In 2015, unknown hackers snuck malware onto thousands of apps on the iPhone App Store. At the time, researchers believed the hack had the potential to impact hundreds of millions of people, given that it affected around 4,000 apps, according to researcher estimates.

Apple refused to reveal how many people had been hit by the hack. This was presumably because it wanted to maintain the illusion that its App store was totally secure.

However, XCodeGhost looks like it was the largest hack against iPhones – at least that we know about.

But now, thanks to emails published as part of Apple's trial against Epic Games, we finally know how many iPhone users were impacted: 128 million in total, of which 18 million were in the US.

"In total, 128M customers have downloaded the 2500+ apps that were affected LTD. Those customers drove 203M downloads of the 2500+ affected apps LTD."

Dale Bagwell, who was Apple's manager of iTunes customer experience at the time, wrote in one of the emails. Another Apple employee wrote in the emails that "China represents 55 percent of customers and 66 percent of downloads.

'As you can see, a significant number (18 million customers) are affected in the US." The emails also show that Apple was scrambling to figure out the impact of the hack and working on notifying the victims.