Storage partitioning represents a fundamental shift in how modern web browsers protect user privacy by isolating browsing data across different websites. Rather than blocking third-party cookies entirely, which can break legitimate website functionality, storage partitioning creates isolated storage containers for each top-level website a user visits, preventing third-party trackers from building comprehensive profiles across the entire web. This privacy-preserving technology has been implemented by all major browsers including Chrome (since version 115), Firefox (since version 103), Safari, and Edge, marking a coordinated industry effort to combat cross-site tracking while maintaining web functionality. The approach represents a balance between user privacy and web compatibility, allowing legitimate third-party services to function within individual websites while eliminating their ability to track users across unrelated sites. As of October 2025, storage partitioning has become the default behavior across major browsers, fundamentally restructuring how browser state is managed and establishing new paradigms for privacy-preserving web interactions that both protect users and enable legitimate business functionality.
Understanding Cross-Site Tracking and Privacy Vulnerabilities
The Traditional Problem: Unpartitioned Storage
Before storage partitioning became standard, web browsers stored all data using a simple single-key system based on the origin or domain that set it. This architecture, while straightforward, created significant privacy vulnerabilities that enabled widespread cross-site tracking. When a user visited multiple websites that each embedded the same third-party service—such as an analytics provider, advertiser, or widget provider—that third-party service could access the same cookie or local storage across all those different websites. This allowed trackers to identify the same user across numerous unrelated sites and build detailed profiles of their browsing habits, interests, and behaviors over time. For example, if a third-party advertising domain set a cookie containing a unique user identifier when a user visited a retail website, that same identifier could be retrieved and used when the user subsequently visited a news site, a sports site, or any other website that embedded the same third-party content. The tracker could then correlate all these visits and create a comprehensive behavioral profile spanning the entire web.
The mechanics of traditional cross-site tracking relied on the fundamental property of how cookies were keyed and retrieved. When an iframe from `example.com` was embedded on `a.com` and later on `b.com`, the `example.com` domain could store a unique identifier in a cookie and successfully retrieve that same identifier on both sites. This occurred because historically, storage was keyed only by the domain or origin that set it, not by where it was embedded. Trackers exploited this behavior by storing user identifiers when users first visited their sites directly, then accessing those same identifiers when their tracking code was embedded on other websites. Additionally, even more sophisticated attackers could use side-channel techniques such as timing attacks, XS-Leaks, and COSI (Cross-Origin State Inference) to infer information about a user’s state on other websites without needing to access stored cookies directly. These side-channel attacks could reveal whether a user was logged into a particular service, had visited certain websites, or possessed specific information, all without explicit data sharing.
Privacy Threats Beyond Traditional Tracking
The vulnerabilities created by unpartitioned storage extended beyond simple cookie-based tracking. Network state—including HTTP caches, DNS lookups, connection pooling, font caches, and other browser-level infrastructure—could also be exploited for tracking purposes. These resources were traditionally keyed only by their origin, allowing attackers to infer information about a user’s browsing across websites by observing which resources were cached or how quickly certain network operations completed. Furthermore, communication APIs like BroadcastChannel, SharedWorker, and service workers could be abused to enable cross-site communication and coordination between same-origin contexts embedded in different top-level websites. A single tracker could coordinate information gathering across multiple websites through these communication channels, effectively synchronizing data collection efforts even when direct cookie access was theoretically isolated. The cumulative effect of these vulnerabilities was that despite users visiting different websites on what appeared to their perspective to be separate browsing sessions, their data was continuously aggregated and correlated across all these sites, enabling comprehensive behavioral tracking and profiling.
Fundamentals of Storage Partitioning Technology
Core Principles and Key Mechanisms
Storage partitioning operates on a deceptively simple but powerful principle: instead of keying browser storage by a single origin, storage is double-keyed by both the origin that sets it and the top-level site where the origin is embedded. This additional “partition key” ensures that data stored by a third-party service in one browsing context remains completely isolated from data stored by the same service in other browsing contexts, effectively creating separate storage compartments for each combination of third-party origin and top-level site. When storage partitioning is enabled, if `example.com` is embedded on `a.com` and later embedded on `b.com`, these two instances of `example.com` have completely separate storage spaces. Data stored by `example.com` when embedded on `a.com` is inaccessible to the instance of `example.com` embedded on `b.com`, and vice versa. This architectural change means that trackers can no longer use a single persistent identifier across multiple websites because their storage is automatically segregated by top-level site.
