The security bug allows a malicious actor to craft a RTF (Rich Text Format) document that would download and execute a Visual Basic script containing PowerShell commands. Microsoft has already addressed the flaw, but not quick enough to prevent malware such as the Dridex banking Trojan from abusing it in attacks.

The exploit leverages Office’s Object Linking and Embedding (OLE) functionality to link to an HTA (HTML Application) file hosted on a remote server. When the user opens the RTF document received via spam email, winword.exe issues a HTTP request to retrieve the malicious HTA file, which loads and executes the malicious Visual Basic script. In turn, the script downloads and executes malware.

According to FireEye, the malicious scripts used in these incidents were also observed terminating the winword.exe processes (to hide a prompt from OLE2link) and loading decoy documents.

Another attack abusing this vulnerability consisted of two malicious stages, and distributed a variant of the dropper known as WingBird (which has similar characteristics as FinFisher). Heavily obfuscated, the malware packs several anti-analysis measures, including a custom VM to slow analysis, and was recently associated with the activities of a threat group known as NEODYMIUM.

Office users are advised to apply the newly released patches as soon as possible, to ensure they are protected from these attacks.