HACKERS love to be party-poopers, as Apple has just discovered. A group dubbed “AntiSec”, which is affiliated with Anonymous, a prominent group of hackers, has leaked a file containing over 1m unique device identifiers (UDIDs) from the tech giant. UDIDs are sequences of letters and numbers assigned to specific iPhones, iPads other gizmos made by the company. AntiSec claims to have purloined over 12m of Apple’s during a hacking attack it says it staged in March.

Quite where it got the identifiers from remains something of a mystery. The hackers say they penetrated the laptop of an agent of America’s Federal Bureau of Investigation (FBI) and discovered the UDIDs in a file on it. But the FBI has publicly pooh-poohed this claim in turn, saying it never had the identifiers and that reports that one of its laptops has been compromised are “totally false”.

Some IT specialists who have checked the data released by AntiSec say it is genuine. The hackers have claimed that, in some cases, personally identifiable data, such as people’s names and mobile-phone numbers, were linked to some of the UDIDs it purloined. This is worrying. Aldo Cortesi and other security researchers have been giving warning for some time that hackers who get hold of UDIDs and personal information associated with them could use the data to create fake user accounts on social-media sites, say. Mr Cortesi has called the AntiSec leak a “privacy catastrophe”.

AntiSec's move seems to have been timed to embarrass Apple, which is due to launch its latest iPhone and other iGadgets at an event in San Francisco on September 12th. However, the company may not be the source of the stolen data. Plenty of app developers and online-advertising networks also collect UDIDs as part of their activities. So it is perfectly possible that the hackers managed to nick files from one or more of these outfits—or that the FBI collected data from them, only to have it swiped by AntiSec’s crafty programmers. (The latter scenario raises the question of why the FBI might be gathering UDIDs of Apple devices in the first place.)