PDF Spam Adopts CAPTCHA Test Style Text to Fool Filters

It's only been a few weeks since I came across the first spam emails containing their "payload" within PDF files appeared. At SpamButcher, we reacted quickly to get the problem under control. However, spammers are already shifting their tactics in an effort to stay one step ahead of email filtering systems.

The Register is reporting that some PDF spam messages are using CAPTCHA style obfuscation techniques. That is, the text inside the PDF file is heavily distorted. While still generally readable by a human, it's virtually impossible for a computer program to decipher it. This makes the text effectively invisible to any application trying to eliminate such messages.

This would seem to be a preemptive strike by spammers as at this time I'm not aware of any spam utilities that are capable of performing OCR (optical character recognition) on PDF files. The good news is that our monitoring indicates that the changes aren't impacting SpamButcher's ability to detect and these emails.

It appears the spammers haven't caught on to certain details in their messages that make them readily detectable. We're obviously not going to publish that information here.

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