The partition key typically consists of the scheme (https/http) and the top-level registrable domain (eTLD+1) of the website the user is currently visiting. So if a user visits `https://retail.example` which embeds a chat service from `https://support.chat.example`, the partition key for the chat service would be `(“https”, “retail.example”)`. If the user then visits a different website that also embeds the same chat service, that embedded instance would receive a different partition key reflecting the new top-level site. The browser enforces that only cookies and storage with matching partition keys can be accessed, creating a natural isolation boundary. This approach maintains a crucial distinction from simple cookie blocking: it allows legitimate third-party services to continue functioning within individual websites (they can set and retrieve cookies within their assigned partition) while simultaneously preventing them from tracking users across websites (they cannot access data stored in other partitions).
Handling Complex iframe Hierarchies
The implementation of storage partitioning becomes significantly more complex when iframes are nested, particularly when the same origin appears multiple times within the iframe chain. Browser vendors, particularly Chrome, have had to develop sophisticated mechanisms to handle these edge cases correctly while maintaining both security and privacy protections. Consider a scenario where site A1 contains an iframe for site B, which in turn contains an iframe for site A2, and both A1 and A2 belong to the same registrable domain. If partitioning only considered the top-level site and the current frame’s origin, site A2 might be mistakenly treated as first-party storage because it shares a domain with the top-level context A1, despite the cross-site iframe B in between. This could create a security vulnerability like clickjacking, where A2 would have unpartitioned access to storage that could be manipulated from an untrusted cross-site context.
To address this complexity, Chrome introduced an “ancestor bit” to the storage partition key. This bit is set to true if any document between the current iframe and the top-level site is from a different cross-site origin, and false otherwise. In the example above, because site B is cross-site relative to both A1 and A2, the ancestor bit would be set for A2’s storage, causing it to be partitioned separately from A1’s storage even though they share the same registrable domain. Conversely, when an iframe chain consists solely of same-site contexts—for example, A1 containing A2, which then contains A3—the ancestor bit remains false and their storage remains shared, keyed only by their common origin and top-level site, since there are no security or privacy barriers between same-origin contexts that have synchronous script access to each other. This nuanced approach ensures that storage partitioning provides strong privacy protections while maintaining backward compatibility with legitimate same-site iframe architectures.
Browser Implementations and Deployment Status
Google Chrome and Chromium-Based Browsers
Google Chrome began rolling out storage partitioning to all users starting with version 115 in August 2023, making it the first major browser to implement this technology by default for all users. The feature isolates most storage and communication APIs in third-party contexts, including local storage, IndexedDB, service workers, and communication APIs like BroadcastChannel and SharedWorker. Edge, which is based on Chromium, began implementing storage partitioning with version 119, inheriting the Chrome implementation through the shared Chromium codebase. Chrome has continued refining the implementation through subsequent versions, with ongoing updates and extensions to the system. For example, starting with Chrome 137 (released May 27, 2025), Blob URLs became partitioned for all uses except top-level navigations, preventing cross-partition blob URLs from being used with fetch operations or as src attributes for HTML elements, while still allowing top-level navigations with `noopener` enforcement.
Chrome’s approach includes several important technical details. The implementation affects storage quota systems, with the quota system now managing each partition as a separate bucket to determine how much storage space is permitted and when clearing occurs. The Web Storage API (local storage and session storage) is partitioned by top-level site, and the Origin Private File System and Storage Bucket API are also partitioned, reinforcing data privacy across multiple storage layers. Additionally, the Clear-Site-Data header, which allows servers to request deletion of stored data, now only clears data within the affected partition rather than clearing all instances of data for a domain. Chrome has also provided a deprecation trial mechanism allowing sites additional time to migrate away from unpartitioned storage if they have legitimate use cases requiring cross-site storage access.
Mozilla Firefox’s State Partitioning
Mozilla implemented storage partitioning through its “State Partitioning” mechanism, which became enabled by default for all Firefox users starting with version 103 in September 2022. Firefox’s approach is conceptually similar to Chrome’s but was implemented first and uses slightly different terminology, referring to the practice as “double-keying” storage by both the origin and the top-level site. Firefox’s implementation applies to every embedded third-party resource regardless of whether it is classified as a tracker, providing universal privacy protections rather than relying on lists of known trackers. This represents a more aggressive privacy stance than Firefox’s earlier Enhanced Tracking Protection (ETP) policy, which only blocked known trackers based on the Disconnect list. By applying partitioning universally, Firefox protects users from tracking domains that have not yet been identified and classified, and protects against tracking techniques developed by new trackers before they appear on any blocklist.
