Archive for July 18th, 2007

PDF Spam

I’m sure that many of you have received spam containing incomprehensible gibberish text and an image attachment containing “tips” for trading on the stock market. Often this tip is mildly obscured in an effort to prevent computers from reading the text and recognising it as spam…in much the same way that CAPTCHAs (those things where you have to enter a bunch of letters and numbers in a box when filling out a form) are used to prevent computers from automatically filling out forms.

Over time, the images in both the spam and the CAPTCHAs have gradually become more difficult to read, as the automated readers have become more sophisticated. The spammers have predictably needed to change their tactics as their message are now verging on unreadable, and today I came across the latest change in their tactics…the images are now being sent in PDF format.

Until now PDFs have almost always been used for legitimate documents, they have been a convenient way to preserve formatting and content…and they don’t generally get scanned for spam-like content. Unfortunately they will now need to be scanned in the same way images are, and unfortunately, scanning image content in PDFs is bound to take more processing power than merely scanning images…probably not much more processing power, but enough to slow down spam checking (and therefore mail delivery) by a notch or two.

On the plus side, the spammers still aren’t very good at sending convincing text with the spam (text which would make you think the attached document was important or from someone you know), so it is still relatively easy to see that the message can be ignored.


3 comments July 18th, 2007 at 10:52am


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