Logical Bug in Microsoft Word’s ‘Online Video’ Allows Code Execution

Microsoft Office is impacted by a logical bug that allows an attacker to abuse the “online video” feature in Word to execute malicious code, Cymulate security researchers warn.

The issue, which supposedly impacts all users of Office 2016 and older, can be exploited without special configuration, the security researchers say. Furthermore, no security warning is presented to the user when a malicious document abusing the flaw is opened. 

According to Cymulate, the vulnerability is created when the user uses the ‘online video’ feature to embed a video into their document. The bug resides in the associated document.xml file, which contains a parameter called embeddedHtml (under WebVideoPr) that refers to a YouTube iframe code. 

The issue, the researchers say, is that an attacker could replace the YouTube iframe code with malicious HTML/JavaScript code that would run in the background. Instead of linking to the actual YouTube video, the payload would open Internet Explorer Download Manager with the embedded code execution file.

The researchers also created a proof-of-concept code that contains the embedded executable as a blob of base64. Once executed, the code uses the msSaveOrOpenBlob method to trigger the download of the executable via Internet Explorer Download Manager, with the option to either run or save the file.

The bug was reported to Microsoft three months ago, Avihai Ben-Yossef, co-founder and CTO of Cymulate, told SecurityWeek via email. However, the Redmond-based software giant did not acknowledge it as a security vulnerability, he also revealed. 

“As this specific evasion could also be considered as a vulnerability, we’ve submitted this to Microsoft 3 months ago before we’ve implemented it in our platform. They didn’t acknowledge it as a flaw,” Ben-Yossef says. 

Attackers looking to abuse this flaw could hide executable code into Word documents and then abuse social engineering to trick users into opening those documents as part of phishing attacks. 

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