Firefox implements storage partitioning with an emphasis on both storage and network partitioning. Network partitioning in Firefox has been permanent and non-relaxable since Firefox 85, affecting HTTP cache, image cache, favicon cache, connection pooling, stylesheet cache, DNS, HTTP authentication, Alt-Svc, speculative connections, fonts and font cache, HSTS, OCSP, intermediate CA cache, TLS client certificates, TLS session identifiers, prefetch, preconnect, and CORS-preflight cache. Storage partitioning itself came later but provides similar comprehensive coverage. Firefox’s implementation uses the scheme and registrable domain (eTLD+1) of the top-level site as the additional key, exactly paralleling Chrome’s approach. Prior to becoming the default in Firefox 103, storage partitioning was enabled for users with “Strict” privacy protection settings in Firefox 86 and in private browsing mode starting with Firefox 90, allowing Firefox to gather real-world compatibility data before enabling it universally.
Apple Safari and Other Browsers
Apple Safari has implemented storage partitioning as part of its Intelligent Tracking Prevention (ITP) framework, which Safari has been developing and refining since version 13.1. Safari’s approach to blocking third-party cookies is comprehensive and strict: Safari blocks all third-party cookies by default with no exceptions, even if the user previously interacted with the embedded domain. Additionally, Safari limits the lifespan of first-party cookies used for tracking purposes to a maximum of seven days, and deletes all other script-writeable storage after seven days of no user interaction with the website. This storage partitioning, combined with Safari’s aggressive first-party tracking limitations, creates one of the most privacy-protective browsing environments available. When legitimate third-party embeds need storage access in Safari, they must use the Storage Access API and explicitly request permission, which typically requires a user gesture or interaction to be granted.
CHIPS: The Opt-In Partitioning Model
Introduction to Partitioned Cookies
Cookies Having Independent Partitioned State (CHIPS), also known as partitioned cookies, represents a standardized approach to allowing developers to explicitly opt cookies into partitioned storage. Rather than forcing all cookies to be partitioned by default (which could break legitimate third-party services that expect unpartitioned behavior), CHIPS allows third-party services to voluntarily adopt partitioned storage by setting a `Partitioned` attribute on their cookies. This opt-in model emerged from practical experience with storage partitioning: when Firefox and Safari implemented automatic partitioning without developer opt-in, some third-party services that had built their systems expecting unpartitioned cookies experienced unexpected bugs and functionality breakage. By providing an opt-in mechanism, CHIPS enables a smoother transition period where services can gradually migrate to partitioned cookies while maintaining backward compatibility with existing systems that may still expect unpartitioned behavior.
CHIPS is supported by default in Chrome 114 and higher and has seen broad adoption across browsers, with support becoming available in all major browsers through various mechanisms and timelines. The technology introduces a new `Partitioned` attribute for the Set-Cookie HTTP header, allowing developers to explicitly mark cookies as partitioned. A cookie set with the `Partitioned` attribute follows a specific technical structure: it must include the `Secure` attribute, can optionally use the `__Host-` prefix to restrict it to the current domain, and should be combined with `SameSite=None` to ensure proper cross-site transmission. An example would be: `Set-Cookie: __Host-name=value; Secure; Path=/; SameSite=None; Partitioned;`
How CHIPS Technical Implementation Works
The technical implementation of CHIPS involves double-keying cookies by both their host key and a partition key, creating unique cookie storage spaces for each combination. The host key is simply the hostname or domain of the site that set the cookie (for example, `(“support.chat.example”)`), while the partition key is the site of the top-level URL the browser was visiting at the start of the request that set the cookie. Consider a practical example: a user visits `https://retail.example` which embeds a chat widget from `https://support.chat.example`. The chat widget sets a cookie with the `Partitioned` attribute. This cookie is stored with the key `{(“support.chat.example”), (“https”, “retail.example”)}`, combining both the host key (support.chat.example) and the partition key (https://retail.example). If the user subsequently visits `https://shopping.example`, which also embeds the same chat widget, the chat widget cannot access the cookie set when embedded on retail.example because the partition key doesn’t match—it would be `{(“support.chat.example”), (“https”, “shopping.example”)}` instead. The browser enforces that partitioned cookies are only sent in requests where the partition key matches the current top-level site.
When a user visits the chat service directly as a top-level website (for example, navigating to `https://support.chat.example`), the partitioned cookies set when the service was embedded in other sites would not be sent in that request either, because those cookies’ partition keys refer to different top-level sites. This is the fundamental privacy benefit of CHIPS: while the service can maintain state within individual websites where it’s embedded, it cannot use cookies to link a user’s activities across multiple websites or to correlate the user’s direct visits with their embedded visits on other sites. The opt-in nature of CHIPS is critical: only cookies explicitly marked with the `Partitioned` attribute receive this treatment. Unpartitioned cookies continue to work according to traditional rules (subject to browser restrictions on third-party cookies), allowing services time to migrate to the new model without immediate breakage.
