Intelligent Tracking Prevention (ITP) is the anti-tracking system built into WebKit, the browser engine behind Safari on iPhone, iPad, Mac and Vision Pro. It exists to stop companies from recognising the same person as they move between unrelated websites. It classifies domains that behave like trackers, then deletes, isolates or shortens the life of the data they store. Apple introduced it on June 5, 2017. For advertisers, it is the main reason a Safari visitor who clicked an ad last week can look like a stranger today.

How the classifier works

ITP does not rely on a fixed blocklist. A machine learning model, running on the device, counts how a domain behaves across the sites a person visits. According to WebKit's documentation, it weighs three signals: the number of unique first-party sites on which a domain loads as a third-party subresource, the number on which it appears as a third-party iframe, and the number of cross-site redirects it performs.

Two further patterns trigger classification. Bounce tracking covers domains that briefly redirect a visitor through themselves to read or set a cookie in a first-party context. Tracker collusion covers domains that redirect to an already classified domain.

What follows depends on whether the person has interacted with the domain directly, as a first party. WebKit deletes all website data for classified domains without such interaction in the last 30 days of browser use, counting days of use rather than calendar days.

The caps that affect marketing

Most of the friction marketers attribute to ITP comes from time limits, several of which hit first-party data.

  • Third-party cookies are blocked outright. "ITP by default blocks all third-party cookies. There are no exceptions to this blocking," according to WebKit. An embedded widget can ask for access through the Storage Access API, which requires a user gesture.
  • Script-written storage is capped at seven days. Cookies created with JavaScript's document.cookie, along with localStorage, IndexedDB, SessionStorage and Service Worker caches, are deleted after seven days of Safari use without interaction with the site.
  • Link decoration is capped at 24 hours. If a classified domain sends a visitor to a page whose URL carries a query string or fragment, such as a click identifier, cookies written by JavaScript on that landing page expire after 24 hours.
  • CNAME and IP cloaking trigger a seven-day cap on cookies set in HTTP responses. This applies when a subdomain such as metrics.shop.example maps through DNS to a third-party server, or resolves to an IP address far from the main site's.

A worked example shows how these combine. A person clicks a social media ad that lands on shop.example with a click ID appended. The social network's domain is classified, so the landing page's JavaScript cookie that stores the click ID survives 24 hours rather than the 90 days the tag may request. If the purchase happens on day three, the browser-side tag sees a new visitor. Direct visits every few days, by contrast, reset the seven-day clock.

Where it sits in the ad transaction

ITP acts inside the browser, before any bid request is formed, so a supply-side platform cannot attach a cookie-based user ID. Cookie syncing, which matches IDs between ad tech companies through redirect chains, has no cookies to match in Safari. Attribution, frequency capping and retargeting break for the same reason.

On the sell side, publishers absorb lower prices for unaddressable impressions. Permutive's vendor data put the penalty for impressions without an identity signal at 41% lower CPMs, an unaudited figure. Buyers have shifted toward server-to-server conversion APIs, which bypass the browser.

Origin and version history

The original 2017 rules, described by John Wilander of Apple's WebKit team, allowed a classified domain's cookies to work in third-party contexts for 24 hours after a user interaction, partitioned them for up to 30 days, then purged them. ITP shipped with Safari 11 in September 2017, and Google said Google Analytics would keep working because it used first-party cookies.

On September 14, 2017, six US trade groups including the Association of National Advertisers (ANA) and the Interactive Advertising Bureau (IAB) signed an open letter saying the feature would "sabotage the economic model for the Internet," according to MarTech. "Machine-driven cookie choices do not represent user choice; they represent browser-manufacturer choice," the letter said. Criteo, the retargeting company, said on December 14, 2017 that ITP could cut its 2018 revenue ex-TAC (revenue after traffic acquisition costs) by about 22% if its workaround failed, up from a 9% to 13% estimate, according to its US Securities and Exchange Commission filing.

Each later version closed a workaround.

