Web Platform Security

Web Platform Security

Audit the browser-enforced security primitives that every web app depends on, regardless of framework. These are the controls the browser itself implements — getting them right makes entire vulnerability classes (XSS impact, clickjacking, CSRF, third-party data exfiltration, cross-origin data leaks) much harder to exploit.

The skill is grounded in first-party sources: web.dev articles, developer.chrome.com, the Fetch / HTML / Cookies / WebAuthn specs, and Chromium intent-to-ship records. When upstream guidance changes (e.g., Chrome rolls out Private Network Access enforcement, CHIPS partitioned cookies graduate from origin trial), this skill's checks update before the next scheduled review.

When this skill applies

• Reviewing HTTP response headers on a deployed web app
• Designing or hardening a Content Security Policy
• Auditing cookie attributes for session, CSRF, and tracking cookies
• Enabling cross-origin isolation to use SharedArrayBuffer, high-resolution performance.now(), or Cross-Origin-Embedder-Policy: require-corp
• Replacing third-party-cookie SSO with FedCM
• Integrating WebAuthn / passkeys
• Preparing for Chrome's Private Network Access enforcement (preflight from public to private IPs)
• Migrating to CHIPS (Partitioned cookies) for cross-site embedded contexts

Use other skills for:

• App-layer XSS sink review → react-security, vue-nuxt-security, etc., or saas-code-security-review
• Framework-specific header injection patterns → nextjs-security, svelte-sveltekit-security, etc.
• API rate limiting and webhook signatures → saas-api-security
• Server-side TLS / certificate config → iac-container-security

This skill is the deep-dive platform reference; the others can defer to it for browser primitives.

Workflow

Follow ../_shared/audit-workflow.md. Web-platform-specific notes below.

Phase 1: Scope confirmation

• Production domain(s) and any preview/staging origins (CSP allowlist will need them)
• Embedded contexts: does the app appear in iframes elsewhere? Do you embed third-party iframes?
• Auth model: cookie-based session, Bearer token, OAuth via redirect, OIDC via iframe, WebAuthn?
• Browser baseline: evergreen-only, or do you support Safari ≤ 15, Firefox ESR, etc.? (Affects which features can be required.)

Phase 2: Inventory

# Headers on a representative URL
curl -sIL -H 'Accept: text/html' https://app.example.com/ \
  | grep -iE 'content-security-policy|strict-transport-security|x-frame-options|x-content-type-options|referrer-policy|permissions-policy|cross-origin-opener-policy|cross-origin-embedder-policy|cross-origin-resource-policy|set-cookie|x-permitted-cross-domain-policies'

# CORS surface for an API endpoint
curl -sI -X OPTIONS \
  -H 'Origin: https://probe.example' \
  -H 'Access-Control-Request-Method: POST' \
  -H 'Access-Control-Request-Headers: content-type,authorization' \
  https://api.example.com/v1/users

# Cookies issued at login (use a real test account)
curl -sI -c - https://app.example.com/login -d 'email=...&password=...' \
  | grep -i set-cookie

External evaluators worth running:

• https://csp-evaluator.withgoogle.com — paste the CSP, see weaknesses
• https://securityheaders.com — overall grade
• https://hstspreload.org — HSTS preload eligibility
• Chrome DevTools → Application → Frames → top → Security — shows cross-origin isolation state, COOP/COEP report-only violations, cookie audit

Phase 3: Detection — the checks

CORS — see references/cors-and-pna.md

• WPS-CORS-1 Access-Control-Allow-Origin uses an explicit origin allowlist, never reflects the Origin header without validating it against a literal list.
• WPS-CORS-2 Access-Control-Allow-Origin: * is never paired with Access-Control-Allow-Credentials: true (browsers reject the combination, but the intent is the smell).
• WPS-CORS-3 Vary: Origin is set on any response that varies CORS headers — required for correct CDN caching.
• WPS-CORS-4 Access-Control-Allow-Methods and Access-Control-Allow-Headers are explicit lists, not *, when credentials are involved.
• WPS-CORS-5 Preflight responses (OPTIONS) return 204 with appropriate headers and Access-Control-Max-Age (1h–24h is reasonable).
• WPS-CORS-6 Wildcard subdomain matching (*.example.com) is implemented as a hostname suffix check on a parsed URL, not a regex that allows evil-example.com.
• WPS-CORS-7 Private Network Access: any request from a public page (https or http on a routable address) to a private IP (RFC1918, link-local) sends Access-Control-Request-Private-Network: true on preflight and the server returns Access-Control-Allow-Private-Network: true only when intentional. Chrome enforces this; failing to handle it breaks LAN-targeted apps.

