- Feb 4, 2016
Threat analysts have discovered a recent malware distribution campaign using PDF attachments to smuggle malicious Word documents that infect users with malware.
The choice of PDFs is unusual, as most malicious emails today arrive with DOCX or XLS attachments laced with malware-loading macro code.
However, as people become more educated about opening malicious Microsoft Office attachments, threat actors switch to other methods to deploy malicious macros and evade detection.
In a new report by HP Wolf Security, researchers illustrate how PDFs are being used as a transport for documents with malicious macros that download and install information-stealing malware on victim's machines.
Embedding Word in PDFs
In a campaign seen by HP Wolf Security, the PDF arriving via email is named "Remittance Invoice," and our guess is that the email body contains vague promises of payment to the recipient.
When the PDF is opened, Adobe Reader prompts the user to open a DOCX file contained inside, which is already unusual and might confuse the victim.
Because the threat actors named the embedded document "has been verified," the Open File prompt below states, "The file 'has been verified." This message could trick recipients into believing that Adobe verified the file as legitimate and that the file is safe to open.