  • ITP 1.1 (March 2018) added the Storage Access API.
  • ITP 2.0 (June 4, 2018) removed the 24-hour window, and detected bounce tracking and collusion.
  • ITP 2.1 (February 21, 2019) ended partitioned cookies and introduced the seven-day cap on cookies set through document.cookie.
  • ITP 2.2 (April 24, 2019) capped JavaScript cookies at one day after navigation from a classified domain to a decorated URL.
  • ITP 2.3 (September 23, 2019) extended the cap to localStorage and other script-writable storage in that scenario and trimmed document.referrer.

Numbered releases stopped there. On March 24, 2020, with Safari 13.1 and iOS 13.4, WebKit blocked all third-party cookies by default and applied the seven-day cap to all script-writable storage. WebKit called Safari "the first mainstream browser" to do so. The CNAME cloaking defence followed on November 12, 2020 with Safari 14, alongside a "SameSite=Strict jail" for domains that bounce through at least 10 unique sites.

The IP address variant is harder to date. WebKit gives no version; vendor Addingwell places it in Safari 16.4 in April 2023, while Stape says Apple announced it in autumn 2022. Under the rule, server-set cookies keep their lifetime only if the server's IPv4 address shares its first 16 bits with the site's, according to Stape.

Why it matters for marketers

Cloudflare data put Safari at 15.1% of global browser traffic in the third quarter of 2025, against 66.3% for Chrome. ITP also explains much of the industry's move to the server. Cookies set by a server on the first-party domain escaped the seven-day limit for years, which made server-side tagging attractive. Google launched a first-party mode beta for its tags on October 9, 2024 The CNAME and IP caps target such set-ups when a third party runs the server.

Safari 26, released in September 2025, made Advanced Fingerprinting Protection the default for all browsing, cutting scripts classified as fingerprinters off from high-entropy APIs, long-lived storage and query parameters.

Limitations and disputes

The first criticism is that ITP shifts power to large first parties. Companies with logged-in users, such as Google, Meta and Amazon, keep identity because people visit them directly, and walled gardens gain as open-web identity weakens.

The second is that tracking prevention can itself leak information. Google researchers disclosed in 2019 that sites could detect how ITP treated their content and use that state as a tracking signal. WebKit's December 2019 fix, in Safari 13.0.4 and iOS 13.3, applied its referrer and cookie restrictions to all third parties regardless of classification.

The third is opacity. Apple's 2026 blocks rely on lists that are not public, and WebKit linked them to its Tracking Prevention Policy rather than to ITP by name, leaving open whether they belong to ITP at all.

Measurement also suffers. Because ITP deletes data silently, analytics tools overcount new Safari users, and browser-based attribution windows beyond seven days become unreliable.

Commonly confused terms

  • App Tracking Transparency (ATT) is Apple's permission prompt for apps, which took effect with iOS 14.5 on April 26, 2021. It covers apps; ITP covers websites and asks the user nothing.
  • Enhanced Tracking Protection (ETP) is Mozilla's equivalent in Firefox. It blocks trackers from a list supplied by Disconnect rather than an on-device classifier, and became the Firefox default in September 2019.
  • Privacy Sandbox was Google's set of proposed Chrome APIs to replace third-party cookies. Google retired most of them on October 17, 2025. ITP offered no targeting replacement.
  • Link Tracking Protection is a separate Safari feature, announced in June 2023 for iOS 17, that strips known click identifiers such as gclid and fbclid from URLs in Mail, Messages and Private Browsing. ITP shortens cookies after a decorated link; this feature removes the decoration.

Recent developments

WebKit commit 307525, merged on February 13, 2026, added an 11-line hook that lets Apple builds block listed domains before any tracker exemption applies. iOS 27 began rolling out on September 14, 2026, and on September 21 Ian Meyers of The Trade Desk filed a WebKit bug listing nine blocked domains, including adsrvr.org, uidapi.com, id5-sync.com and rlcdn.com. A July 2026 code review had shown Safari 27 checking destination IP addresses to block LinkedIn and Bing tags, which first-party proxies do not avoid.

