Apple's latest privacy crackdown left the online ad industry scrambling for answers. Now, the tech giant has finally offered some. Late on Monday, Apple published a blog post laying out its rationale for new restrictions that block technology used to identify and track consumers across websites. The changes are implemented through WebKit, the browser engine that powers Safari and other browsers on iPhones and iPads.

Apple said the change was designed to target "identity resolution," a method adtech companies use to connect separate data points about users — such as email addresses, phone numbers, IP addresses, browsing history, or information about the device they're using — to identify and track them across devices. The method grew in popularity as many web browsers, including Apple's Safari, began phasing out third-party tracking cookies, the small files dropped on a user's device that were traditionally used for web tracking.

The ad industry deploys tracking to precisely target specific audiences and measure whether their campaigns are working. In its blog post, Apple said that while users might readily offer their email address to to a website or , most wouldn't expect it to also be shared with other companies for invisible tracking purposes. "It's not enough to block cross-site cookies to prevent cross-site tracking today because the tracking industry has not stopped trying to track users on the web," Apple said. "They have ushered into what they call the post cookie era and in our latest releases we have put strong protection in place."

The blog post explains that Apple is implementing "domain-level" blocking, a broad intervention designed to ensure that identity providers can't circumvent the restriction by changing how they send data. Apple took the adtech industry by surprise earlier this month when it came to light that it had begun blocking the domains operated by several companies that offer alternative identifiers, without warning.

The blocked domains included those operated by The Trade Desk, LiveRamp, ID5, Permutive, and Audigent. The initial rollout also blocked The Trade Desk's ability to serve ads at all on Apple-operated browsers. Apple rolled out a fix a few days later that restored The Trade Desk's ad-serving domain from being blocked alongside its advertising identifier, Unified ID 2.0. The ad industry is now bracing for impact as more users begin updating their devices to iOS 27.

Ad pros previously told CMO Insider that the change will make it harder for advertisers to identify and reach people across the open web. Industry insiders said that by effectively shrinking the pool of users they can target, advertisers are expecting less reach, higher prices to reach the audiences they can still identify, and more difficulty in controlling how often people see an ad and measuring whether their campaigns worked. Identity resolution has become widespread across the open web.

An academic paper published in September that analyzed 600,000 websites over 41 months found that 83.76% of those sites were using alternative identifiers by May 2025. If advertising becomes less effective on the open web, marketers could funnel more of their budgets into so-called "walled garden" platforms like Meta and Google, which aren't as heavily reliant on alternative identifiers because they have reams of their own logged-in user data.

Apple, too, has a growing advertising business within its App Store and apps, including News and Maps. There are still unanswered questions about Apple's exact definition of an identity resolution provider, and the company has already demonstrated it can add domains to its block list without warning. There's also an indication Apple is only getting started. First reported by AdExchanger, Apple has also created a broader block list that can dynamically block potentially hundreds of adtech, martech, and data companies.

Affected companies may not realize they've been added to the list until they begin experiencing disruptions. In Monday's blog post, Apple linked to its long-standing tracking-prevention policy, which acknowledges that its blocking rules may have unintended impacts. Those include disrupting the funding of websites that use targeted or personalized advertising, advertising measurement, and fraud prevention. The policy states that while Apple will try to limit unintended impact, "when faced with a tradeoff, we will typically prioritize user benefits over preserving current website practices."

Ever since Apple launched its Intelligent Tracking Prevention feature in 2017, the tech giant has been locked in a cat-and-mouse game with the adtech industry, which has persistently developed workarounds to its privacy updates. Adtech companies have argued that these techniques were necessary to help publishers effectively monetize their websites with ads while still maintaining user privacy. Apple, which has long held that "privacy is a fundamental human right," makes it clear in its policy that it won't allow workarounds to proceed on its watch.

"We treat circumvention of shipping anti-tracking measures with the same seriousness as exploitation of security vulnerabilities," the Apple policy states. "If a party attempts to circumvent our tracking prevention methods, we may add additional restrictions without prior notice."