There has been a huge surge in the volume of PDF spam this week.
About 10 percent to 15 percent of all spam in one day arrived with PDF attachments, according to the Israeli security company Commtouch Software 's estimates.
"Given the fact that these messages are nearly four times bigger than standard spam messages, this increases overall global spam traffic by 30 percent to 40 percent," said Rebecca Herson, senior director of marketing.
So far, the outbreak has involved 14 billion to 21 billion PDFs and shows no signs of slowing, Herson said.
An analysis of the outbreak over a six-hour period showed it to be a truly global zombie-distributed spam attack, Herson said. About 24 percent of the spams were from the US, 14 percent were from Taiwan, and China and Russia accounted for 10 percent and 4 percent, respectively, she said. In all, PDF spam emails are being distributed by computers in 167 countries, she said.
According to Herson, the technique of sending messages as PDF attachments is relatively new and was first detected only a few weeks ago. The current outbreak shows that spammers have widely adopted the technique, she said.
"The popularity of the PDF format for legitimate business communications makes it difficult for traditional anti-spam solutions to block effectively without causing massive false positives," she said.
Spammers seem to be aware of this fact and don't even appear to be trying to disguise their messages, she said. Unlike image spam messages, which were relatively easy to detect, "these look like standard business letters until you look at the contents and see they are about organ enhancers and stock tips," she said.
The spike in PDF spam comes even as there are reports of a steady decline in image spam, which in January constituted more than half of all spam. Symantec, which publishes a monthly spam report, noted a continuing drop in image spam to just over 16 percent of all unwanted messages in May, compared with 27 percent in April and 37 percent in March.