Unpatched Critical Bug in Microsoft Word Online Video Feature Allow Attacker to Deliver Powerful Malware

An unpatched bug that abusing Microsoft word Online Video future that allow an attacker to deliver malicious files into the victim’s system.

A bug that existing in the JavaScript code execution within the office-embedded video component leads attackers to execute the malicious code.

This flaw affected Office 2016 and older versions and it will not produce any security warning while victims opening the document.

Researchers built a Proof-of-concept for this attack using youtube video link with word document and demonstrate the infection process.

This flaw allows let an attacker execute the powerful malware or ransomware also they will use the evasion technique to avoid the security software detection.

How Does This Attack Works

Malicious hackers having an embedded video link inside of the Microsoft word document and send to victims via phishing mail that trick users to open it.

Embedded video contains a link that pointed to YOUTUBE and the hidden html/javascript code that will be running in the background.

According to cymulate, This attack is carried out by embedding a video inside a Word document, editing the XML file named document.xml, replacing the video link with a crafted payload created by the attacker which opens Internet Explorer Download Manager with the embedded code execution file.

Embed an online video option within the word document and link any YouTube video and save the document.

Later unpack the word document using unpacker or change the extension as zip and unzip it where you can find the several files along with word folder.

Word document contains a file called document.xml and find the  embeddedHtml parameter in order to identify the Youtube iframe code and Replace the current iframe code with any html code / javascript to be rendered by Internet Explorer.

A researcher from cymulate created a PoC that contains the embedded executable (as a blob of a base64). Once run, this code will use the msSaveOrOpenBlob method to trigger the download of the executable by opening Internet Explorer Download Manager with the option to run or save the file.

Mitigation:

Block Word documents containing the tag: “embeddedHtml” in the Document.xml file of the word documents.

Block word documents containing an embedded video.

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