Hosting provider Namecheap said Monday hackers compromised some of its users' accounts, likely using a recently disclosed list of 1.2 billion usernames and passwords compiled by Russian hackers.
The "vast majority" of login attempts have failed, wrote Matt Russell, vice president of hosting, on a company blog.
The attackers are trying brute-force attempts to gain control of accounts, which involves repeatedly trying different usernames and passwords until the right combination grants access.
"As a precaution, we are aggressively blocking the IP addresses that appear to be logging in with the stolen password data... as well as making this data available to law enforcement," Russell wrote.
Namecheap suspects the attackers are using a list of credentials publicized last month by Hold Security, a Milwaukee-based security company that tracks stolen data on underground cybercriminal forums.
It wasn't immediately clear why Namecheap suspects that list is being used. Company officials did not have an immediate comment.
Hold Security founder Alex Holden said via email that he did "not see any data supporting Namecheap's claim." Brute-force attacks are commonly used to compromise accounts, he added.
Russell wrote the attackers are using software that emulates the Chrome, Firefox and Safari Web browsers to simulate a real login attempt. Such a method might help avoid security defenses that could detect, for example, repeated fast guesses.
Namecheap will issue new login credentials to accounts that have been frozen. People who used the same password for their Namecheap account and others may be vulnerable, Russell wrote.
All Namecheap customers are advised to change their passwords or enable two-factor authentication, a strong defense that involves entering a one-time passcode in order to log in.
Data breaches at websites are often a source for usernames and passwords, and hackers have long been collecting lists of credentials that they hope will unlock other Web services. Security experts advise people to not reuse passwords for this reason.
The findings from Hold Security, which also uncovered the data breaches at Adobe Systems and the retailer Target, was notable because the 1.2 billion list of unique credentials was so large. All told, the gang possesses 4.5 billion records, but may of those are duplicates.
The company spent seven months researching the gang that collected the list, which it nicknamed "CyberVor." "Vor" means thief in Russian.
It wasn't clear what the hackers who collected the credentials cache plan to do with it, although the list would have value in underground circles.
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