privacy

name	privacy
description	Action-oriented guidelines for privacy by design, data minimization, third-party audits, and modern browser privacy APIs. Use this skill when dealing with user data, cookies, tracking, third-party scripts, or browser privacy APIs.

Privacy

Web application architects and developers must treat privacy not as an compliance afterthought, but as a foundational architectural design requirement. Modern web platforms are shifting away from implicit tracking toward explicit, user-consented, browser-mediated identity and permission exchanges.

1. Privacy by Design and Data Minimization

Reducing the digital footprint to limit breach exposure and foster user trust.

DOs:

• DO collect only adequate, relevant, and strictly necessary data for a stated purpose.
• DO use lower granularity ("fuzzing") where precise data is not required (e.g., age brackets vs. exact birthdates).
• DO offer guest checkouts to avoid forced account creation.
• DO explain why data is collected inline, using aria-describedby to link inputs to their explanations.
• DO use the Clear-Site-Data HTTP header upon logout to wipe client-side cookie caches and local storage.

DON'Ts:

• DON'T collect data speculatively "just in case" it becomes useful.
• DON'T rely on dark patterns or pre-checked opt-ins to force consent.

Code Examples:

Clear-Site-Data (HTTP)

Send this header on the page after logout confirmation.

Clear-Site-Data: "cache", "cookies", "storage"

Inline Transparency (HTML)

<div>
  <label for="email">Email address*</label>
  <input id="email" type="email" name="email" required aria-describedby="whyemail">
  <a href="#whyemail">Why do we need this?</a>
  
  <aside id="whyemail">
    We need this email to send password resets. We will not use it for marketing unless you opt-in.
  </aside>
</div>

2. Third-Party Audits and Mitigations

Limiting leakage introduced by external scripts, embeds, and tracking pixels.

DOs:

• DO audit third parties technically using DevTools (Network panel) and HAR logs.
• DO use the Façade Pattern (Lazy Loading) to load static thumbnails first, only loading heavy widget iframes upon user click.
• DO replace third-party social buttons with static HTML sharing links (e.g., zero tracking SDKs).
• DO set strict HTTP Referrer policies (strict-origin-when-cross-origin or no-referrer) to prevent leaking sensitive URL query parameters.
• DO use rigid Permissions-Policy to lock down powerful APIs (geolocation, camera) globally or for subframes.
• DO use Content-Security-Policy-Report-Only for continuous automated audits of where third-party scripts are attempting to send data.

DON'Ts:

• DON'T use default tracking SDKs if a static hyperlink suffices.

Code Examples:

Referrer-Policy and Permissions-Policy (HTTP)

Referrer-Policy: strict-origin-when-cross-origin
Permissions-Policy: accelerometer=(), camera=(), fullscreen=*

3. Cookie Deprecation and Partitioned Storage

Architecting for a web without unpartitioned third-party cookies.

DOs:

• DO use CHIPS (Cookies Having Independent Partitioned State) by appending the Partitioned attribute for 1:1 embeds that do not share state across top-level sites.
• DO use the Storage Access API (SAA) when cross-site state sharing is functionally critical (such as SSO portals).
• DO trigger SAA permission requests (requestStorageAccess()) via direct user interaction (click/keypress).

DON'Ts:

• DON'T rely on unpartitioned SameSite=None cookies as they are being systematically blocked by modern browser engines.

Code Examples:

CHIPS Set-Cookie (HTTP)

Set-Cookie: session_id=abc123; SameSite=None; Secure; Path=/; Partitioned; HttpOnly

Storage Access API (JavaScript)

document.getElementById('login-btn').addEventListener('click', async () => {
  try {
    const hasAccess = await document.hasStorageAccess();
    if (!hasAccess) {
      await document.requestStorageAccess();
    }
    // Access granted: unpartitioned cookies are now attached to fetch()
  } catch (err) {
    console.error('Storage access denied', err);
  }
});

Third-Party State Matrix

Mechanism	Scope	Requires Interaction	Use Case
CHIPS	1:1 Partitioned	No	Embeds (Maps, Widgets)
Storage Access API (SAA)	Cross-site	Yes	SSO Portals, Analytics
FedCM	Identity Federation	Yes	"Sign In with..."