Content Security Policy — see references/csp-and-trusted-types.md

• WPS-CSP-1 Content-Security-Policy header is present and enforcing (not just Content-Security-Policy-Report-Only).
• WPS-CSP-2 script-src uses nonces ('nonce-…') with 'strict-dynamic', or hashes for static apps. No 'unsafe-inline' and no 'unsafe-eval' (or each documented with rationale).
• WPS-CSP-3 No wildcard hosts in script-src or object-src (*, https:).
• WPS-CSP-4 object-src 'none' and base-uri 'none' (or 'self') — closes two common XSS bypass vectors.
• WPS-CSP-5 frame-ancestors 'self' (or specific allowed embedders) — replaces X-Frame-Options and prevents clickjacking.
• WPS-CSP-6 form-action 'self' (or specific) prevents form-submission hijacking via injected ``.
• WPS-CSP-7 upgrade-insecure-requests is present so subresource HTTP URLs are upgraded to HTTPS.
• WPS-CSP-8 report-uri or report-to is configured AND the receiving endpoint is monitored (an unmonitored report endpoint catches nothing).
• WPS-CSP-9 Trusted Types: require-trusted-types-for 'script' and trusted-types are set on apps that use DOM sinks. Converts DOM XSS from a runtime bug into a thrown error.
• WPS-CSP-10 The same enforcing policy is applied to every HTML response (middleware/edge), not per-route ad-hoc.

Cross-Origin isolation — see references/cross-origin-isolation.md

• WPS-COOP-1 Cross-Origin-Opener-Policy: same-origin (or same-origin-allow-popups if you intentionally open cross-origin popups) — severs the opener relationship for cross-origin navigations, preventing tabnabbing and cross-origin window manipulation.
• WPS-COEP-1 Cross-Origin-Embedder-Policy: require-corp (or credentialless when you embed cross-origin resources you can't control) — required to be "cross-origin isolated" (gates SharedArrayBuffer, high-resolution timers, performance.measureUserAgentSpecificMemory()).
• WPS-CORP-1 API responses set Cross-Origin-Resource-Policy: same-origin (or same-site) to opt out of cross-origin embedding by attacker pages.
• WPS-ISO-1 Pages that need cross-origin isolation actually achieve it (verify crossOriginIsolated === true in console). COEP without COOP doesn't isolate.

Cookies — see references/cookies-modern.md

• WPS-COOK-1 Session cookies set HttpOnly (JS cannot read), Secure (HTTPS only), and SameSite=Lax (or Strict for high-value sessions).
• WPS-COOK-2 Session cookies use the __Host- prefix when scoping permits: requires Secure, no Domain attribute, Path=/. Subdomain takeover cannot read or replace.
• WPS-COOK-3 Sensitive cookies use the __Secure- prefix at minimum (browser enforces Secure flag).
• WPS-COOK-4 No long-lived authentication cookies (access cookie ≤ 1h; pair with rotated refresh).
• WPS-COOK-5 Logout invalidates session server-side, not just clears the cookie.
• WPS-COOK-6 Cross-site embedded contexts use Partitioned (CHIPS) cookies — per-top-level-site cookie jars. Avoid SameSite=None without Partitioned when third-party cookies are being phased out.
• WPS-COOK-7 Cookies with sensitive data never use a wider Domain=.example.com than necessary.

Permissions Policy

• WPS-PP-1 Permissions-Policy header denies powerful features not used by the app: camera=(), microphone=(), geolocation=(), payment=(), usb=(), bluetooth=(), serial=(), accelerometer=(), gyroscope=(), magnetometer=(), xr-spatial-tracking=().
• WPS-PP-2 Features the app DOES use are scoped to the origin: geolocation=(self), not geolocation=*.
• WPS-PP-3 Iframes carrying user content set allow="" (empty) to drop all delegated permissions.

Subresource Integrity (SRI)

• WPS-SRI-1 Every ` and loaded from a third-party CDN (cdnjs, unpkg, jsdelivr) includes integrity="sha384-..." and crossorigin="anonymous"`.
• WPS-SRI-2 First-party scripts loaded from your own CDN do not require SRI (you control them) but it's a defense-in-depth.
• WPS-SRI-3 Tag managers (GTM) that inject third-party scripts cannot retrofit SRI — restrict GTM tag inventory via tag manager permissions instead.

Mixed content + HSTS

• WPS-HST-1 Strict-Transport-Security: max-age=63072000; includeSubDomains; preload on production HTTPS responses; the apex domain is submitted to https://hstspreload.org.
• WPS-HST-2 No mixed content: every subresource (`, , , fetch()) loaded over HTTPS. CSP upgrade-insecure-requests` catches stragglers.
• WPS-HST-3 X-Content-Type-Options: nosniff is set on all responses to prevent MIME sniffing.

