Fuzzy text, or Captcha, that is used to make text unrecognizable to computers (to prevent automated registrations) is now being used by spammers to prevent spam filters from recognizing images hidden in PDF files. An article from Register reports on a spam message that arrives as a distorted text image in PDF format.
Here's a quote from the article:
Neil Cook, European technology chief at anti-spam specialist Cloudmark, called it "a kind of Turing test for spam filters."
Indeed, the technique could force spam filters to include image recognition as an integral part of the system. PDF-based filtering itself poses a serious challenge to spam filters. Also, image-based spam has been gaining ground in recent times. Spam consumes a major chunk of network resources world-wide and now spam filter creators have to realize that any technique that targets computers can be potentially used against their filtering systems as well!
While techniques are figured out to conquer these new kind of attacks, users and enterprises must ensure that the latest updates are applied across spam filters at their organization.