App developers are exploring surreptitious new forms of user tracking to evade Apple’s new privacy rules, which threaten to upend the mobile advertising industry in the coming months.

Early in 2021, an iPhone update will prevent apps from using advertising identifiers known as IDFA without obtaining each user’s explicit consent for targeting. Developers expect more than two-thirds of users will block tracking when they see a pop-up appear within their apps.

Some app makers say they plan to use invasive tracking techniques such as “device fingerprinting” to work around the new restrictions—even though doing so risks getting them thrown off the App Store if they are caught.