At McAfee, we have put significant efforts in hunting attacks such as advanced persistent threats and “zero days.” Yesterday, we observed suspicious activities from some samples. After quick but in-depth research, this morning we have confirmed these samples are exploiting a vulnerability in Microsoft Windows and Office that is not yet patched.
This blog post serves as a heads-up for our customers and all Office users to protect against this zero-day attack.
The samples we have detected are organized as Word files (more specially, RTF files with “.doc” extension name). The exploit works on all Microsoft Office versions, including the latest Office 2016 running on Windows 10. The earliest attack we have seen dates to late January.
The exploit connects to a remote server (controlled by the attacker), downloads a file that contains HTML application content, and executes it as an .hta file. Because .hta is executable, the attacker gains full code execution on the victim’s machine. Thus, this is a logical bug, and gives the attackers the power to bypass any memory-based mitigations developed by Microsoft. The following is a part of the communications we captured:
The .hta content is disguised as a normal RTF file to evade security products, but we can find the malicious Visual Basic scripts in a later part of the file:
The successful exploit closes the bait Word document, and pops up a fake one to show the victim. In the background, the malware has already been stealthily installed on the victim’s system.
The root cause of the zero-day vulnerability is related to the Windows Object Linking and Embedding (OLE), an important feature of Office. (Check our Black Hat USA 2015 presentation, in which we examine the attack surface of this feature.)
We strongly suggest Office users take the following actions to protect or mitigate against this zero-day attack before Microsoft issues an official patch. We notified the Microsoft Security Response Center as soon as we found the suspicious samples, and we will continue to work with them to protect Office users.
Do not open any Office files obtained from untrusted locations.
According to our tests, this active attack cannot bypass the Office Protected View, so we suggest everyone ensure that Office Protected View is enabled.