Third-Party Cookie Deprecation
Prepare for cookie-less third-party contexts and understand privacy-preserving alternatives
Introduction: The End of Third-Party Cookies and Why It Matters
Have you ever browsed a product on one website, only to see ads for that exact item following you across the internet for days? That unsettling experience—and the invisible tracking infrastructure behind it—is finally coming to an end. The web is undergoing one of its most significant privacy transformations since its inception, and whether you're a developer, business owner, or simply someone who cares about digital privacy, this shift will affect you. To help you master these concepts, we've included free flashcards throughout this lesson to reinforce your understanding of key terms and principles.
At the heart of this transformation lies a simple piece of technology that has powered both the convenience and the invasiveness of the modern web: the third-party cookie. But before we explore why browsers are systematically eliminating them, we need to understand what makes a cookie "third-party" versus "first-party."
Understanding the Cookie Divide
When you visit a website like example.com, that site can set a first-party cookie—a small piece of data stored in your browser that belongs to the domain you're actually visiting. These cookies serve legitimate purposes: remembering your login state, storing your shopping cart, or saving your preferences. They're the foundation of user authentication and personalized experiences on individual sites.
A third-party cookie, however, is set by a domain different from the one you're visiting. When example.com embeds content from tracker.com—perhaps an advertisement, analytics script, or social media widget—that third party can set its own cookie in your browser. Here's the critical issue: when you later visit another-site.com that also includes resources from tracker.com, that same cookie is accessible, allowing tracker.com to recognize you across both sites.
User Journey with Third-Party Tracking:
Day 1: Visit shop.com Day 2: Visit news.com
↓ ↓
[shop.com sets [news.com sets
first-party cookie] first-party cookie]
↓ ↓
[ad-tracker.com sets [ad-tracker.com reads
third-party cookie SAME cookie from Day 1
ID: user12345] ID: user12345] ← Tracking!
↓ ↓
Records: user12345 Knows: user12345 visited
viewed hiking boots shop.com yesterday
🎯 Key Principle: The distinction isn't about where the cookie is stored (always in your browser) but about which domain set it and which domains can access it across different sites.
The Deprecation Timeline: How We Got Here
The movement to eliminate third-party cookies didn't happen overnight. It's the culmination of years of growing privacy concerns and regulatory pressure:
Safari's Intelligent Tracking Prevention (ITP) led the charge, with Apple beginning to restrict third-party cookies in 2017 and implementing increasingly aggressive blocking measures through 2020. Today, Safari blocks third-party cookies by default.
Firefox's Enhanced Tracking Protection (ETP) followed suit, enabling third-party cookie blocking by default for all users in 2019. Mozilla's approach focused on maintaining a list of known trackers to block while preserving functionality for legitimate use cases.
Chrome's Privacy Sandbox represents the most consequential shift due to Chrome's dominant market share (over 60% of web users). Google announced plans to phase out third-party cookies in 2020, though the timeline has shifted multiple times. As of 2024, Chrome has begun restricting third-party cookies for a percentage of users, with full deprecation planned for late 2024 or early 2025.
🤔 Did you know? Chrome's delayed timeline wasn't just about technical challenges—Google's business model relies heavily on advertising, making the transition particularly complex for the company to navigate.
Why This Matters: The Privacy Crisis
The push to deprecate third-party cookies stems from legitimate privacy concerns that have reached a tipping point. Cross-site tracking has enabled advertising networks and data brokers to build detailed profiles of users' browsing behavior, interests, and even sensitive attributes without meaningful consent.
Consider the scope: a single ad network embedded across thousands of websites can track a user's journey through medical information sites, financial services, political content, and personal interests—building a comprehensive user profile that exists outside any individual user's control. This data becomes a commodity, bought and sold, potentially leaked in breaches, and used in ways users never anticipated.
💡 Real-World Example: In 2019, researchers demonstrated that advertising trackers could identify individual users' medical conditions by tracking their visits to health information sites. A person researching diabetes treatments would be tagged with that interest across dozens of ad networks, potentially affecting insurance rates, employment prospects, or simply invading their privacy.