ID5 chief executive Mathieu Roche wrote on October 3 that the move was "an attack against the business model of the web" and that regulators should examine it. On October 5, Wilander asked The Trade Desk to test an iOS 27.2 beta; the ticket stayed open, and ID5 remained on the list as of October 8. IAB Tech Lab pushed its Trusted Server edge runtime in response.

On October 19, 2026, Wilander, Charlie Wolfe and Matthew Finkel published WebKit's rationale, saying the company blocks third parties that turn email addresses into tracking IDs. "For the majority of users, their email address serves as a globally unique ID that they keep for years, even decades," the post said. It named no companies. As of October 2026, iOS 27.2 has no public release date.

Timeline

  • 2003 - Safari 1.0 ships with a default policy that stops third parties without existing cookies from setting new ones.
  • June 5, 2017 - WebKit announces Intelligent Tracking Prevention.
  • September 14, 2017 - Six US ad trade groups publish an open letter against ITP.
  • September 2017 - ITP ships with Safari 11 and iOS 11.
  • December 14, 2017 - Criteo estimates a 22% net hit to 2018 revenue ex-TAC if mitigation fails.
  • March 2018 - ITP 1.1 adds the Storage Access API.
  • June 4, 2018 - ITP 2.0 removes the 24-hour window and adds bounce tracking detection.
  • February 21, 2019 - ITP 2.1 introduces the seven-day cap on document.cookie cookies; Do Not Track removed.
  • April 24, 2019 - ITP 2.2 adds the one-day cap after decorated navigation.
  • September 23, 2019 - ITP 2.3 extends caps to script-writable storage and trims document.referrer.
  • September 2019 - Firefox turns on Enhanced Tracking Protection by default for all users.
  • December 10, 2019 - WebKit details fixes prompted by Google's disclosure of ITP state leaks.
  • March 24, 2020 - Safari 13.1 and iOS 13.4 block all third-party cookies by default.
  • November 12, 2020 - Safari 14 adds the CNAME cloaking cap and the SameSite=Strict jail.
  • April 26, 2021 - App Tracking Transparency takes effect with iOS 14.5.
  • April 2023 - IP address cookie cap reported in Safari 16.4 (vendor sources; Apple undated).
  • June 2023 - Apple announces Link Tracking Protection for iOS 17.
  • September 2025 - Safari 26 makes Advanced Fingerprinting Protection the default for all browsing.
  • October 17, 2025 - Google retires most Privacy Sandbox technologies.
  • February 13, 2026 - WebKit merges the unconditional domain-blocking hook.
  • September 14, 2026 - iOS 27 rolls out.
  • September 21, 2026 - The Trade Desk files WebKit bug 324771.
  • October 5, 2026 - Apple asks The Trade Desk to test an iOS 27.2 beta.
  • October 19, 2026 - WebKit publishes "Blocking Identity Resolution Infrastructure".

Summary

Who: Apple's WebKit team, led on privacy by John Wilander, builds and maintains ITP. It affects ad tech vendors, identity providers such as ID5 and LiveRamp, demand-side platforms such as The Trade Desk, publishers and every advertiser measuring Safari traffic.

What: An on-device system that classifies cross-site trackers and deletes their data, blocks all third-party cookies, caps script-written storage at seven days, caps cookies after decorated links at 24 hours and limits server-set cookies from cloaked third-party servers to seven days.

When: Announced June 5, 2017, revised through version 2.3 in September 2019, extended to full third-party cookie blocking in March 2020 and supplemented in 2026 by domain and IP blocking in Safari 27.

Where: In Safari on iOS, iPadOS, macOS and visionOS, acting inside the browser before any ad request is sent.

Why: Apple says the goal is to stop cross-site tracking without user consent. Critics, from the 2017 trade groups to ID5 in 2026, argue it entrenches large platforms and Apple's own app ecosystem while weakening the ad-funded open web.