As spam filters get increasingly effective, spammers are changing their tactics to foil anti-spam software and get through to your inbox. Recently, this has involved a shift from the use of text-based spam to the use of embedded images and PDF file attachments as the preffered delivery method for their spammy intrusions.
The prevalence of this form of spam increased in 2006, primarily as a means for advertising penny stocks. It involved the use of a picture or graphic embedded in the body of the junk email. The junk email’s message is displayed as an image.
Because most anti-spam filters are text-based, image spam was relatively successful. This led to its use in advertising everything from sexual enhancement to fake pharmaceuticals.
One serious effect of image spam has had is to further clog up Internet bandwidth, and drive up costs to businesses. This is because the average size of each junk email almost doubled. In fact, this increased size and the sheer volume of image spam forced many businesses to block all emails that contained embedded or attached images.
By early 2007, image spam reached an all-time high, accounting for almost two-thirds of all junk email. However, as spam filter technology has adapted to detect image spam, its use has since declined to less than 15% of all junk email. Instead, spammers are turning to PDF spam
Spammers are increasingly using PDF files to bear their spam messages. The practice begun in mid-2007, primarily as a scam to fool recipients into investing their money in the stock of a particular company.
With this type of spam, the junk email is sent out with a PDF file attachment, which most anti-spam filters cannot or do not read. These attachments range from rudimentary to professional-looking documents. The text in the body of the email is usually nonsensical gobbledygook that the spam-filter does not recognize as junk mail.
For the spammer, the use of PDF files is advantageous because PDF files are so commonly used in the business world. In fact, several companies allow or even require their business email systems to deliver these documents to the recipient. This makes it very likely that this PDF spam will reach the user’s inbox.
The use of junk mail with PDF attachments takes up even more Internet bandwidth. This is because PDF files are generally much larger than the embedded pictures and graphics used in image spam. Image spam is typically in GIF format; PDF files are upto 3 times the size of these files.
The upside to the use of image and PDF spam is that so far, there is no hard evidence that either one can be used to embed malicious software on the recipient’s computer. The only harm is done to those who do what the message says. Spammers have also begun to experiment with attachments in different file types such as excel and zip files.
The advent and decline of the different types of spam attest to the cat-and-mouse game that goes on between the spammers and the security experts. As anti-spam technology catches up to their techniques, they continue to innovate and change tactics to deliver their spam messages.