The Collateral Damage: Legitimate Use Cases at Risk
While eliminating tracking is the goal, third-party cookies have powered several legitimate web functionalities that now require reimagining:
🔧 Advertising and Attribution: Publishers rely on ad revenue to fund free content. Third-party cookies enabled targeting and measuring ad effectiveness—capabilities that need replacement, not just elimination.
📊 Cross-Domain Analytics: Businesses with multiple domains (e.g., company.com and company-shop.com) used third-party cookies to understand unified user journeys.
🎵 Embedded Content: Video players, music widgets, and interactive content embedded on other sites often relied on third-party cookies for preferences and state management.
🔒 Single Sign-On (SSO): Enterprise SSO solutions and social login buttons ("Sign in with Google") depend on cross-domain identity mechanisms that traditionally used third-party cookies.
⚠️ Common Mistake: Assuming all third-party cookies are malicious. Many legitimate services depended on them, which is why the transition requires new technologies, not just blanket blocking. ⚠️
Why Developers Must Act Now
This isn't a theoretical future concern—it's happening now. If you're building web applications today and haven't considered third-party cookie deprecation, you may already have broken functionality for Safari and Firefox users (over 20% of the market). And with Chrome's phased rollout underway, ignoring this shift means risking critical breakage for the majority of your users.
❌ Wrong thinking: "I'll wait until Chrome fully deprecates third-party cookies to make changes."
✅ Correct thinking: "I need to audit my application's cookie usage now and plan migration strategies for each use case."
The end of third-party cookies represents more than a technical change—it's a fundamental reimagining of how the web balances functionality, business models, and user privacy. Understanding this shift is essential for anyone building for the modern web, and in the sections that follow, we'll explore the technical mechanisms replacing third-party cookies and practical strategies for adapting your applications to this new reality.
💡 Pro Tip: Start your migration journey by auditing your application's cookie usage with browser DevTools. In Chrome, open DevTools → Application → Cookies, and look for any cookies with a domain different from your current site. These are your third-party cookies that need attention.
Technical Mechanisms and Alternatives to Third-Party Cookies
As browsers phase out third-party cookies, understanding the technical mechanisms behind cookie blocking and the emerging alternatives becomes essential for web developers. The transition isn't simply about removing a feature—it's about replacing it with privacy-preserving technologies that maintain legitimate functionality while protecting users.
How Browsers Block Third-Party Cookies
Browsers identify third-party cookies by examining the context in which they're set and accessed. When you visit example.com and it embeds content from tracker.com, any cookies from tracker.com are considered third-party because the domain doesn't match the top-level site in your address bar.
The SameSite attribute provides granular control over cookie behavior across contexts. This attribute accepts three values:
SameSite=Strict → Cookie only sent to same-site requests
SameSite=Lax → Cookie sent to same-site + top-level navigation
SameSite=None → Cookie sent everywhere (requires Secure flag)
🎯 Key Principle: Modern browsers default cookies to SameSite=Lax, meaning developers must explicitly opt into cross-site cookie sending by setting SameSite=None; Secure. This "secure by default" approach is the foundation of third-party cookie blocking.
Cookie partitioning takes this further by creating separate cookie jars for each top-level site. Even if tracker.com is embedded on both news.com and blog.com, the cookies it sets are partitioned:
Top-level: news.com Top-level: blog.com
└─ tracker.com cookies └─ tracker.com cookies
(Partition A) (Partition B)
These cannot be linked together!
This prevents cross-site tracking while still allowing cookies to function within their embedded context.
Storage Access API: Requesting Permission
The Storage Access API provides a way for legitimate embedded content to request access to its unpartitioned cookies with user consent. This solves scenarios like embedded payment widgets or authentication frames that genuinely need cross-site state.
The API follows a permission-based model:
// Check if storage access is already granted
const hasAccess = await document.hasStorageAccess();
if (!hasAccess) {
// Request access (triggers browser permission prompt)
await document.requestStorageAccess();
// Now can access unpartitioned cookies
}
💡 Real-World Example: A social media "Like" button embedded across websites might use the Storage Access API. When a user clicks the button, it requests storage access, shows a browser prompt explaining why, and only after user approval can it access the authentication cookies to record the like.
