State Partitioning

Introduction: The Privacy Evolution of Browser Storage

Modern web applications rely heavily on client-side storage to deliver seamless user experiences--from remembering shopping cart contents to maintaining authenticated sessions. However, the same mechanisms that enable convenience have historically provided trackers with powerful tools to monitor user behavior across the web.

State Partitioning is a browser privacy mechanism that isolates storage access based on the combination of the resource's origin and the top-level site context in which it's embedded. Rather than allowing third-party content to access a unified storage bucket keyed solely by origin, browsers now partition storage so that the same third-party embedded across different websites cannot share or correlate user data between those contexts.

This approach emerged from recognizing that traditional cookie and storage policies--which typically operated on binary allow/block decisions--created both privacy vulnerabilities and compatibility challenges for legitimate third-party integrations.

The Problem: Cross-Site Tracking Through Shared State

Traditional Storage Behavior

To appreciate what State Partitioning accomplishes, it's important to understand the tracking vulnerability it addresses. In the traditional web storage model, browsers keyed client-side state--cookies, localStorage, IndexedDB, and other mechanisms--primarily by the origin of the resource accessing them.

This meant that when a third-party iframe from analytics.example was embedded in both site-a.com and site-b.com, that iframe could read and write a shared storage bucket keyed simply as analytics.example. A user visiting both websites would present the same identifier to the embedded analytics script, enabling the tracker to build a comprehensive profile of browsing behavior across unrelated sites.

The Tracking Exploitation Mechanism

The exploitation mechanism was deceptively simple but powerful. When a user visited a website containing embedded content from a tracker--often through advertising networks, analytics scripts, or social media widgets--the tracker would deposit a unique identifier into the browser's storage keyed to its own domain. When the user subsequently visited any other website containing embedded content from the same tracker, that third-party iframe could retrieve the previously deposited identifier, establishing a connection between the two visits.

How State Partitioning Works

The Double-Key Mechanism

State Partitioning addresses this vulnerability by fundamentally changing how browsers key client-side storage. Rather than using a single key based on the resource's origin, browsers now use a compound key that includes both the origin of the resource accessing storage and the top-level site context in which that access occurs.

This means that when the same third-party iframe from analytics.example is embedded in both site-a.com and site-b.com, it now receives two completely separate storage buckets: one for the analytics.example embedded in site-a.com context, and a different one for the analytics.example embedded in site-b.com context.

The top-level site is determined by the scheme and registrable domain of the page the user is actively viewing--the first-party context. This partitioning logic applies to every storage access, ensuring that third-party content receives isolation that matches the privacy expectations of users browsing different sites.

Affected Browser APIs

Storage APIs Under Partitioning

The scope of State Partitioning extends across virtually every browser storage mechanism:

Communication APIs Under Partitioning

Beyond traditional storage, State Partitioning affects browser APIs that enable communication between browsing contexts:

• BroadcastChannel - Communication restricted to same-partition contexts
• SharedWorker - Worker threads isolated by partition
• Web Locks API - Lock coordination per partition
• Service Workers - Third-party SWs fully partitioned

Storage API Updates

The Storage API, which provides standardized access to storage estimates and quotas, reports information specific to the current partition. The navigator.storage.estimate() method returns quota and usage for the current partition only.

Browser Implementation Status

Chrome and Chromium-Based Browsers

Chrome began rolling out storage partitioning incrementally starting in 2023, with full availability for all users from Chrome 115 onward. The implementation affects all Chromium-based browsers, including Edge (from version 119), Opera, and others.

Chrome's implementation includes a deprecation trial mechanism that provides temporary relief for sites needing additional migration time. Sites can opt into a trial that allows unpartitioned storage access for embedded third-party content for a limited period.

Testing flag: --disable-features=ThirdPartyStoragePartitioning (development use only)

Firefox Implementation

Firefox implemented State Partitioning under the name "dynamic partitioning" and enabled it by default starting with Firefox 103. Mozilla's implementation reflects the organization's commitment to privacy as a core browser value.

Firefox provides comprehensive console logging that helps developers understand when storage is being partitioned, including messages indicating when storage access is partitioned, when unpartitioned access is granted through heuristics, and when the Storage Access API is used.

