React Rsc Data Boundary Review

React RSC Data Boundary Review

Purpose

Review React Server Components (RSC) code for the concrete, documented ways sensitive server-side data leaks across the server-to-client serialization boundary: a secret passed as a prop such as password={SECRET_API_KEY} on a Client Component, a server-only data module with no server-only guard that a client bundle could accidentally pull in, a 'use server' Server Action that mutates data with no session/ownership check (missing the session?.user check pattern), a 'use client' module reading a non-NEXT_PUBLIC_ environment variable, or a value that should have been marked with experimental_taintUniqueValue and narrowed before crossing the boundary. This skill exists so the review stays anchored to these five documented defect classes instead of drifting into a general "React code review" of component design, hooks usage, or rendering performance.

When to use

Use this skill when the user asks to:

• review a Server Component that passes props to a Client Component,
• audit a data-access module (lib/, data/, or similar) that is meant to run only on the server,
• review a 'use server' Server Action or Server Function for authorization gaps,
• check whether a 'use client' module is reading environment variables safely,
• perform a pre-launch security review of a Next.js App Router or other RSC-based application's data flow.

Do not use this skill for:

• generic client-side XSS review (dangerouslySetInnerHTML, unsanitized innerHTML, DOM-based injection) — that is a distinct defect class covered by frontend-dom-xss-csp-review, not this skill,
• Next.js rendering/caching-strategy review (revalidation, cache tags, fetch cache options) with no data-boundary security angle — use nextjs-rendering-caching-review or nextjs-app-router-data-fetching-review instead,
• confirming that a leak has actually been exploited in production — static analysis proves the structural risk (a secret is reachable across the boundary) and not that an attacker has already captured it; that requires live request/network interception, which this skill does not perform.

Context7 Documentation Protocol

• Resolve the React library ID with resolve-library-id (matched result: /reactjs/react.dev) before citing any experimental_taintUniqueValue or Server/Client Component serialization claim. Use query-docs against it to confirm the exact tainting API shape and the documented anti-pattern (a secret such as process.env.API_PASSWORD passed directly as a prop) before flagging a finding as documentation-based.
• Resolve the Next.js library ID with resolve-library-id (matched result: /vercel/next.js) before citing any server-only package usage, NEXT_PUBLIC_ environment-variable convention, or Server Action authorization pattern claim. Use query-docs against it for the server-only install/import pattern and the documented Server Action authorization example (auth() + ownership check) before labeling a finding documentation-based.
• experimental_taintUniqueValue is an experimental React API, not yet stable — label any finding or fix sketch that depends on it as documentation-based (experimental API), and note that the equally valid non-experimental fix is simply omitting the sensitive field from the object passed to the Client Component.
• Confirm which surface is in scope before citing a documented rule: a "Server Component passing props" claim only applies where a Server Component (no 'use client' directive, or an RSC-context module) actually renders a Client Component; a 'use server' claim only applies to a function or file carrying that exact directive; a NEXT_PUBLIC_ claim is Next.js–specific and does not automatically apply to a different meta-framework's env-var convention.
• If Context7 is unavailable, fall back to the official_docs URLs in this skill's metadata.json and label the claim documentation-based, unverified against current release.

Lean operating rules

• All five defect categories default to HIGH severity. This is a security-scoped skill: do not downgrade an untraced secret-prop pass, a missing server-only guard, or an unauthorized use server mutation to MEDIUM just because it has not been observed exploited yet — the risk is in the structure, not in whether someone has already hit it.
• Trace every finding to a concrete file:line and a concrete data-flow path. A finding that says "this prop might leak a secret" without naming the specific prop, the specific Server Component that renders it, and the specific Client Component receiving it is not a valid finding — it is a guess.
• Do not flag every prop passed from a Server Component to a Client Component. Only a prop whose value traces back to an environment variable, a credential, a token, a full unfiltered config/response object, or any other server-only sensitive value is a finding. A plain string, number, or narrowed non-sensitive field (e.g. config.SERVICE_API_VERSION) is not a finding.
• Do not approve a 'use server' action that mutates or deletes data unless a session check (the session?.user pattern) and, where the mutation targets a specific resource, an ownership check comparing the resource's owner to the authenticated user, are both visibly present on that exact function's path. A session check existing in a different action does not clear this bar — trace the specific function under review.
• Do not treat a server-only import anywhere in the codebase as covering every server-only module. Confirm the exact file that reads the sensitive value (process.env.*, a database credential, an internal API token) has server-only imported at its own top, not merely that some other file in the project has it.
• Watch for whole-object prop forwarding: a Server Component that fetches a config/response object and forwards it unnarrowed as a same-named prop (e.g. config={config}) crosses the boundary with whatever sensitive fields the object holds, even if no single field is individually named "secret" or "password" in the forwarding code. Narrowing to specific non-sensitive fields, or applying experimental_taintUniqueValue to the sensitive fields before any possible pass-through, are the two documented mitigations.
• Never execute, build, or run application code, and never send live requests, as part of this review; this is a static-review skill (Read/Grep/Glob only).
• Load only the reference needed for the concern in scope.

References

Load these only when needed:

• Review workflow and findings contract — use for the step-by-step review procedure, the decision tree across all five defect categories, and the required output shape.
• Boundary data leaks and the taint API — load only when the review scope includes a prop passed from a Server Component to a Client Component, a server-only guard question, or a value that should have been tainted.
• Server Action authorization and client environment exposure — load only when the review scope includes a 'use server' directive or a 'use client' module reading process.env.

Response minimum

Return, at minimum:

• the Server Component(s), Client Component prop boundaries, 'use server' action(s), and/or 'use client' module(s) in scope,
• ranked findings with file:line evidence, defect category (boundary-data-leak, missing-server-only-guard, action-authz-missing, env-exposure-in-client, or taint-boundary-violation), the concrete data-flow trace (the sensitive value's origin and every hop to the boundary crossing or missing guard), and a fix sketch matching React's or Next.js's documented pattern,
• guard status per finding: an explicit statement of whether a server-only import, an experimental_taintUniqueValue call, or a session/ownership check is present on the traced path — never approve on the assumption one exists elsewhere,
• evidence level per finding (repo evidence, documentation-based, or structural-risk), with structural risk findings explicitly labeled as structural risk, not as confirmed-exploited,
• verdict (approve / approve-with-notes / block),
• open questions or scope the review could not cover (e.g., "confirming data leakage requires live request interception (network inspection), not static review" or "taint API coverage requires a Context7 audit of all async boundaries where promises serialize, beyond this review's scope").