⚠️ Common Mistake: Calling requestStorageAccess() must happen in response to user interaction (like a click). Calling it on page load will fail silently. ⚠️
CHIPS: Partitioned Cookies by Design
CHIPS (Cookies Having Independent Partitioned State) allows developers to explicitly opt into cookie partitioning. Instead of requesting full unpartitioned access, CHIPS lets third-party contexts maintain separate state per top-level site.
Setting a partitioned cookie looks like this:
Set-Cookie: session=abc123; SameSite=None; Secure; Partitioned
The Partitioned attribute tells the browser to key this cookie by both the setting domain and the top-level site:
Cookie Key: (tracker.com, news.com, "session") → "abc123"
Cookie Key: (tracker.com, blog.com, "session") → "xyz789"
💡 Pro Tip: Use CHIPS for embedded widgets that need state but don't require cross-site identity. Examples include embedded maps that remember zoom level, chat widgets that maintain conversation state, or embedded shopping carts—all scoped to the site they're on.
Privacy Sandbox: Purpose-Built APIs
Google's Privacy Sandbox initiative provides specialized APIs for specific use cases previously handled by third-party cookies:
Topics API
The Topics API enables interest-based advertising without tracking individual users. The browser observes sites you visit and assigns you to broad interest categories ("topics") like "Fitness" or "Travel." Advertisers can query these topics—but the browser only reveals 3 topics, keeps them for 3 weeks, and adds random noise.
// Advertiser requests topics
const topics = await document.browsingTopics();
// Might return: ["Fitness", "Technology", "Cooking"]
🤔 Did you know? The Topics API uses a hierarchical taxonomy with ~350 categories, all public and human-readable. Users can see and remove topics, giving them direct control over interest signals.
Protected Audience API (FLEDGE)
The Protected Audience API enables remarketing (showing ads to previous site visitors) entirely in the browser. Instead of sending "this user visited my site" to ad networks, the browser remembers interest groups locally and runs ad auctions in a privacy-preserving environment.
// Site adds user to interest group
await navigator.joinAdInterestGroup({
owner: 'https://advertiser.com',
name: 'hiking-boot-shoppers',
biddingLogicUrl: 'https://advertiser.com/bid.js',
ads: [{renderUrl: 'https://cdn.com/ad.html'}]
}, 30 * 24 * 60 * 60 * 1000); // 30 days
The browser runs ad auctions locally without revealing which groups the user belongs to.
Attribution Reporting API
The Attribution Reporting API measures ad effectiveness (clicks leading to conversions) without identifying individual users. It reports conversions with intentional noise and aggregation:
Ad Click (news.com) → Conversion (shop.com)
↓ ↓
Source event Trigger event
↓ ↓
└──────── Browser ─────────┘
↓
Aggregated report
("~150 conversions from
Campaign X this week")
First-Party Alternatives
Many use cases don't require special APIs—they just need architectural changes:
🔧 Server-side session management: Instead of third-party cookies, embedded widgets can communicate with their backend through the parent page, using postMessage for secure cross-frame communication.
🔧 Authentication tokens: OAuth flows and JWT tokens can replace session cookies for federated authentication scenarios.
🔧 FedCM (Federated Credential Management): This API streamlines identity federation without third-party cookies. When a site wants to "Sign in with Google," FedCM provides a browser-native flow that doesn't require tracking:
const credential = await navigator.credentials.get({
identity: {
providers: [{
configURL: 'https://idp.com/fedcm.json',
clientId: 'client123'
}]
}
});
The browser mediates the authentication, showing a clear UI about what's happening.
📋 Quick Reference Card:
| 🎯 Solution | 📊 Use Case | 🔒 Privacy Model |
|---|---|---|
| Storage Access API | Authenticated embeds | User consent required |
| CHIPS | Stateful widgets | Per-site partitioning |
| Topics API | Interest advertising | Coarse categories + noise |
| Protected Audience | Remarketing | On-device auctions |
| Attribution Reporting | Conversion tracking | Aggregate + noise |
| FedCM | Federated login | Browser-mediated |
❌ Wrong thinking: "I need to pick one replacement technology for all my third-party cookie use cases." ✅ Correct thinking: "Different use cases need different solutions—authentication might use FedCM, analytics might use Attribution Reporting, and widgets might use CHIPS."
