX.
• Logging: redact attStmt, cert PEMs, and challenges.

6. Implementation quality

Apply code-quality-standards: typed options; accept/reject matrix unit tests; virtual-authenticator integration tests; prefer library attestation + MDS support over hand-rolled CBOR/crypto.

Routing

Need	Skill
Attestation conveyance, trust store, AAGUID policy	This skill
Full passkey register/assert, RP ID, origin, UV, counter	passkeys-webauthn-basics
Authenticator-app TOTP (not WebAuthn)	totp-mfa-implementation
MFA skip, backup codes, step-up gaps	mfa-bypass-methodology
Session/JWT amr/acr after assert	api-auth-and-jwt-abuse
Secure coding, tests, review baseline	code-quality-standards

Selection: attestation policy/verify/trust → this skill. Ceremony lifecycle → passkeys-webauthn-basics. OTP MFA → totp-mfa-implementation. Fixes → code-quality-standards.

Output Checklist

• Requested conveyance and RP assurance goal documented
• Server verifies or explicitly ignores attestation (no silent skip)
• Trust anchors / MDS / enterprise CA source, refresh, pin strategy
• Per-fmt accept/reject; self/none policy explicit
• AAGUID allow/deny or log-only; missing-AAGUID rule
• Downgrade (omitted statement): reject vs allow
• Storage minimized; certs redacted; EA/BYOD fallback documented
• Tests: good chain, bad root, wrong AAGUID, none fmt, verify-fail reject
• Handoffs: ceremony → passkeys-webauthn-basics; TOTP → totp-mfa-implementation