Heuristic Rule: Use CHIPS for isolated widgets (un-shared state), and SAA for shared identity state requiring explicit user consent.

4. Privacy-Preserving Identity (FedCM)

Moving from opaque navigational redirects to explicit native UI-mediated federation.

DOs:

• DO use the Federated Credential Management API (FedCM) to mediate "Sign-In" flows natively, preventing IdP tracking of Relying Parties prior to user consent.
• DO verify the FedCM-returned un-falsifiable token on your backend.

DON'Ts:

• DON'T bounce users through opaque redirect URL chains if FedCM can fulfill the use-case natively.

Code Examples:

FedCM Sign-In (JavaScript)

try {
  const credential = await navigator.credentials.get({
    identity: {
      providers: [{
        configURL: "https://idp.example/fedcm.json",
        clientId: "rp-client-id-123"
      }]
    }
  });
  authenticateWithBackend(credential.token);
} catch (error) {
  console.error("FedCM login failed", error);
}

5. Fingerprinting & User-Agent Reduction

Shifting from passive device broadcasting to explicit feature inspection.

DOs:

• DO use Feature Detection (e.g., 'createImageBitmap' in window) over User-Agent string parsing.
• DO use User-Agent Client Hints (UA-CH) if you must differentiate environments.
• DO explicitly request only the minimum required high-entropy hints using the Accept-CH header.
• DO use the Vary header (e.g., Vary: Sec-CH-UA-Platform) if your server caches vary based on UA-CH.

DON'Ts:

• DON'T parse navigator.userAgent for non-critical logic.
• DON'T request a high volume of high-entropy hints simultaneously (interpretable as malicious fingerprinting).

Code Examples:

Feature Detection vs Sniffing (JavaScript)

// AVOID: if (navigator.userAgent.includes("Chrome")) ...
if ('createImageBitmap' in window) {
  // Use modern API
}

6. Contextual Request Defenses with Fetch Metadata

Using unforgeable headers to reject unauthorized cross-origin requests server-side.

DOs:

• DO inspect Sec-Fetch-Site, Sec-Fetch-Mode, and Sec-Fetch-Dest headers before processing state-changing requests.
• DO implement a Resource Isolation Policy (middleware) to automatically reject cross-site calls not intended as simple navigations.

DON'Ts:

• DON'T process state-changing requests if the origin relationship (Sec-Fetch-Site) is cross-site and the mode is not navigate.

Code Examples:

Resource Isolation Middleware (Express / Node.js)

app.use((req, res, next) => {
  const site = req.get('Sec-Fetch-Site');
  const mode = req.get('Sec-Fetch-Mode');

  if (!site) return next(); // Fallback for legacy browsers

  if (site === 'same-origin' || site === 'same-site') return next();

  // Allow standard outside user navigations (GET link clicks)
  if (site === 'cross-site' && mode === 'navigate' && req.method === 'GET') {
    return next();
  }

  res.status(403).json({ error: 'Cross-origin request forbidden' });
});

7. Fine-Grained Capability Control (Permissions API)

Querying capabilities before hitting users with automatic prompts.

DOs:

• DO query navigator.permissions.query() before requesting access to powerful APIs (geolocation, camera).
• DO present polite explanations in UI explaining why the prompt exists before triggering the browser's native blocking prompt.

DON'Ts:

• DON'T trigger browser native prompts automatically on page load without context.

Code Examples:

Query Permission Status (JavaScript)

navigator.permissions.query({ name: 'geolocation' }).then((result) => {
  if (result.state === 'granted') {
    loadMap();
  } else if (result.state === 'prompt') {
    showPolitePermissionExplanation(); // trigger requestStorageAccess upon button click
  }
});