Ticketfly says that a cyberattack has exposed information belonging to roughly 27 million accounts.
Information including names, email addresses, physical addresses, and phone numbers connected to the accounts has been compromised.
However, as some customers purchase tickets using multiple accounts, it is possible that the number of impacted customers is lower than 27 million.
One week ago, the Eventbrite-owned ticket sales platform took its website offline. The company said it believed it was the "target of a cyber incident," and this has turned out to be true.
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Last Thursday, Ticketfly customers attempting to purchase tickets through the platform were instead met with the main "V for Vendetta" character, which has become something of an icon for the hacktivist collective Anonymous.
The threat actors behind the attack also defaced the Ticketfly landing page.
According to sister site CNET, hacker IShAkDz took responsibility for the cyberattack and demanded 1 Bitcoin to resolve the attack at the time, which was worth $7500 at previous market rates.
A Ticketfly spokesperson told ZDNet that credit card and debit card information was not accessed.
Account passwords are also believed to be safe, but Ticketfly launched a forced reset over the weekend out of caution. The company says that it is "possible" the hashed values of password credentials may have been accessed.
Users of Ticketfly which have not logged into their accounts recently are asked to do so in order to follow through on password resets.
"Upon first learning about this incident we took swift action to secure the data of our clients and fans," the spokesperson said. "We take privacy and security very seriously and regret any disruption this has caused. We're extremely grateful for the patience and support of the Ticketfly community."
The main Ticketfly platform, Ticketfly Backdoor, and the "vast majority" of venue and promoter websites are back online.
ZDNet has reached out to Ticketfly with additional queries and will update if we hear back.