Use Cases and Business Applications
CHIPS enables several important use cases for legitimate third-party services that genuinely need to maintain state within individual websites. Chat widgets embedded on a website can use partitioned cookies to maintain conversation history and user preferences across different pages of that same website, improving the user experience within that single domain without enabling cross-site tracking. Map embeds can persist user location preferences and zoom settings within a particular website. Analytics services embedded on a website can use partitioned cookies to track user interactions within that specific website without gaining the ability to track users across unrelated websites. Payment processors embedded in checkout flows can use partitioned cookies to maintain session information and security tokens. Headless CMS providers can use partitioned cookies to manage cache and configuration information for content served on multiple related sites. Subresource CDN providers can use partitioned cookies for load balancing decisions without enabling cross-site tracking.
These use cases represent legitimate business functionality that would be unnecessarily constrained by absolute cookie blocking. By enabling partitioned cookies, CHIPS allows these services to function effectively while still preventing the cross-site tracking that represents the core privacy concern. This represents a pragmatic balance between privacy protection and web functionality, acknowledging that not all third-party cookies are inherently harmful—the problem is when they enable tracking across unrelated sites. The `__Host-` prefix used with partitioned cookies additionally strengthens security by binding cookies only to the current domain or subdomain, preventing them from being shared inappropriately between subdomains unless explicitly desired.
The Storage Access API and Recovery Mechanisms
Requesting Unpartitioned Storage
For legitimate third-party embeds that genuinely need unpartitioned storage access—such as federated login providers where a user needs to authenticate through an embedded frame and maintain that authentication across multiple related websites—browsers provide the Storage Access API. This JavaScript API allows embedded content to request access to unpartitioned cookies and storage through an explicit API call, enabling users to grant permission on a case-by-case basis rather than all third-party storage being accessible by default. The Storage Access API provides two primary methods: `document.hasStorageAccess()` (also available as `document.hasUnpartitionedCookieAccess()` in Chrome 125 and later) checks whether the embedded content already has storage access, and `document.requestStorageAccess()` requests access permission from the user.
When `requestStorageAccess()` is called, the browser’s behavior depends on the specific browser and the context in which the request is made. In Safari, storage access always requires a direct user interaction (a gesture like a click), and the browser presents a permission prompt to the user for every new embedded origin that requests storage access. In Firefox, the browser only prompts users after an origin has requested storage access on more than a threshold number of sites, implementing a heuristic that assumes if an origin is requesting access on many sites, the user may be more likely to trust it. In Chrome, the browser shows prompts for most embedded content that hasn’t previously received storage access, but may automatically grant access if the embedded content and embedding site are part of the same Related Website Set. Once granted, storage access permissions are typically valid for a limited period—usually 30 days of browser usage—and this period extends with continued user interaction with the embedded content. The time-limited nature of storage access grants further constrains their usefulness for cross-site tracking while still enabling legitimate functionality.
Technical Implementation and Browser Variations
The implementation details of the Storage Access API vary significantly across browsers, reflecting different privacy philosophies and technical constraints. Chrome requires that cookies sent through storage access must have `SameSite=None` explicitly set (as Chrome defaults to `SameSite=Lax`), must have the `Secure` attribute set, and must specify the domain appropriately. Storage access grants in Chrome are phased out after 30 days of browser usage have passed without user interaction, though interaction with the embedded content extends this limit by another 30 days. Firefox’s implementation tracks storage access grants more granularly: if an embedded origin has already obtained access on a top-level origin, it receives immediate access on subsequent visits within 30 calendar days without requiring another prompt. This speeds up the user experience for common scenarios while still requiring initial user approval.
Safari’s implementation is particularly strict and user-focused: storage access grants only persist for 30 days of browser usage without interaction, similar to Chrome, but Safari requires a user gesture (click or other interaction) for every storage access request. Furthermore, in Safari, access granted through the Storage Access API is always per-frame—every separate embed has to call the API independently and receive user permission independently, ensuring granular user control over which embeds receive which permissions. When storage access is granted, the embedded content gains access to its entire first-party cookies and storage—the data it would have access to if visited as a top-level site—but only for that specific top-level site. So an embedded chat service granted storage access on example.com gets access to its full cookie jar as if it were the top-level site, but only when embedded on example.com.
