Specifies trust boundaries, tokens or credentials, abuse controls, expiry, revocation, and threat mitigations for account linking. It fixes adoption boundaries and observable failure evidence while keeping environment-specific limits configurable.
Package status: reference context ready for human review. The contract and test scenarios are complete, but no claim is made that an adopting implementation has passed them.
Decision
For Account linking, adopt a server-side credential and session lifecycle with explicit assurance. Enforce the control on the trusted server side, default to denial, and define threat assumptions, abuse limits, evidence, and recovery. This security block focuses specifically on security protocol. This package is a reference contract: the adopting team must choose numeric limits, providers, jurisdictions, and operational owners from evidence in its own environment.
Scope
- The Account linking actors, inputs, outputs, states, policy or schema versions, and externally visible outcomes required for the security protocol focus of this security block.
- Primary and alternate interfaces, background work, caches, integrations, support paths, and evidence that can exercise the same boundary.
- Adoption-specific configuration, rollout, recovery, and verification responsibilities needed to apply the reference safely.
Outside this block
- Choosing a universal vendor, framework, jurisdiction, numeric threshold, retention period, or service-level objective for every adopter.
- Claiming that packaged scenarios have run against a downstream implementation or that this reference grants legal, security, or accessibility certification.
Contract
- Store only protected credential material, rotate signing and recovery secrets, and keep credentials out of URLs, logs, analytics, and support views.
- Bind issuance, use, expiry, revocation, recovery, and replay handling to a named subject and intended action.
- Return enumeration-resistant public responses while retaining sufficient internal reason and correlation data for support and abuse response.
- Re-evaluate sensitive actions after subject, device, authenticator, risk, or session state changes instead of trusting a stale login forever.
- Treat all client, callback, file, model, and telemetry fields as untrusted until authenticated and validated for the intended action.
- Require a second independent safeguard for high-impact paths where one parser, credential, or provider compromise would otherwise be decisive.
- Require recent authenticated intent for both identities, prove control through their authoritative issuers, and bind the link to stable issuer and subject identifiers rather than matching email text alone.
- Preview conflicts in profile, credentials, organizations, entitlements, consent, and history, prevent cross-tenant takeover, and make link, merge, unlink, and recovery independently authorized and auditable.
- For the security protocol variant, enforce this declared boundary: specifies trust boundaries, tokens or credentials, abuse controls, expiry, revocation, and threat mitigations for account linking.
Implementation guidance
- Threat-model normal, replay, enumeration, cross-tenant, automation, and degraded-dependency paths before implementation review.
- Model Account linking inputs, outputs, actors, states, invariants, side effects, and evidence before selecting framework or vendor details.
- Store the applicable Account linking contract or policy version with material state so migrations, replay, and support decisions remain attributable.
- Introduce the path behind controlled rollout, compare expected and observed outcomes, and keep a tested recovery route until adoption evidence is complete.
Failure handling and safeguards
- Missing, expired, replayed, or contradictory authentication evidence denies the action and follows a rate-bounded recovery path.
- Invalid or contradictory Account linking state is rejected or quarantined; the implementation never guesses a value that broadens authority or duplicates an effect.
- Retries are bounded and use durable identity; after exhaustion, work reaches an inspectable terminal state with an accountable owner.
Verification and operations
- Use paired allow and deny cases at every trust boundary and confirm that loss of the control dependency fails closed.
- Monitor Account linking success, denial, validation failure, dependency failure, retry exhaustion, and recovery by contract version without sensitive payload dimensions.
- Re-run paired positive and negative fixtures after policy, schema, dependency, migration, or boundary changes and preserve the resulting evidence.
Adoption assumptions
- Names and values in the example are a concrete fixture, not universal defaults; adopters replace them through documented evidence and ownership.
- The adopting system can provide authenticated identity, durable operation or record versions, bounded telemetry, and a controlled path for change.
The executable-looking examples in this package are fixtures and acceptance contracts. Run the collection validator to check structure and metadata, then translate and execute the scenarios in the target repository before recording implementation evidence.
References
- NIST SP 800-63B-4, Authentication and Authenticator Management (applies as of 2026-09-08)