Microsoft has finally patched a zero-day vulnerability in Microsoft Word that has been exploited by cybercriminals for months. Recently, the vulnerability has been exploited by the gang behind the Dridex banking Trojan.
The remote code execution vulnerability (CVE-2017-0199) affects the Windows Object Linking and Embedding (OLE) application programming interface. The vulnerability is a logic flaw rather than a programming error, which makes defending against attacks difficult.
The bug affects RTF files. The spam email campaigns use RTF files containing an embedded OLE2Link object, which downloads an HTA (HTML Application) file containing malicious code when the document is opened. No user interaction other than opening the file is required to infect the end user’s device.
There is some debate as to how long the vulnerability has been actively exploited in the wild. Attacks may have been occurring as early as November 2016 according to SophosLabs, although certainly since the start of 2017. Over the past two months, the vulnerability has been extensively exploited to deliver the Dridex banking Trojan.
The zero-day vulnerability in Microsoft Word has been exploited for espionage purposes in Russian speaking countries, while FireEye observed the exploit being used to distribute Latentbot malware. Latentbot is an information stealer with the ability to corrupt hard drives.
Many security companies have been tracking the vulnerability, although it was McAfee that announced the existence of the actively exploited flaw on Friday last week. The flaw exists in virtually all Microsoft Word versions and does not require macros to be enabled in order for malicious code to run.
Employees are advised never to enable macros on documents unless they are 100% certain that a document is legitimate; however, this zero-day exploit does not rely on macros. Simply opening the Word document on an unpatched Office installation is likely to result in infection.
This makes the vulnerability particularly dangerous. Any end user that opens a specially crafted Word document would automatically run the code which would see the Dridex Trojan (or another malware) downloaded. One protection that can prevent the malicious code from running is to enable Protected View mode. However, the code will run when Protected View is turned off.
The malicious emails sent out in at least one campaign have the subject line “scanned data” with the RFT file attachments containing the word ‘scan’ followed by a random string of numbers, according to Proofpoint.
To protect against this exploit, the patches for both Office and Windows that were released by Microsoft on Tuesday April 11, 2017 should be applied. However, in order to apply the security update, Service Pack 2 for Office 2010 must be installed.
If it is not possible to apply the Microsoft updates immediately, you can configure your spam filter to block RTF files or add RTF files to the list of documents to block in the Microsoft Office Trust Center.