Extensions and Future Developments
The Storage Access API continues to evolve with new extensions and capabilities. As of Chrome 125, support was added for requesting access to non-cookie storage through the `requestStorageAccess({localStorage: true})` syntax, allowing embeds to request unpartitioned access to localStorage, IndexedDB, and other storage mechanisms beyond cookies. The browser can also integrate with the Permissions API through the `”storage-access”` feature name, allowing developers to query whether storage access has already been granted using standard permission-checking mechanisms, reducing the need for trial-and-error API calls. Firefox has been experimenting with automatic unpartitioning through heuristics in certain common scenarios, particularly single-sign-on flows where a popup is opened that needs to communicate back to the opening page. Firefox detects these patterns and automatically grants storage access without requiring an explicit user prompt, improving the user experience for common authentication workflows while still maintaining security through pattern recognition rather than blanket permissions.
Looking forward, browsers are experimenting with extending the Storage Access API to handle more complex scenarios, such as providing unpartitioned access across chained iframes for services that need this functionality for legitimate reasons. Chrome is working on extensions to the Storage Access API that would allow explicit opt-in to unpartitioned storage in complex iframe hierarchies, recognizing that some legitimate use cases genuinely require this capability and should be supported through explicit consent rather than automatic access.
Comprehensive Protection Mechanisms Beyond Cookies
Network State Partitioning
Storage partitioning extends beyond just cookies and localStorage to encompassing network state—the collection of caches, connection data, and other networking infrastructure that browsers maintain to optimize performance. This represents a critical protection layer because sophisticated trackers have developed techniques to exploit network state for cross-site tracking, using timing analysis of network operations to infer information about users’ states on other websites. Firefox pioneered network partitioning, implementing it by default since Firefox 85, making it permanent and non-relaxable by websites or users. The network APIs partitioned by Firefox include HTTP cache (preventing cached resources from being used across different top-level sites), image cache, favicon cache, connection pooling, stylesheet cache, DNS queries, HTTP authentication, Alt-Svc headers, speculative connections, fonts and font caches, HSTS settings, OCSP responses, intermediate CA cache, TLS client certificates, TLS session identifiers, prefetch and preconnect resources, CORS-preflight cache, WebRTC device IDs, and backward/forward cache (bfcache).
Chrome has implemented similar network partitioning, though the details and timeline differ from Firefox. The key principle underlying network partitioning is that each top-level site gets its own isolated network state, preventing an attacking site from using network timing variations to infer whether a victim site has cached particular resources or completed particular network operations. For example, if a resource loads significantly faster on the second access (indicating it was cached), an attacker could previously infer that a victim user had also visited the site that cached that resource. With network partitioning, each top-level site maintains its own cache, so an attacker cannot gain information about what other sites have cached. The impact of network partitioning on browser efficiency is non-trivial: partitioned caches consume more storage space, network requests that could previously be served from cache now miss the cache and require network roundtrips, and bandwidth usage may increase because resources cannot be shared across sites. However, browsers have determined these efficiency costs are acceptable tradeoffs for the privacy protection achieved.
Communication API Partitioning
Beyond storage and network APIs, browsers also partition communication APIs that allow contexts to communicate across origin boundaries. The BroadcastChannel API, which allows windows, tabs, and iframes of the same origin to broadcast messages to each other, is partitioned by top-level site. This prevents same-origin contexts embedded in different top-level sites from communicating with each other through broadcast channels. Similarly, SharedWorker—which provides a worker that can be accessed across multiple browsing contexts of the same origin—is partitioned by top-level site, preventing coordination between same-origin shared workers embedded in different contexts. The Web Locks API, which allows code in one context to acquire locks for shared resources that other contexts of the same origin can respect, is also partitioned. These partitionings prevent sophisticated trackers from coordinating data collection and sharing information across multiple same-origin embeds on different top-level sites through these communication channels.
Service Workers, which can respond to network requests and alter navigation timing, represent a particular concern because they can be exploited for history sniffing and other side-channel attacks. Service Workers registered from a third-party context are now partitioned, preventing an embedded third-party service worker from being invoked or from accessing stored data in ways that could enable cross-site requests or information leakage. Extension pages, which use the `chrome-extension://` scheme and can be embedded on sites across the web, continue to have access to their top-level partition to maintain functionality, but websites embedded within extensions access only their own partition. This careful partitioning of communication APIs reflects the recognition that tracking is not limited to cookie-based techniques—sophisticated attackers exploit the full breadth of browser capabilities for reconnaissance and information leakage, requiring equally comprehensive privacy protections.
Service Worker and Quota Management
Service workers, which provide powerful capabilities for background processing and offline functionality, are now partition-aware and respect storage partitioning boundaries. Service workers registered in a third-party context operate within their own partition and cannot intercept or modify requests for unrelated top-level sites. The quota management system, which determines how much storage a website can use, now also respects partitioning, with each partition receiving its own quota allocation. This means that a third-party service cannot consume an unlimited amount of storage across multiple top-level sites—each partition has its own limits. The Clear-Site-Data header, which servers can use to request deletion of stored data when users leave a site, now also operates partition-specifically, clearing only data within the affected partition rather than all instances of that domain’s data.
