In the span of about 24 hours, three major websites have requested that their users change their passwords following apparent heists of millions of credentials.

　　Music website Last.fm is the latest to fall prey. On Thursday, the U.K.-based company released a statement announcing it was looking into the matter.

　　“We are currently investigating the leak of some Last.fm user passwords,” the statement read. “As a precautionary measure, we're asking all our users to change their passwords immediately.”

　　The news comes on the heels of the leak of some 6.5 million passwords belonging to users of LinkedIn, in addition to possibly another 1.5 million passwords connected to members of dating website eHarmony. The passwords, which were protected by an easily crackable encryption format, were posted to a Russian forum by a hacker who was seeking assistance in decoding them.

　　Although there is no evidence that directly ties the three attacks, Chester Wisniewski, senior security adviser at SophosLabs, said the timing may imply that they are.

　　“There's no evidence so far, but it certainly feels like it,” Wisniewski said. “That's just a gut feeling.”

　　While the LinkedIn data was cloaked using a cryptographic hash function, SHA-1, it can still be decoded because additional encryption methods such as salting, which adds a sequence of symbols to passwords before they're hashed, were not implemented. Meanwhile, eHarmony's passwords were disguised using MD-5, a cryptographic hash function that has been known for years to be vulnerable.

　　“They shouldn't even have bothered using it,” Wisniewski told SCMagazine.com Thursday. “It's almost as bad as storing them in plain text.”

　　Users with profiles or accounts on these websites should replace their passwords with more “complex” characters even if they weren't compromised, Lance James, director of intelligence at Vigilant, a company that offers security monitoring solutions, said in an email to SCMagazine.com.

　　“As an example, use a sentence that can be memorized and add symbols and numbers that substitute certain letters,” he said.

　　In all three cases, it is unclear how the attackers stole the data. Security experts have offered up a number of possibilities of how the thieves may have gotten to the passwords, including through a defective web application or as a trusted insider.

　　A spokesperson for Last.fm did not respond to a request for comment.

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