HP's cybersecurity folks have uncovered an email campaign that ticks all the boxes: messages with a PDF attached that embeds a Word document that upon opening infects the victim's Windows PC with malware by exploiting a four-year-old code-execution vulnerability in Microsoft Office.
Booby-trapping a PDF with a malicious Word document goes against the norm of the past 10 years, according to the HP Wolf Security researchers. For a decade, miscreants have preferred Office file formats, such as Word and Excel, to deliver malicious code rather than PDFs, as users are more used to getting and opening .docx and .xlsx files. About 45 percent of malware stopped by HP's threat intelligence team in the first quarter of the year leveraged Office formats.
"The reasons are clear: users are familiar with these file types, the applications used to open them are ubiquitous, and they are suited to social engineering lures," Patrick Schläpfer, malware analyst at HP, explained in a write-up, adding that in this latest campaign, "the malware arrived in a PDF document – a format attackers less commonly use to infect PCs."