Apple has begun rejecting apps that access UDIDs – an alphanumeric string, unique to each Apple device, which can be used by developers and ad networks to identify handsets and gauge usage.

According to TechCrunch, app submissions – including one from an unnamed “publicly-traded, multibillion dollar company” – have already started to be declined for using UDIDs.

Mobile ad networks use UDIDs to track users across multiple apps, to better target ads at individual consumers. So, whats the alternative?

Mike Dewhirst, CTO at StrikeAd – which already uses a UDID-less solution via HTML5 and cookies – embraces the decision. “This change will hit some mobile ad networks hard, as they use UDIDs to make their advertising better targeted, tracking consumers to learn how they respond to different ads and which apps they use most often,” says Dewhirst. “The UDID-less approach has always provided better results for our CPA campaigns, delivering more precise attribution data and allowing us to buy across mobile web and in-app, whilst many other media buyers who relied on the UDID could only run on in-app traffic for CPD campaigns.”

UDIDs cant be cleared or deleted by the user, as web cookies can, and previously didnt require any consent to be leveraged by developers. Apple indicated the change six months ago, with iOS documentation that it was going to deprecate UDIDs, but – amid pressure from the media and US Congress over the issue – this move against UDIDs has come quicker than anticipated.