Invaders at the Gate, and Slipping Past It

A previously little-seen type of spam, containing images that foil many filters, now makes up roughly a third of all spam handled by the e-mail monitoring company Postini, which processes some 1.3 billion messages a day.

So-called image spam consists of a digital image, or a constellation of images, generally depicting a page of text with a spamlike message — exhorting recipients to buy a stock, for example. Since the message is conveyed in an image, it eludes spam filters that are designed to scan only a message’s text.

Some filters try to compensate by scanning e-mailed pictures for images of text. But the process is slow, and spammers make this difficult by lacing the messages with eccentricities like colored letters or unseen images that swirl in the background.

A side effect has been to drive up sharply the size, in bytes, of the spam that e-mail providers must handle. The volume of spam filtered by Postini has more than doubled since October 2005, rising 128 percent. But the size of that spam has risen 227 percent.

Source : New York Times