An unusual attack technique has enabled spammers to distribute more than 1 million messages an hour using legitimate, free file storage services available on the Internet, security researchers said this week.

Researchers at AppRiver, a security services provider, say they have spotted spam messages being distributed at a rate of more than 1 million per hour for the past four days via free file storage sites. This attack accounted for as much as 5 to 6 percent of all spam on the Web this week, they say.

"This is not the first time we've seen this approach, but we've never seen it in this type of volume," says Troy Gill, security analyst at AppRiver.

In a blogpublished earlier this week, AppRiver describes the unusual approach. In a nutshell, spammers have created an automated method for creating accounts in popular free file storage services -- including groups.yahoo.com, groups.google.com, and livejournal.com -- and are using those accounts to host their spam content. The use of these sites makes the spam appear to be legitimate, enabling it to bypass commonly used blacklists.

In addition, the spammers have used the same account-creation capability to set up hundreds of Hotmail accounts, which they use to distribute their spam messages, Gill says.

The attacks from the various free file storage services have been going strong for most of the week, but traffic from groups.google.com and livejournal.com appears to have tailed off this afternoon, Gill says. Spam traffic coming out of groups.yahoo.com is still flowing at a high rate, he says.

"I don't see it slowing down until the spammers get tired of it or until Yahoo does something about it on their end," Gill says.

AppRiver has tweaked its own defenses to detect the new spam attack and block it, Gill says, but traditional spam tools that blacklist IP addresses or domains will not block the new attacks.

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