Defines assets, threats, trust boundaries, required controls, secure defaults, and implementation guidance for XSS. This reference fixes the adoption boundary, failure behavior, and observable evidence while leaving 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 XSS, adopt a threat model and server-enforced control at the protected boundary. Enforce the control on the trusted server side, default to denial, and define threat assumptions, abuse limits, evidence, and recovery. 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 XSS actors, inputs, outputs, states, policy or schema versions, and externally visible outcomes described by 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
- Name assets, actors, trust boundaries, attacker capabilities, abuse cases, assumptions, and residual risk before selecting controls.
- Fail closed for protected operations when identity, policy, integrity, freshness, or resource scope cannot be established.
- Bound work and attempts per actor, credential, tenant, resource, and infrastructure capacity without trusting client-supplied identity alone.
- Keep secrets and sensitive payloads out of logs while retaining control version, correlation, decision, and safe reason as evidence.
- 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.
- Encode untrusted data for its exact HTML, attribute, URL, JavaScript, CSS, or DOM context and use vetted sanitization when user-authored markup is intentionally supported.
- Avoid dangerous sinks and string-built code, apply a restrictive defense-in-depth content policy, and test stored, reflected, DOM, template, SVG, and rich-content paths.
Implementation guidance
- Threat-model normal, replay, enumeration, cross-tenant, automation, and degraded-dependency paths before implementation review.
- Model XSS inputs, outputs, actors, states, invariants, side effects, and evidence before selecting framework or vendor details.
- Store the applicable XSS 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
- Control degradation preserves the protected boundary, raises an owned incident signal, and never grants access solely to maintain availability.
- Invalid or contradictory XSS 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 XSS 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.