Referrer Policy

• WPS-REF-1 Referrer-Policy: strict-origin-when-cross-origin (default in modern browsers) or stricter. Avoid no-referrer-when-downgrade (default in old browsers) and unsafe-url.
• WPS-REF-2 Pages that handle tokens in URL fragments / query string set Referrer-Policy: no-referrer to avoid leaking via Referer header.

postMessage / cross-frame trust

• WPS-PM-1 Every window.addEventListener('message', …) handler validates event.origin against an allowlist before reading event.data.
• WPS-PM-2 postMessage(message, targetOrigin) calls use a specific origin, not '*', when the payload contains anything sensitive.
• WPS-PM-3 Message handlers validate the payload shape (schema check) before dispatching to business logic.
• WPS-PM-4 Iframes hosting third-party content set the sandbox attribute restricting capabilities (sandbox="allow-scripts allow-same-origin" is the minimum useful; weigh removing allow-same-origin).

Modern authentication

• WPS-AUTH-1 WebAuthn / Passkeys: discoverable credentials registered with residentKey: required and userVerification: required for passwordless flows. The relying-party ID matches the eTLD+1 of the production domain.
• WPS-AUTH-2 FedCM (navigator.credentials.get({identity: …})) used in place of third-party-cookie SSO flows where applicable. IDP configuration is fetched from /.well-known/web-identity and the IDP serves the FedCM config.
• WPS-AUTH-3 Origin-bound one-time codes (@bound: ) used in SMS-OTP flows to prevent cross-origin code phishing.

Service Worker

• WPS-SW-1 Service worker scope is the narrowest path that works (declared via Service-Worker-Allowed header if needed) — not the entire site root by accident.
• WPS-SW-2 Service worker does not cache responses with Authorization headers or user-specific data unless cache-keying includes the user identity.
• WPS-SW-3 Service worker source is served with Cache-Control allowing updates within minutes, not days.
• WPS-SW-4 A registered service worker can be unregistered remotely if compromised (kill-switch).

Phase 4: Triage

Critical class examples for this skill:

• No CSP at all on the production app, or script-src 'unsafe-inline' 'unsafe-eval' *
• Session cookie without HttpOnly (XSS = session steal)
• CORS reflects Origin with Allow-Credentials: true (cross-site exfiltration of authenticated responses)
• No HSTS on production HTTPS (network attacker downgrade)
• Open redirect in auth callback

High class:

• COOP missing on a page that uses popup-based OAuth (tabnabbing)
• COEP missing on a page expecting cross-origin isolation
• Third-party scripts from public CDN with no SRI
• frame-ancestors allows attacker origins

Phase 5: Report

Use ../_shared/findings-schema.md. Prefix IDs with WPS- (Web Platform Security). The granular category sub-prefixes (WPS-CORS-3, WPS-CSP-9, etc.) shown above are recommended for cross-report aggregation.

Outputs

1. Markdown audit report following the unified findings schema.
2. (Optional) A drop-in headers.conf / middleware snippet for the user's edge / framework that sets the recommended headers.
3. (Optional) A CSP rollout plan — start in Report-Only, observe for 1-2 weeks, switch to enforcing, then tighten.

References

• references/cors-and-pna.md — CORS allowlist patterns, credentials gotchas, subdomain wildcards, Private Network Access preflight (sourced from web.dev, Fetch spec, Chrome PNA intent-to-ship)
• references/csp-and-trusted-types.md — Strict CSP design, nonce vs hash, strict-dynamic, Trusted Types policies, report endpoint hygiene (sourced from web.dev Strict CSP guide, W3C CSP3 / Trusted Types specs)
• references/cross-origin-isolation.md — COOP / COEP / CORP, credentialless, why isolation gates SharedArrayBuffer, debugging report-only rollout (sourced from web.dev Cross-Origin Isolation, developer.chrome.com)
• references/cookies-modern.md — SameSite, __Host- / __Secure- prefixes, CHIPS Partitioned cookies, the Lax-by-default migration, third-party cookie phase-out (sourced from web.dev, RFC 6265bis)

Source attribution

Authoritative sources for this skill:

• web.dev (Google's web platform docs)
• developer.chrome.com (Chrome team posts, intent-to-ship)
• WHATWG Fetch spec (CORS, preflight)
• W3C CSP3, Trusted Types, Permissions Policy specs
• RFC 6265bis (cookies)
• W3C WebAuthn Level 3 and FedCM working drafts
• Chrome status entries for newly-shipped features (PNA, CHIPS)

When upstream guidance changes (e.g., browser default flips, spec stabilizes a new header), update the relevant check above and bump last_verified in .github/tech-inventory.yml for the corresponding entry.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: hlsitechio
• Source: hlsitechio/claude-skills-security
• License: MIT

Install and usage instructions live in the source repository linked above.