Image Spam on the Rise - Blocking Image Spam
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Fortunately, when it comes to catching image spam, all is not lost. Paul Bacca, a spam and virus researcher at Sophos, notes that "We see a lot of image spam and we know which computers are sending it," so they simply block mail from those computers. "We think we catch about 80% of image spam using these conventional techniques."
Other techniques take advantage of the fact that these are, after all, images, and many of them are scanned into a computer. That means they contain information connected to the scanner that was used, such as the number of colors or pixels it uses. Adjust the spam filter to look for those colors and numbers, and suddenly you have a new metric for deciding whether or not a message is spam.
A third technique acknowledges that, image or not, these emails contain text (in the sense of something that humans can read) - and tries to find ways for computers to recognize that text. This means using optical character recognition (OCR) techniques that can extract the text from the image. Once the text is extracted, conventional text filtering techniques can be applied to it. Sadly, that level of OCR is said to be a long way off. "You're looking at technology that is anything from 10 to 30 years away," notes Luis von Ahn of Carnegie Mellon University, one of the developers of the CAPTCHA technique.
Spam filters can also detect and block image spam by examining certain attributes of the sending computer, message envelope and headers. As a general point, Scott Petry, Postini's founder and CTO, recommends that companies pay attention to the volume of incoming messages with image attachments. If a significant portion of these messages are getting through, the IT department may want to restrict their delivery because image spam is a potentially huge bandwidth problem. "You don't want those messages to undermine the availability of data in your enterprise," Petry explained. "It might mean some grumpy users, but at least the mail server will remain up and running." In short, to combat image spam, you will need to use filters that focus on both the content and the origin of the message.
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