The Origin Private File System, provided by the File System Access API, is also partitioned, ensuring that private file system access for one top-level site doesn’t grant access to the file system contents stored for embedded content on other sites. The Storage Bucket API, an emerging standard that consolidates various storage APIs into a unified bucket model, respects partitioning from the ground up, ensuring that buckets created in one partition are not accessible from other partitions. These comprehensive protections across multiple layers of browser functionality reflect the sophisticated nature of modern tracking techniques and the need for privacy protections to be applied comprehensively rather than piecemeal.
Impact on Web Functionality and Compatibility Challenges
Real-World Breakage and Migration Issues
The deployment of storage partitioning has revealed genuine compatibility challenges, particularly for services that historically relied on unpartitioned third-party storage to function. Organizations using cross-domain messaging, authentication, and data synchronization have experienced disruptions when storage became partitioned by default. For example, services attempting to maintain unauthenticated consumer identity and state across different domains and subdomains face significant challenges, as storage partitions prevent data set on one domain from being accessed on another domain, even when both are part of the same brand’s ecosystem. In particular, unpartitioned cross-domain web messaging has been affected, as local storage values can no longer be passed through Workers or iframes when different top-level domains are involved. This impacts scenarios where a shopping cart stored on one domain needs to be accessible on another domain’s checkout page, or where a user’s preferences set on one brand’s site should be accessible when they navigate to another brand’s site.
Authentication providers, particularly single-sign-on (SSO) providers, have experienced challenges when storage partitioning eliminates their ability to maintain sessions across multiple related websites. Browser enrollment systems that relied on storage-based tokens have encountered situations where enrollment credentials stored in one domain’s partition became inaccessible when the browser was upgraded and partitioning was enabled, effectively logging users out unexpectedly. Some services have had to implement fallback mechanisms or browser extensions to maintain functionality across these scenarios. However, browsers have provided migration assistance through mechanisms like deprecation trials, allowing sites to request temporary exemptions from storage partitioning during a defined transition period to allow their systems to be updated to use new APIs like the Storage Access API or Related Website Sets.
Legitimate Use Cases and Solutions
The genuine compatibility challenges have led to the development of several mechanisms for supporting legitimate use cases that require cross-site storage access. The Storage Access API provides one solution, enabling embeds to request user permission for unpartitioned access when needed for authentication or other legitimate functionality. Related Website Sets represent another approach, allowing related domains (such as multiple properties of the same company) to maintain shared storage access within their set without requiring full unpartitioned access to all third-parties. The `requestRelatedWebsitePartition` API proposal would extend this capability, allowing third-party embeds to maintain sessions across websites in the same related website set without triggering user prompts or providing full unpartitioned access.
For services genuinely needing unpartitioned storage, the deprecation trial mechanism provides temporary relief. Chrome offered the `DisableThirdPartyStoragePartitioning` deprecation trial from Chrome 113 to 126, allowing sites to opt into unpartitioned storage, service workers, and communication APIs for specific third-party contexts during a migration window. This trial has been renewed, with Chrome offering extended deprecation trial support for users on Chrome 127 to 132, allowing sites that were previously enrolled and already have valid tokens to request renewal for an additional 6 milestones. However, these trials are explicitly temporary measures designed to facilitate migration, not permanent solutions. Browsers are clear that partitioning represents the permanent direction for the web platform, and developers should update their systems to be partition-aware.
Browser-Specific Impact Variations
The impact of storage partitioning varies significantly across browsers based on their specific implementations and user bases. Safari users have experienced the longest duration of partitioning effects, as Safari implemented aggressive storage partitioning through Intelligent Tracking Prevention years before other browsers made it default. Firefox users experienced partitioning as a default in 2022-2023, with the ability to adjust settings through preferences for those with specific needs. Chrome and Edge users experienced the transition more recently, with Chrome 115 (August 2023) and Edge 119 representing the rollout timeline. The impact was particularly acute for Chrome users due to Chrome’s market dominance—websites and services built on assumptions about unpartitioned cookies had to confront compatibility issues affecting their majority user base simultaneously.
For services that don’t adapt, users experience broken functionality: messaging features that require state persistence across domains stop working, shared shopping carts become domain-specific, and authentication flows requiring cookies across multiple domain boundaries fail. However, the incentive structure for services is clear: they must either adapt to storage partitioning or lose functionality for users across major browsers. This has accelerated adoption of standards-based solutions like the Storage Access API and Created related website infrastructure changes, as service providers recognize they must build systems that work within the partitioned storage model rather than expecting to maintain unpartitioned access indefinitely.
