iOS 27 has added five named programmatic platforms — The Trade Desk, ID5, LiveRamp, Audigent and Permutive — to a WebKit block list that disables parts of their business in Safari. Apple lifted the restriction on one company this week.

The development surfaced within the past month, when The Trade Desk found itself unable to deliver ads to Apple device owners running the latest iOS. The culprit sits inside WebKit's block list of data brokers and ad tech vendors that conduct cross-site user tracking.

This week, Apple removed The Trade Desk's ad-serving conflict from a new iOS 27 beta, sparing the demand-side platform a worst-case scenario. ID5, LiveRamp, Audigent and Permutive remain blocked.

What sits on WebKit's Safari block list?

Five vendors have surfaced publicly:

• The Trade Desk and its Unified ID 2.0 identity initiative
• ID5
• LiveRamp
• Audigent
• Permutive

Apple accused all five of cross-site user tracking through Safari. None of the vendors confirmed the basis of that designation.

Why carve out The Trade Desk?

The Trade Desk alerted Apple to its ad-serving failure, according to industry reporting. A fresh iOS 27 beta removes the DSP from the block list. None of the other four companies appear in the fix.

What about WebKit's larger list?

WebKit maintains a second block list spanning multiple categories of ad tech and martech. The full scope stays opaque. Apple publishes neither the entries nor the criteria. A company has to download, decompile and audit WebKit source code to find out whether it sits on the list.

Why keep the list secret?

Senior Editor James Hercher said Apple does not "think very highly" of the ad tech industry's ability to self-regulate. The closed list runs separately from Apple's own advertising ambitions.

Editor-in-Chief Allison Schiff added that the iOS 27 block list "hits different" than AppTrackingTransparency. ATT stopped ad tech players from using identifiers without consent. WebKit's domain list blocks companies from parts of their business entirely.

Schiff observed the industry has not "really earned much benefit of the doubt on data practices."

What changes for practitioners?

For SSPs, DSPs and measurement teams, the shift demands immediate inventory and identity audits:

• Confirm whether ID5, LiveRamp, Audigent or Permutive IDs run inside any campaign stack
• Audit proprietary SDKs and tracking pixels against WebKit source code for hidden entries
• Route Safari iOS traffic through server-side conversion APIs, the workaround that emerged after AppTrackingTransparency
• Map identity-graph redundancies for Unified ID 2.0 in case Apple re-blocks The Trade Desk later

ID5, LiveRamp, Audigent and Permutive face continued inability to serve Safari inventory on iOS 27 until Apple reverses course. None of the four have confirmed a fix publicly.

Could the block list expand?

Yes. WebKit's second, broader list suggests Apple can extend restrictions from a handful of scaled vendors across the entire ecosystem. iPhone users sit behind Safari on every default-browsing session. Apple's silent edit of the permitted-vendor list redraws which programmatic companies reach iOS audiences on Apple's installed base.

Historical precedent

The episode rhymes with Apple's AppTrackingTransparency rollout. ATT paralyzed the mobile ad industry until solutions and workarounds — including conversion APIs — gained traction. The iOS 27 move lacks ATT's public framework. Ad tech vendors cannot build around a published rule they cannot see.

The pattern repeats: announce quietly, wait for workarounds, tighten through future iOS releases. Martech teams that treated WebKit as a niche mobile concern now need to treat it as a first-tier delivery channel.