You might not have thought of – much less visited – Myspace in years. (Yes, it’s still around. Time, Inc. acquired it and other properties when it bought Viant earlier this year.) But user data never really dies, unfortunately. For Myspace’s new owner, that’s bad news, as the company confirmed just ahead of the Memorial Day holiday weekend in the U.S., that it has been alerted to a large set of stolen Myspace username and password combinations being made available for sale in an online hacker forum.

The data is several years old, however. It appears to be limited to a portion of the overall user base from the old Myspace platform prior to June 11, 2013, at which point the site was relaunched with added security.

Time, Inc. didn’t confirm how many user accounts were included in this data set, but a report from LeakedSource.com says that there are over 360 million accounts involved. Each record contains an email address, a password, and in some cases, a second password. As some accounts have multiple passwords, that means there are over 427 million total passwords available for sale.

Despite the fact that this data breach dates back several years, the size of the data set in question makes it notable. Security researchers at Sophos say that this could be the largest data breach of all time, easily topping the whopping 117 million LinkedIn emails and passwords that recently surfaced online from a 2012 hack.

That estimation seems to hold up – while there are a number of other large-scale data breaches, even some of the biggest were not of this size. The U.S. voter database breach included 191 million records, Anthem’s was 80 million, eBay was 145 million, Target was 70 million, Experian 200 million, Heartland 130 million, and so on.

The issue with these older data breaches is that they’re from an era where security measures were not as strong as today. That means these passwords are easily cracked. LeakedSource notes that the top 50 passwords from those cracked account for over 6 million passwords – or 1.5 percent of the total, to give you a sense of scale.

The passwords were stored as unsalted SHA-1 hashes, as LinkedIn’s were, too.

That allowed Time, Inc. to date the data breach to some extent, as the site was relaunched in June 2013 with strengthened account security, including double-salted hashes to store passwords. It also confirmed that the breach has no effect on any of its other systems, subscriber information, or other media properties, nor did the leaked data include any financial information.

Myspace is notifying users and has already invalidated the passwords of known affected accounts.

The company is also using automated tools to attempt to identify and block any suspicious activity that might occur on Myspace accounts, it says.

“We take the security and privacy of customer data and information extremely seriously—especially in an age when malicious hackers are increasingly sophisticated and breaches across all industries have become all too common,” said Myspace’s CFO Jeff Bairstow, in a statement. “Our information security and privacy teams are doing everything we can to support the Myspace team.”

However, while the hack itself and the resulting data set may be old, there could still be repercussions. Because so many online users simply reuse their same passwords on multiple sites, a hacker who is able to associate a given username or email with a password could crack users’ current accounts on other sites.

Of course, it’s not likely users even remember what password they used on Myspace years ago, which makes protecting your current accounts more difficult. A better option is to always use more complicated passwords, reset them periodically, and take advantage of password management tools like Dashlane or LastPass to help you keep track of your logins.

Myspace also confirmed that the hack is being attributed to the Russian cyberhacker who goes by the name “Peace.” This is the same person responsible for the LinkedIn and Tumblr attacks, too. In Tumblr’s case, some 65 million plus accounts were affected. But these passwords were “salted,” meaning they are harder to crack.

Myspace is working with law enforcement as this case is still under investigation, the company says.