Detection and Response Mechanisms
Detecting Third-Party Cookie and Storage Blocking
Website developers need to detect when third-party storage is blocked or partitioned to implement appropriate fallbacks or alternative functionality. As of May 2025, several detection mechanisms exist with varying degrees of reliability and cross-browser compatibility. The Storage Access API provides `document.hasStorageAccess()` and `document.hasUnpartitionedCookieAccess()` methods that allow embedded content to query whether it currently has storage access, returning a Promise that resolves with a boolean indicating access status. This represents the most direct approach but only indicates current access state, not whether partitioning is affecting functionality.
The most reliable practical detection method involves embedding a hidden iframe from the third-party domain, attempting to set a test cookie within that iframe, and using `window.postMessage()` to communicate back to the parent window whether the cookie was successfully set and retrieved. This approach simulates a real third-party scenario, works across all major browsers when properly configured, requires no server-side components, and directly tests the functionality that matters. The key limitation is that this approach detects blocking but doesn’t necessarily distinguish between cookies being blocked outright (as in Safari) versus being partitioned (as in Firefox and Chrome with default settings). Chrome introduced the `Sec-Fetch-Storage-Access` HTTP request header starting in version 133, which is sent with cross-site requests to indicate whether the iframe has access to unpartitioned cookies, but this header is only visible to servers, not to JavaScript, and is not yet supported by other browsers.
Browser-Specific Detection Differences
Detection results vary significantly across browsers. In Safari, the iframe plus postMessage detection method almost always fails unless the embedded content explicitly requests storage access through the Storage Access API and the user grants permission. In Firefox, the test cookie may be set and retrieved successfully (allowing detection of successful cookie setting), but the cookies may be useless for cross-site authentication because they’re partitioned—the cookie exists but is ineffective for the tracking functionality the test is checking for. In Chrome, the detection typically succeeds with default settings, allowing partitioned cookies to be set and retrieved, but developers must understand that these cookies are partitioned and not accessible from other top-level sites. These differences mean that a single detection script cannot make reliable assumptions about capability across browsers without incorporating browser detection logic.
Recent Industry Evolution and October 2025 Updates
Privacy Sandbox Redirection and API Retirements
In October 2025, Google announced significant updates to its Privacy Sandbox initiative, reorienting the project after recognizing that several proposed APIs had achieved limited adoption despite years of development. The company announced the retirement of multiple Privacy Sandbox technologies including the Attribution Reporting API, IP Protection, On-Device Personalization, Private Aggregation (including Shared Storage), Protected Audience, Related Website Sets (including `requestStorageAccessFor`), SelectURL, SDK Runtime, and Topics. These retirements reflect a recognition that despite substantial investment in developing privacy-preserving alternatives to third-party cookies, many of these APIs failed to achieve meaningful ecosystem adoption or deliver sufficient value to justify continued development. The announcement emphasized that Google would continue supporting CHIPS and FedCM (Federated Credential Management), which have seen broad adoption across browsers and represent the successful privacy-preserving solutions worth maintaining.
Most significantly, Google announced that it would maintain its current approach to third-party cookies in Chrome rather than implementing the previously announced phased elimination. This represented a major pivot from the 2019 announcement that third-party cookies would be phased out by 2023 (later extended to 2024 and then 2025). The regulatory landscape, particularly the CMA and ICO engagement, along with ecosystem feedback from publishers, developers, and advertisers expressing divergent perspectives on eliminating third-party cookies, led to the decision to maintain user choice through browser settings rather than forcing a single approach. This development underscores that while storage partitioning provides fundamental privacy protections that browsers have deployed universally, the question of whether to eliminate third-party cookies entirely remains unresolved and subject to regulatory and market forces.
Continued Support for Storage Partitioning and Core Privacy Features
Notably, storage partitioning was not subject to reconsideration in the October 2025 update. Storage partitioning remains a core, committed privacy feature across all major browsers, representing a successful consensus on preventing cross-site tracking while maintaining web functionality. The differentiation between retiring complex attribution and personalization APIs while maintaining storage partitioning reflects a philosophical consensus: preventing trackers from following users across the web is a fundamental privacy requirement that should be universal and non-negotiable, whereas complex privacy-preserving measurement and personalization systems remain in active exploration and development. Going forward, Chrome indicated continued focus on enhancing tracking protections in Incognito mode (which already blocks third-party cookies by default), with plans to launch IP Protection in Q3 2025 to extend protections against network-level tracking.
