Koobface is designed to spread itself by checking to see if person is logged into a social network including Facebook and MySpace. It will then post fraudulent messages on the person's profile trying to entice friends to click the link, which then leads to a malicious website that tries to infect the PC.
Twitter is the latest site to be targeted by a Koobface variant, said Rik Ferguson, senior security advisor for Trend Micro. Other sites have included Bebo, Hi5, Friendster and LiveJournal, says to the US Computer Emergency Readiness Team.
"Koobface has a long, inglorious history and has been relatively successful at infecting machines," Ferguson said.
At least a couple hundred accounts have been infected by Koobface's latest efforts, according to Ryan Flores, an advanced threats researcher, writing on Trend's blog. When it made its first appearance a couple of weeks ago on Twitter, Koobface was just sending out three shortened URLs leading to malware. Flores wrote that Koobface is sending out more bad links this time around.
The use of URL shortening services on Twitter have made it difficult for people to tell what website they'll end up at, Ferguson said. However, Twitter tools such as TweetDeck will show the full URL, which can help make people make a better security judgement, he said.
Some of Koobface's bad links have advertised, for example, videos of Michael Jackson, where the malware writers are trying to pique people's interest in current news events, said Graham Cluley, senior technology consultant for Sophos.
If a person followed the link, it would lead to a website asking the user to download an upgrade for their Flash multimedia players but is actually Koobface, he said.
But Twitter has been fairly quick at shutting down accounts of people who are infected with Koobface and resetting their passwords, Cluley said.
Malware has also spread on Twitter via fake accounts that have been registered using automated tools. Ferguson said Twitter could somewhat guard against that by sending a verification link to an email address during registration, making it more difficult to register dummy accounts en masse.
"That's real low-hanging fruit for them to address," Ferguson said.
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See also: Koobface virus spreads to Bebo