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According to tech analysts, Apple is keeping private a certain application programming interface (API) that would make it easier for ad blockers to hide content in web browsers from Mozilla and others.

With the release of its iOS 9 operating system, Apple has for the first time approved some ad blocking apps with the inclusion of a new API that makes it easier for ad blockers to hide content in Safari. Many new applications do just that, but others have taken this a step further and installed root certificates on devices in an attempt to block app-based ads. Such behavior, critics warn, could be a threat to other apps’ functionality.

ios 9 ad blocker api appleFor instance, the Been Choice app claimed to be the first app that could block ads in Facebook, Pandora, The New York Times, and even Apple News by routing web traffic through a VPN, at which point in time its servers would perform a deep packet inspection and use pattern matching to remove ads.

Been Choice has been removed twice from Apple’s App Store. Its developers plan to resubmit a version of the app with the in-app blocking capability disabled.

However, according to Mozilla Firefox’s VP Mark Mayo, Apple has maintained a certain degree of privacy for its ad-blocking API with regards to web browsers other than Safari, meaning that Firefox and others are left with limited options when it comes to blocking content on iOS 9.

“There are other ways in which browsers can block content on iPhones,” journalist Thomas Fox-Brewster notes in an article for Forbes. “They could bake the blocking into the browser itself or could build their own APIs to support ad blockers. But that would take far more engineering time than incorporating the ready-made API.”

Such tools would also slow down users’ browsing experience.

As of this writing, Apple has yet to comment on Mayo’s comments.

News of Apple’s private ad-blocking API follows just a day after the tech giant announced it had removed 256 apps from its App Store for using private APIs to gather user information. It also comes less than a month after Apple removed hundreds, if not thousands, of apps that used a counterfeit version of Apple’s software–dubbed XCodeGhost–to be downloaded by developers.