Identifier for Advertisers (IDFA)

The Identifier for Advertisers (IDFA) is a string Apple assigns to each individual iPhone and iPad so apps and ad networks can recognize the same device when they measure and target ads. Since iOS 14.5, an app can read the real IDFA only after the user taps allow in the App Tracking Transparency (ATT) prompt.

What is IDFA?

Apple describes the IDFA as a UUID unique to each device and meant only for advertising: frequency capping, install attribution, conversion tracking, estimating unique users and fraud detection (Apple Developer, advertisingIdentifier, accessed October 2026).

Without permission, the API returns a string of all zeros. That happens when the app has not shown the ATT prompt yet, when the user declines, when the device is configured to block tracking, and always on the Simulator. Apple recommends not storing the IDFA but reading it fresh every time, because users can change their choice at any moment under Settings > Privacy > Tracking.

What changed from 2021 to 2026?

iOS 14.5 was released on April 26, 2021 (Apple Support, Apple security updates 2020 to 2021). Before then, apps could read the IDFA without asking. After it, any app that wants to track users across other companies’ apps and websites has to ask for permission through ATT.

Apple also closed the workaround. Apps may not combine device signals to build a fingerprint in place of the IDFA, and an app that uses an SDK doing this can be rejected (Apple Developer, User Privacy and Data Use, accessed October 2026).

Installs without an IDFA are measured with SKAdNetwork and AdAttributionKit. These two APIs return aggregated, delayed results and do not require ATT.

How is IDFA used in app measurement?

When the user opts in, the MMP records the IDFA at the moment of the ad click, then compares it with the IDFA the SDK sends back when the app is opened for the first time. If the two match, the install belongs to that click. Adjust calls this device matching and uses it as its main attribution method (Adjust Help Center, Attribution methods, accessed October 2026).

Matching at the device level tells you which campaign a user came from and on which day after the install they bought something. Without the IDFA, that question can only be answered in aggregate.

Example

Hypothetically, a food delivery app spends 300 million VND on iOS app install ads in a month and gets 12,000 installs. If 25% of users accept ATT (a hypothetical figure, since real rates vary by app), the MMP can match 3,000 installs to individual clicks. The other 9,000 show up only through SKAdNetwork or AdAttributionKit postbacks, which arrive at least 24 to 48 hours late and carry no user-level data. Reading cost per install from the IDFA portion alone will give you the wrong answer.

IDFA vs. IDFV vs. Android Advertising ID

On Android, Google says that since a Google Play services update in late 2021, apps receive only a string of zeros once a user deletes their advertising ID. Apps targeting Android 13 or higher must declare the AD_ID permission in the manifest (Play Console Help, Advertising ID, accessed October 2026).

Frequently asked questions

Has Apple retired the IDFA completely?

Not yet. Apple’s documentation as of October 2026 still describes the API. The only difference is that apps must ask permission before reading it.

Can an app offer coins or unlock features to get users to opt in?

No. Apple points to guideline 5.1.2(i) of the App Review Guidelines: apps may not tie features or rewards to consent. An app is allowed to explain its reasons before showing the prompt, as long as it is honest about how the data will be used.

If you run app install ads on iOS and need to make sense of your numbers when most installs come without an IDFA, see how Adtek handles it on the App User Acquisition page.