Image Spam on the Rise
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Normal spam is bad enough, but spammers have come up with a trick that lets them get around many older spam filters. Itís called image spam. It has spiked over the past year. What is it, and what can we do about it?
When I first started spotting stock spam in my email inbox, I didn't think much of it - until the volume started increasing. I'd see this screen that displayed apparently normal text for a second, then flash over to what looked like a formatted HTML message. My spam filters weren't catching these messages, no matter how many of them I threw in the junk box after the fact. What was going on?
It turns out that I wasn't alone, and what I took to be a formatted HTML message was in fact an image. It's the latest weapon in the modern spammer's arsenal for getting around spam filters. It's also painfully on the rise.
Image spam has been around for about four years, but it didn't start taking up serious space in inboxes until last year. Postini, a messaging management company, notes that image spam was even on the decline in 2005, from 12 percent of all spam at the beginning of the year to five percent of all spam in November 2005. December 2005, however, saw the beginning of a sharp spike (I'll explain one of the possible reasons for that spike in the next section, when I explain why image spam works so well).
While Postini places that spike at 25 percent of all spam, IT security company Sophos estimated that image spam made up about 18 percent of all spam at the beginning of 2006. Sophos thinks the problem getting much worse than Postini estimates, however. According to Carole Theriault, a senior consultant at Sophos, image spam now makes up 40 percent of all spam. "That's a big increase," Theriault notes, with a gift for understatement.
The IronPort Threat Operation Center noted that global spam has nearly doubled in the past year. In October 2005, spammers sent 31 billion pieces of unsolicited bulk email every day. As of mid-November 2006, that number had reached 61 billion. Whether we're looking at 25 percent or 40 percent of that many messages, that's an awful lot of bandwidth (an issue which I'll talk more about when I discuss the special problems involved in dealing with image spam).
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