An unpatched critical vulnerability in Microsoft Office is being exploited by malicious actors to achieve full code execution on target machines, McAfee and FireEye security researchers warn.
The vulnerability resides in the Object Linking and Embedding (OLE) functionality in Office and can be abused to create malicious RTF (Rich Text Format) documents that link to HTA (HTML Application) files hosted on remote servers. These HTA files load and execute a final malicious Visual Basic script.
“Because .hta is executable, the attacker gains full code execution on the victim’s machine,” McAfee explains, adding that the malicious RTF samples they observed were using the .doc extension.
Both McAfee and FireEye explain that this logical bug allows attackers to bypass memory-based mitigations developed by Microsoft, as well as other security products. The malicious documents are used to download and execute malicious payloads pertaining to various well-known malware families.
The HTA files used in the observed attacks were masquerading as normal RTF files to trick users and evade detection. When successful, the exploit closes the original Office document, then opens a new one and displays it to the victim, while the malicious code is being installed in the background.
“In both observed documents the malicious script terminated the winword.exe process, downloaded additional payload(s), and loaded a decoy document for the user to see. The original winword.exe process is terminated in order to hide a user prompt generated by the OLE2link,” FireEye explains.
The vulnerability was initially observed in January, but attacks that leverage it continue to surface, McAfee says. The security company said that all Office versions are affected by this issue, including Office 2016 on Windows 10.
According to FireEye, they too have been aware of the vulnerability for some time, but they have been coordinating with Microsoft for several weeks to release information on the matter only after a patch was available. Microsoft’s next set of security patches is scheduled to roll-out as soon as this Tuesday.
Users are advised to avoid opening Office files that come from unknown sources and to leave Office Protected View enabled to ensure no malicious code runs without their knowledge. Apparently, the vulnerability can’t bypass Protected View.