No sooner had PDF started to emerge as the format of choice for spammers than it has started to mutate to become a little harder to detect.
Security companies are warning that not only is PDF spam on the increase, but that spammers have already started to use related formats to deliver their pitches.
Symantec recently reported that while image spam went down to 15% of spam in July, PDF spam had emerged to constitute about 2% to 8% of spam.
And McAfee is now reporting that spammers have started using the .FDF format, a subset of PDF dealing exporting form data, as spam attachments, maybe to bypass filters based on file extension.
PDF spam is effective because files in PDF format have long been considered an acceptable way for businesses to transfer information, said Andrew Klein, senior product manager at SonicWALL.