Live Nation Entertainment said on Friday it was investigating a data breach at its Ticketmaster unit that it discovered on May 20, the latest in a string of high-profile corporate hacks in the past year.

In a filing with the U.S. Securities and Exchange Commission, Live Nation said it found “unauthorized activity” in a third-party cloud database that mainly contained Ticketmaster data, and was working with forensic investigators.

Last week, a hacking group named ShinyHunters said it had stolen user data of over 500 million Ticketmaster customers, according to various media reports.

Some media reports quote the group as saying it’s in possession of names, addresses, phone numbers — and in some cases, partial credit card information — of Ticketmaster users around the world.