Spammers love free hosting, so it's no surprise that they continue to hammer free online storage and file-sharing services -- these sites are attractive to spammers because they have legitimate URLs and are typically immune to blacklisting.
Microsoft's Windows Live SkyDrive Beta was the latest such service to be victimized by spammers. Earlier this week, McAfee Avert Labs captured thousands of spam messages that contained links to SkyDrive. The links redirected you to an online pharmacy service. McAfee estimates that the spammers uploaded tens of thousands of files to the site.
Spammers are always looking for cheap and easy ways to distribute their unwanted traffic. "Any free Web host will be vulnerable to this in theory," says Richi Jennings, an independent consultant who specializes in spam and email technology. "Blogspot, for example. And spammers have used Geocities for years. I've seen them use Google's Picasa free hosting, too."
Jennings says spammers use these sites as a redirector to their own Websites. "They're just using it as a redirector to wherever their 'spamvertised' Website is at the moment," Jennings says. "Because that might change if their host kicks them off."
Botnets are a popular means of spewing spam, he says, but even with fast-flux Domain Name Services (DNS), bot availability isn't always a given since client machines are often turned on and off. (See On the Trail of 'Fast Flux' Botnets and Attackers Hide in Fast Flux.) "So spammers keep coming back to this idea of [abusing] free Web hosting," he says. "There is little oversight by 'real' humans, so the time taken to remove abusive pages is long," and they send their spam runs out efficiently, he says.
SkyDrive, for instance, is susceptible because it doesn't do a malware scan, and it's also still in beta, so it's still a bit of a work in progress, according to McAfee researchers. Fee-based services such as Amazon's S3 and Nirvanix are less attractive to spammers because they aren't free. But freebies like SkyDrive, Rapidshare.com, and Filefactory.com have been abused by spammers, says Dave Marcus, security research and communications manager for McAfee's Avert Labs.
Marcus adds that online storage and file-sharing sites that encrypt files make it tough for filters to scan them for spam.
The good news is that the SkyDrive spam run stopped early Tuesday morning, either because it had drawn unwanted attention, or because Microsoft had shut it down, Marcus says. Microsoft had not responded to press inquires at the time of this posting.
The spam payload was a "welcome" HTML file with a redirector that takes the victim to an online pharmacy. McAfee researchers have also seen this redirect technique being used on Blogspot spam blogs (sblogs).
Online storage and file-sharing services should scan any uploads and apply strong spam content filtering to "what's being pushed out," Marcus says.
The latest spam run wasn't the first for SkyDrive. McAfee researchers had previously detected some small-scale spam runs last fall. "Another interesting point is the number of times we trapped each url was interestingly low for such a big campaign, Id therefore estimate they had tens of thousands of files uploaded," a McAfee researcher wrote in the Avert Labs blog. "Im sure it wont be too long before its used to host other unwelcome content types Id like to see more of these online file storage offerings malware scanning downloads too." [sic]
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