A hack on niche online dating service Cupid Media earlier this year has exposed names, e-mail addresses, and—most notably—plaintext passwords for 42 million accounts, according to a published report.

The cache of personal information was found on the same servers that housed tens of millions of records stolen in separate hacks on sites including Adobe, PR Newswire, and the National White Collar Crime Center, KrebsonSecurity journalist Brian Krebs reported Tuesday night. An official with Southport, Australia-based Cupid Media told Krebs that user credentials appeared to be connected to "suspicious activity" that was detected in January. Officials believed they had notified all affected users, but they are in the process of double-checking that all affected accounts have had their passwords reset in light of Krebs' discovery.

The compromise of 42 million passwords makes the episode one of the bigger passcode breaches on record. Adding to the magnitude is the revelation the data was in plaintext, instead of a cryptographically hashed format that requires an investment of time, skill, and computing power to crack. As Krebs noted:

The danger with such a large breach is that far too many people reuse the same passwords at multiple sites, meaning a compromise like this can give thieves instant access to tens of thousands of email inboxes and other sensitive sites tied to a user's email address. Indeed, Facebook has been mining the leaked Adobe data for information about any of its own users who might have reused their Adobe password and inadvertently exposed their Facebook accounts to hijacking as a result of the breach.

Making matters worse, many of the Cupid Media users are precisely the kinds of people who might be receptive to content frequently advertised in spam messages, including male enhancement products, services for singles, and diet pills.

The Cupid Media user records reviewed by Krebs contain the usual assortment of weak passwords. More than 1.9 million accounts were protected by 123456. Another 1.2 million used 111111. Users who used the same e-mail address and password to secure accounts on other sites are vulnerable to hijacking. Word of the Cupid Media compromise follows recent reports of password leaks from a host of other sites or companies, including Adobe (150 million reversibly encrypted passwords), MacRumors forums (860,000), and web software developer vBulletin (number not disclosed).

Ars has long advised readers to use a password manager that stores a long, randomly generated password that's unique for every important site. That way, when breaches hit a particular site, users aren't left scrambling to change credentials for other accounts that used the same password. For more background about password cracking, see Why passwords have never been weaker—and crackers have never been stronger. For a thorough tutorial on good password hygiene, see The secret to online safety: Lies, random characters, and a password manager.