The key to successfully navigating third-party cookie deprecation is matching each specific use case to the appropriate modern alternative, understanding that privacy-preserving design often requires purpose-built solutions rather than one-size-fits-all tracking mechanisms.
Migration Strategies and Common Pitfalls
Successfully migrating your application away from third-party cookies requires careful planning, thorough testing, and awareness of common traps that can derail your transition. This section provides a practical roadmap for navigating this migration.
Auditing Your Application
The first critical step is understanding where and how your application relies on third-party cookies. A comprehensive dependency audit reveals hidden assumptions that might otherwise cause production failures.
🎯 Key Principle: You cannot fix what you cannot see. Exhaustive auditing must precede any migration work.
Start by mapping all cross-domain interactions in your application:
[Your Domain] ──────────────────────┐
│ │
├─> Analytics (3rd party) │
├─> Payment Gateway │ All potential
├─> Social Login Widgets │ 3rd-party cookie
├─> Embedded Widgets │ dependencies
├─> CDN Resources │
└─> SSO Authentication │
┘
💡 Pro Tip: Use browser DevTools to monitor cookie behavior. In Chrome, navigate to Application > Cookies and filter by "SameSite=None" to identify cross-site cookies. Enable "Block third-party cookies" in settings to simulate the future state.
| 🔍 Audit Area | 🔧 Tools & Methods | ⚠️ Red Flags |
|---|---|---|
| 🔒 Authentication flows | Test cross-domain login sequences, check for Set-Cookie headers without SameSite=None | Silent auth failures, infinite redirect loops |
| 📊 Analytics & Tracking | Review third-party script tags, check network tab for cookie requests | Scripts setting cookies in iframes, cross-domain pixel requests |
| 💳 Payment Processors | Test checkout flows with third-party cookies blocked | Embedded payment forms that break, session loss during redirect |
| 🎨 Embedded Widgets | Inventory all iframes, social plugins, chat widgets | Widgets requiring user state, cross-domain communication |
🤔 Did you know? Studies show that over 70% of web applications unknowingly depend on third-party cookies through analytics scripts alone, making comprehensive auditing essential.
Common Migration Mistakes
Even experienced developers make predictable errors during third-party cookie migration. Understanding these patterns helps you avoid costly setbacks.
⚠️ Common Mistake: Mistake 1: Assuming Storage Access API Works Without User Gesture ⚠️
The Storage Access API requires explicit user interaction before granting access to cookies. Many developers implement the API but forget this critical requirement.
❌ Wrong thinking: "I'll call requestStorageAccess() automatically when the iframe loads."
✅ Correct thinking: "I need to call requestStorageAccess() in response to a user click or tap within the iframe."
// ❌ WRONG: Called without user gesture
window.addEventListener('load', async () => {
await document.requestStorageAccess(); // Will fail!
});
// ✅ CORRECT: Called from user interaction
button.addEventListener('click', async () => {
await document.requestStorageAccess(); // Works!
});
⚠️ Common Mistake: Mistake 2: Not Testing Across Browsers with Different Policies ⚠️
Safari has blocked third-party cookies since 2020, Firefox has Enhanced Tracking Protection enabled by default, and Chrome's timeline continues to evolve. Testing in a single browser creates a false sense of security.
💡 Real-World Example: A major e-commerce platform discovered their embedded payment widget worked perfectly in Chrome but completely failed in Safari. They had only tested in Chrome during development, leading to a 15% drop in conversions from Safari users that went unnoticed for weeks.
⚠️ Common Mistake: Mistake 3: Implementing Half-Baked Solutions Under Time Pressure ⚠️
When deadlines loom, teams often implement quick fixes that create technical debt. Rushing to add SameSite=None; Secure to all cookies without understanding the implications can create security vulnerabilities.
Anti-Patterns to Avoid
As third-party cookies disappear, some developers attempt workarounds that violate user privacy or browser policies. These anti-patterns should be avoided:
🔴 Browser Fingerprinting: Attempting to identify users through device characteristics violates privacy expectations and may be blocked by browsers.
🔴 CNAME Cloaking: Disguising third-party requests as first-party through DNS configuration undermines user consent and is increasingly detected and blocked.
🔴 Bounce Tracking: Redirecting users through domains to sync identifiers creates poor user experience and is explicitly targeted by browser privacy protections.
