Microsoft today patched a critcial zero-day vulnerability in Microsoft Office/WordPad that attackers had been exploiting in the wild for months.

CVE-2017-0199 is a remote code execution vulnerability in the Windows Object Linking and Embedding (OLE) application programming interface. The vulnerability already had been weaponized in attacks to distribute the Dridex banking Trojan, as a botnet payload, and in a cyber espionage campaign.

Security firm McAfee published a report on April 7 to warn users of an exploit that has been used to target users since late January. SophosLabs claims attacks actually date back to November 2016, with most occuring between March and April 2017.

There are a few reasons this bug is especially dangerous: it affects most, or all, Windows versions of Microsoft Word, and targets don't have to enable macros for exploits to be effective, explains Bryan Burns, vice president of threat research and engineering at Proofpoint.

Burns says it's a logic bug, which is harder to defend against, he says. Victims are more likely to fall for these attacks because they aren't prompted to enable macros -- something businesses teach their employees to avoid. Instead, the attacks use a dialogue box.

"They've probably been trained over the last several years not to enable macros," says Burns. "This is a different vector. Users haven't seen a dialogue box. They haven't been trained not to click on it."

The attackers attach emails containing Microsoft Word RTF (Rich Text Format) documents. Subject lines read "Scan Data" and attachments were named "Scan_123456.doc" or "Scan_123456.pdf" with "123456" replaced with random numbers, Proofpoint reports.

When launched, the exploit connects to a remote server, downloads a file containing HTML application content, and executes it as a .hta file, McAfee explains. Because .hta is executable, the attacker gains full code execution on the machine. After the malware has been installed, the exploit closes the bait Word document and displays a new one to show the victim.

The exploit uses an embedded OLE2link object in specially created documents.

"Everything is working as designed," says Burns of how the attack deceives targets. "But the way it's designed, it left a corner open so code can run when you wouldn't expect it to be running."

In the case of Dridex, attackers obtained full system control to launch the banking Trojan malware. This enabled them to grab businesses' financial credentials and intercept transactions to steal money. "In this case, it was financially motivated," says Burns of the attackers' goals. "They were trying to infect with this malware to empty out [organizations'] bank accounts."

But Burns warns that this Office flaw would allow for any type of malware to be installed. Ransomware is one example Proofpoint sees frequently distributed.

"We would expect any threat actor who is trying to attack businesses to try this technique," he cautions.

Several security firms aside from Proofpoint observed this vulnerability in the wild ahead of Microsoft's patch. Netskope's Threat Research Labs linked this zero-day to the Godzilla botnet loader. Researchers saw IPs released to the botnet loader serving payloads related to exploits for the vulnerability using malicious Word documents.

In a different scenario, FireEye spotted this flaw being used in a cyber espionage campaign targeting Russian-speaking victims since January 2017 and installing FINSPY, a tool previously associated with the "lawful intercept" company Gamma Group. FireEye in another case saw exploits installing Latentbot, a malware family used by cybercriminals for financial gain.

Now that Microsoft has issued a fix, businesses should protect themselves by patching as quickly as possible, Burns says. He also cautions organizations to be wary of these types of attacks, as email as the "dominant threat vector" hackers will use to infect businesses.

Meanwhile, today is the last day of extended support for Windows Vista. The OS will no longer receive updates or support from Microsoft. 

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