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Office's OLE Leveraged to Hide Malicious Code

Malware authors are switching from macros to object linking and embedding (OLE) to hide malicious Visual Basic (VB) and JavaScript (JS) scripts in Office documents, Microsoft warns.

Just as with malicious macros before, attackers are using various social engineering techniques to trick users into enabling the hidden scripts, which in turn download malware onto the compromised machines. According to Microsoft, the use of OLE might indicate a shift in behavior as administrators and enterprises are adopting better security and new options in Office to prevent this infection vector.

In a blog post, Alden Pornasdoro, Microsoft Malware Protection Center, explains that the OLE-embedded objects and content spotted in recent cases are surrounded by well-formatted text and images, which are meant to trick users into enabling the malicious code. Pornasdoro also notes that the observed files were using malicious Visual Basic (VB) and JavaScript (JS) scripts.

The text encourages users to interact with the script embedded in the document, which results in a warning window to popup, prompting users whether to proceed or not. However, should the user choose to proceed, the malicious script runs and any form of infection can occur. What’s important to note, however, is the fact that user interaction is still required for the malicious payload to be executed, and that nothing happens if the object isn’t enabled.

Some of the malicious documents that employed this technique were seen containing language similar to that used in CAPTCHA and other human-verification tools. Moreover, Pornasdoro notes that malware authors can easily replace the OLE or embedded object in the file, thus switching between VB and JS scripts in no time. What’s more, attackers can embed any image they desire in their malicious documents, because it has nothing to do with the scripting language being used.

TrojanDownloader:VBS/Vibrio and TrojanDownloader:VBS/Donvibs were observed as the malicious payloads in the investigated documents, and Microsoft’s researcher explains that a decryption function set this campaign apart from the typical download-and-execute routine observed in this sort of infection.

Microsoft noticed that the malicious VB script would download an encrypted binary, which allows it to bypass network-based protection usually designed to recognize malicious formats and block them. After the encrypted file is saved to %appdata% with a random file name, the script would decrypt it. The decrypted binary in the analyzed sample turned out to be the Cerber ransomware, Pornasdoro says.

Apparently, the use of malicious Office documents that leverage the OLE capability to hide malware has decreased steadily since discovered in late May. Moreover, Pornasdoro notes that the two threats associated with these infection campaigns, namely TrojanDownloader:VBS/Vibrio and TrojanDownloader:VBS/Donvibs, are not particularly prevalent. However, they show large concentration in the United States.

Given that the embedded objects and content require user interaction to execute the hidden malicious code, Microsoft recommends user education as an important aspect of mitigation. “As with spam emails, suspicious websites, and unverified apps. Don’t click the link, enable the content, or run the program unless you absolutely trust it and can verify its source,” Pornasdoro says.

One of the biggest security threats a decade ago, macro malware was rather dormant until recently, but it has returned stronger than ever, as IBM revealed in December last year. In March this year, Dridex and Locky, which abused macros for distribution, were observed using forms to hide their malicious code. In May, Microsoft researchers noticed that other macro malware families were employing new techniques to hide code and hinder detection efforts.

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