Safari and WebKit

Safari implements storage partitioning as part of its Intelligent Tracking Prevention system. Apple's implementation emphasizes user privacy by default, with partitioning applied automatically based on its tracking detection algorithms.

Debugging and Development Tools

Browser Developer Tools Integration

Modern browser developer tools provide support for inspecting partitioned storage behavior:

• Firefox Storage Inspector - Shows origin and top-level site context for storage
• Chrome Application Panel - Displays storage usage broken down by partition context
• Console Logging - Explicit messages indicate when partitioning affects storage access

Testing Strategies

Testing applications under State Partitioning requires creating scenarios that simulate different embedding contexts:

1. Test embedded in an iframe on a different-origin page
2. Verify all storage mechanisms (cookies, localStorage, IndexedDB, Cache API)
3. Check communication APIs (BroadcastChannel, SharedWorker)
4. Validate Storage Access API integration

Chrome Demo: storage-partitioning-demo.glitch.me provides a visual demonstration of partitioning effects on different storage mechanisms.

For teams building web applications with third-party integrations, understanding these debugging tools is essential for maintaining functionality while respecting user privacy.

Storage Access API and Exceptions

Requesting Unpartitioned Access

The Storage Access API provides a standardized mechanism for embedded content to request unpartitioned storage access:

// Request storage access
document.requestStorageAccess().then(
 () => {
 // Access granted - can use unpartitioned storage
 console.log('Storage access granted');
 },
 () => {
 // Access denied
 console.log('Storage access denied');
 }
);

The API returns a Promise that resolves if the user grants permission. Browsers may present permission prompts or grant access based on user interaction patterns.

Storage Access Heuristics

Browsers implement heuristics that automatically grant unpartitioned access in common scenarios:

• Opener Heuristic - When a partitioned third-party opens a pop-up that the user interacts with, the third-party gains storage access for 30 days
• Navigation Heuristic - If a user navigates between related sites quickly and returns, temporary access may be granted

These heuristics recognize that some cross-site storage patterns are fundamental to web functionality, including federated identity systems and embedded AI agent workflows that require persistent state.

Best Practices for LLM and Agent Applications

Designing for Partitioned Storage

Applications built with LLM agents and AI systems often require sophisticated state management. Under State Partitioning:

Recommended Approach: Design state storage around first-party mechanisms rather than relying on third-party storage that could be partitioned. When an LLM agent widget is embedded in a customer's website, store state on the first-party domain (via postMessage communication) rather than attempting to maintain cross-site state on your own domain.

Our AI automation services help organizations design LLM-powered applications that work correctly with browser privacy mechanisms while delivering powerful functionality to end users.

Federated Identity and Authentication

Identity systems face particular challenges because authentication state has traditionally relied on cross-site cookie access:

• Use the Storage Access API for identity providers to maintain cross-site functionality
• Consider FedCM (Federated Credential Management API) for newer implementations
• Design authentication flows that work with partitioned cookies

Monitoring and Analytics

Analytics embedded as third-party content face significant changes:

• User tracking across sites through shared identifiers no longer functions
• Analytics that should track behavior within a single site work correctly
• Consider server-side aggregation for cross-site analytics

Migration and Compatibility

Auditing Third-Party Dependencies

Before migration, audit all third-party dependencies:

1. Identify advertising networks, analytics providers, chatbot widgets, social media embeds
2. Determine what storage mechanisms each dependency uses
3. Contact vendors about their partitioning migration plans
4. Test each dependency under partitioning conditions

Implementing Fallbacks

For dependencies needing additional migration time:

• Graceful fallbacks - Maintain core functionality without cross-site storage
• First-party storage - Shift state to customer-owned domains
• Server-side state - Implement server-side state management
• Deprecation trial - Chrome's temporary relief mechanism for migration

Long-Term Compatibility

Focus on architectural patterns that work regardless of partitioning status:

• Shift state management to first-party mechanisms
• Implement server-side state for cross-session data
• Design UX that gracefully handles reduced cross-site functionality
• Monitor browser updates and policy changes

Partner with our web development team to audit and update your third-party integrations for the privacy-first web.

Sources

1. MDN Web Docs - State Partitioning
2. Privacy Sandbox - Storage Partitioning
3. Privacy Community Group - Storage Partitioning
4. Chrome Platform Status - Storage Partitioning
5. TrustBuilder - Chrome Storage Partitioning