Towards the end of 2006, we have seen the dramatic rise of image spam as spammers continuously attempt to bypass spam-filtering tools. This forced antispam appliances to improve their detection capabilities, which in turn decreased the number of image-based spam in the recent months. This did not stop spammers from improving their tactics as well though.
What came after is PDF-based spam, which proved effective for spammers because this format is much exhaustive to filter out and traditionally, PDF files have rarely been associated with spam and malware, so very few have been examining PDF files. Of course, PDF also uses text and it was not long before spam filters figured out a way to easily detect them. So spammers thought, “Why not use both PDF and JPEG to send out spam?” Lo and behold, a JPEG-embedded-in-PDF spam emerged. This spells double-trouble for users because this kind of spam can effectively bypass most antispam appliances, which usually have no ability to handle the processing power needed to decode images, much less those encoded inside a PDF file. Trend Micro, however, addresses this problem through its Spam Prevention Solution, which includes image spam detection technology.
Based on a report received by TrendLabs, the said spam comes from fake recipients, which most likely indicates that they are distributed by bot machines. They do not carry malicious payloads though, but we have yet to see the PDF + JPEG + MALWARE combo being exploited by malware authors in the near future. Rest assured, Trend Micro continuously provides cutting-edge approaches to protect users and organizations as these kinds threats evolve.