Malwarebytes' Jerome Segura reports that emails with attachments that are not malicious executables but regular documents that have been exploited are still a big thing.
"Just a couple of days ago, we spotted a new wave of spam emails spewing malicious PDF files. The decoy, which purports to be an invoice, is directly attached to an email targeting small businesses," he shared. Similar emails pretending to deliver an Amazon invoice have also been spotted.
If the recipient is tricked into downloading and running the PDF file, the exploit it contains will do its thing if the user has a vulnerable version of Adobe Reader.
While the user is presented with a fake error message saying that the document could not be opened, the exploit performs a heap spray, downloads additional malware onto the computer (usually the Zeus banking Trojan and, lately, the Cryptolocker ransomware). These files are also made to look like PDF files.
Segura advocates using good protection software, which will detect and block the exploit from running.
"Here’s why exploit protection is also a better solution in this case: malicious documents typically have much lower detection rates than traditional malware binaries (notice how the bad guys didn’t even bother zipping the attachment)," he pointed out.