Attackers have been exploiting an unpatched vulnerability in Microsoft Word for the past few months to compromise computers and infect them with malware.
The first report about the attacks came Friday from antivirus vendor McAfee after the company's researchers analyzed some suspicious Word files spotted a day earlier. It turned out that the files were exploiting a vulnerability that affects "all Microsoft Office versions, including the latest Office 2016 running on Windows 10."
The flaw is related to the Windows Object Linking and Embedding (OLE) feature in Microsoft Office that allows documents to embed references and links to other documents or objects, the McAfee researchers said in a blog post.
When the rogue documents used in this attack are opened, they reach out to an external server and download an HTA (HTML Application) file that contains malicious VBScript code. The HTA file is disguised as an RTF (Rich Text Format) document and is automatically executed.
"The successful exploit closes the bait Word document, and pops up a fake one to show the victim," the McAfee researchers said. "In the background, the malware has already been stealthily installed on the victimâs system."
By searching back through its data, McAfee has tracked down attacks exploiting this vulnerability to late January.
Following McAfee's report, security researchers from FireEye also confirmed that they've been aware of these attacks and exploit for several weeks and have coordinated disclosure with Microsoft.
According to FireEye, the malicious Word documents are sent as email attachments. The company hasn't provided examples of the malicious emails, but because this is a previously undisclosed, zero-day vulnerability, the attacks are likely targeted toward a limited number of victims.
Both McAfee and FireEye noted that the exploit can bypass most memory-based mitigations included in Windows. That's because the vulnerability is a logic bug rather than a programming error.
Microsoft is scheduled to release its monthly security updates on Tuesday, but it's not clear if a patch for this vulnerability will be included. The company did not immediately respond to a request for comment.
In the meantime, users should be wary of documents received from untrusted sources and should enable the Office Protected View mode because it can block this attack.