IDFA

Definition

The IDFA, or Identifier for Advertisers, is a resettable per-device UUID that iOS provides to apps for advertising measurement, and which is only readable when the user has granted App Tracking Transparency permission.

For most of the last decade the IDFA was mobile attribution: an ad network recorded it on the click, the app reported it on the install, and matching the two gave you a deterministic answer. ATT ended that as a default in iOS 14.5. The identifier still exists and still works exactly as it used to — for the minority of users who allow it.

Reading it, and what you get when you cannot

import AdSupport
import AppTrackingTransparency

func advertisingIdentifier() -> UUID? {
    // Reading without authorisation is not an error — it returns
    // the all-zero UUID, which is why unguarded code ships a
    // constant identifier for every unauthorised device.
    guard ATTrackingManager.trackingAuthorizationStatus == .authorized else {
        return nil
    }
    let idfa = ASIdentifierManager.shared().advertisingIdentifier
    return idfa.uuidString == "00000000-0000-0000-0000-000000000000"
        ? nil
        : idfa
}

isAdvertisingTrackingEnabled was the pre-iOS-14 way to check this and is deprecated; on modern iOS it reports the ATT status rather than the old per-device Limit Ad Tracking toggle. New code should read trackingAuthorizationStatus and treat the zero check as a belt-and-braces guard.

IDFA, IDFV and vendor scope

iOS exposes a second identifier that is frequently mistaken for a fallback. It is not one — its scope is deliberately incompatible with attribution.

The IDFV is genuinely useful for linking a user across your own portfolio without consent, and useless for the click-to-install join, because the publisher app computes a different IDFV for the same device. Anyone proposing the IDFV as an ATT workaround has not tested it across two vendors.

What you use instead

The honest framing is that no single replacement exists, because the IDFA's value was precisely that it was one identifier visible to everyone. What replaced it is a set of narrower signals, each valid in a different situation.

For deep linking specifically, the IDFA was never the important part. A deferred deep link can be matched from a first-party click identifier you control, and routing an installed app is handled by the link itself. The measurement question — which paid campaign deserves credit — is the part that genuinely lost its deterministic answer, and Apple's frameworks are the sanctioned substitute.

We do not ship fingerprinting to fill the gap. It violates Apple's rules regardless of ATT status, and the accuracy claims made for it are not verifiable by the advertiser paying for them — see why installs go unattributed for what we report instead.