The October 2025 announcement also clarified that CHIPS and FedCM have seen broad adoption including support from other browsers, validating these as successful standardized solutions for maintaining third-party functionality while preserving privacy. The continued commitment to storage partitioning across all major browsers, combined with the broader ecosystem’s recognition that partitioning provides essential privacy protections without requiring elimination of third-party functionality, suggests that storage partitioning will remain the foundational privacy architecture for the web regardless of how the broader debate about third-party cookies continues to evolve.
Long-Term Implications and Future Directions
Evolution of Privacy-Preserving Web Architecture
Storage partitioning represents a fundamental architectural shift in how the web manages state and privacy, establishing principles that will likely guide web development for years to come. By establishing that storage should be partitioned by top-level site by default, browsers have created a new baseline assumption for web privacy: that users’ activities on different websites should be kept separate unless there is an explicit, intentional reason to link them. This represents a maturation of web privacy thinking, moving beyond binary choices between “allow everything” and “block everything” to a more nuanced model where functionality is preserved while privacy boundaries are established by architectural design rather than relying on user vigilance.
Looking forward, storage partitioning will likely expand to encompass additional storage mechanisms and browser capabilities as they’re developed and as new tracking vectors emerge. The current partitioning covers the most common storage and communication APIs, but browser vendors continue to identify additional mechanisms that could be exploited for tracking if left unpartitioned. For example, as developers create new browser APIs and capabilities, the question of how these should interact with partitioned storage will become increasingly important. The W3C Privacy Community Group’s work on standardizing storage partitioning principles provides a framework for making these decisions consistently across browsers and new technologies.
Developer Education and Ecosystem Adaptation
The widespread deployment of storage partitioning has required significant developer education and ecosystem adaptation. Developers who built services on assumptions of unpartitioned third-party storage have had to learn new APIs (Storage Access API, potentially Related Website Sets, CHIPS) and new patterns for maintaining functionality within partition boundaries. This learning curve has been painful for some services, particularly those that built complex cross-domain functionality without modularity considerations. However, it has also driven innovation: services have developed more sophisticated approaches to maintaining state that don’t rely on third-party cookies, including server-side session management, federated authentication through dedicated sign-on services, and approaches using first-party identifiers within defined ecosystems.
The ecosystem is gradually adapting, with documentation and tools improving to help developers understand and work with partitioned storage. Browser vendor documentation, MDN Web Docs, and privacy community resources now provide comprehensive guidance on storage partitioning, CHIPS, the Storage Access API, and other privacy-preserving APIs. Developer tooling is also improving: browser developer tools now explicitly show partition keys for stored data, allowing developers to understand and debug partitioning effects on their services. This evolving ecosystem of tooling and documentation will continue to improve the developer experience as storage partitioning becomes the norm rather than a special case developers need to understand and accommodate.
Safeguarding Your System with Partitioning
Storage partitioning represents a fundamental and successful solution to the cross-site tracking problem that has plagued web privacy for decades. By isolating storage by top-level site by default across all major browsers—Chrome, Firefox, Safari, and Edge—the technology prevents trackers from building comprehensive behavioral profiles across the web while simultaneously preserving legitimate third-party functionality through mechanisms like CHIPS and the Storage Access API. The technology addresses not just cookies but the full breadth of browser state that could be exploited for tracking, including localStorage, IndexedDB, service workers, communication APIs, and network state such as caches and DNS queries.
For users, storage partitioning provides robust privacy protection without requiring vigilance or configuration—it works automatically in the background, preventing trackers from following them across the web regardless of their technical sophistication or awareness of tracking mechanisms. The protection is particularly strong because it’s architectural rather than based on attempting to identify and block specific trackers: it works against known trackers, unknown trackers, and new tracking techniques that haven’t yet been developed, because the partitioning prevents the fundamental capability that cross-site tracking depends on.
For websites and services, storage partitioning requires adaptation but does not eliminate third-party functionality. Legitimate third-party services can continue to function through partitioned cookies set with the `Partitioned` attribute, through unpartitioned storage access requested through the Storage Access API with user permission, or through federated mechanisms like FedCM for authentication. While services built on unpartitioned storage assumptions have experienced disruption, the transition has been managed through deprecation trials and browser support for standards-based alternatives, and the overall trajectory is clear: the web is evolving toward a model where third-party functionality exists within privacy boundaries by default.
Storage partitioning’s success lies in its pragmatism: it doesn’t attempt to eliminate third-party services from the web, recognizing that many legitimate use cases genuinely require third-party embeds. Instead, it changes the default from “anything is possible” to “functionality is preserved, but cross-site tracking is prevented.” This represents a successful consensus across browsers, standards bodies, and (to the extent possible given divergent interests) the industry, suggesting that storage partitioning will remain the foundational privacy architecture for the web even as specific debates about third-party cookies, attribution APIs, and other technologies continue to evolve.
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