🔴 LocalStorage Sharing Schemes: Using postMessage to share localStorage across domains recreates third-party cookie functionality and defeats the purpose of privacy protections.
🧠 Mnemonic: RESPECT users - Refuse fingerprinting, Embrace APIs, Secure authentication, Prioritize consent, Eliminate workarounds, Check compliance, Test thoroughly.
Best Practices for Common Scenarios
Cross-Domain Authentication Flows
For single sign-on (SSO) and federated authentication, transition to token-based flows that don't rely on cookies:
- Use Authorization Code Flow with PKCE: Send tokens via URL parameters or postMessage rather than cookies
- Implement First-Party Sets (when available): Declare related domains to maintain limited cookie access
- Leverage Storage Access API: For embedded authentication widgets that need session access
- Consider top-level redirects: Instead of iframe authentication, redirect the entire page
Embedded Widget Scenarios
When you control both the parent page and the embedded widget:
🔧 Use postMessage for state synchronization instead of shared cookies
🔧 Request Storage Access API permission on first user interaction
🔧 Design widgets to work in a cookieless mode with degraded functionality
🔧 Provide sign-in prompts within the iframe to trigger user gestures
💡 Pro Tip: Always provide a "Sign in" or "Connect" button within your embedded widget. This serves dual purposes: obtaining user gesture for Storage Access API and improving transparency about cross-domain functionality.
Testing Strategies
Comprehensive testing across browser environments is non-negotiable for successful migration.
📋 Quick Reference Card:
| 🌐 Browser | 🔒 Default Policy | 🧪 Testing Command/Setting |
|---|---|---|
| Chrome | Allowing (transitioning) | chrome://flags/#test-third-party-cookie-phaseout |
| Safari | Blocking (ITP enabled) | Default behavior, no configuration needed |
| Firefox | Blocking (ETP Standard) | Settings > Privacy > Strict mode |
| Edge | Following Chrome | edge://flags/#test-third-party-cookie-phaseout |
Multi-Browser Testing Workflow:
- ✅ Test core functionality with third-party cookies enabled (baseline)
- ✅ Test with third-party cookies blocked in Safari (strictest policy)
- ✅ Test with Firefox Enhanced Tracking Protection in Strict mode
- ✅ Test with Chrome's third-party cookie phase-out flag enabled
- ✅ Test Storage Access API flows with simulated user interactions
- ✅ Verify fallback behavior when APIs are unavailable
- ✅ Monitor console warnings about deprecated cookie usage
💡 Real-World Example: Implement automated browser testing using Playwright or Selenium with third-party cookies disabled by default. This prevents regression and ensures new features work in the privacy-preserving future.
Summary
You now understand how to successfully migrate your application away from third-party cookie dependencies - a critical capability that many developers lack. You've learned to:
✅ Audit systematically: Map all cross-domain dependencies before making changes
✅ Avoid common mistakes: Respect user gesture requirements, test across browsers, and resist quick fixes
✅ Reject anti-patterns: Don't attempt to circumvent privacy protections with fingerprinting or tracking workarounds
✅ Implement properly: Use Storage Access API correctly, adopt token-based authentication, and design resilient embedded widgets
✅ Test comprehensively: Validate functionality across all major browsers with varying policies
⚠️ Critical Reminders:
⚠️ Storage Access API only works with user gestures - design your UX accordingly
⚠️ Safari's Intelligent Tracking Prevention is the strictest policy - if it works there, it works everywhere
⚠️ Browser fingerprinting and CNAME cloaking are anti-patterns that violate user trust and may face regulatory consequences
Practical Next Steps:
🎯 Immediate: Run a dependency audit this week using browser DevTools to identify third-party cookie usage in your application
🎯 Short-term: Set up automated testing with third-party cookies disabled across Chrome, Safari, and Firefox to catch regressions
🎯 Long-term: Redesign authentication flows and embedded widgets to use Storage Access API or token-based approaches, eliminating third-party cookie dependencies entirely
The transition away from third-party cookies represents a fundamental shift toward a more private web. By following these migration strategies and avoiding common pitfalls, you're not just maintaining compatibility - you're building applications that respect user privacy and stand the test of time.