Attackers Leverage Excel File Encryption to Deliver Malware

Researchers from Mimecast have recently observed a campaign where threat actors are using a somewhat rarely seen but easy method to distribute malware using Microsoft Excel's standard file encryption capabilities.

The tactic essentially involves a threat actor hiding malicious code in an Excel file, making the file read-only and then spreading it via phishing email. The attack takes advantage of a default password, "VelvetSweatshop," that is embedded in Excel and can be used to encrypt and decrypt Excel files, Mimecast said in a report Wednesday.

Users who want to encrypt an Excel file before sending it via email have to lock it with a password. The password acts as both the encryption key and a decryption key. To unencrypt a locked Excel file, the recipient has to enter the same password that was used to lock it. Threat actors have for some time taken advantage of how the encryption and decryption process in Excel works to distribute malware, Mimecast said.

The typical modus operandi has been to hide malware in an Excel file, encrypt the file using a password, and then distribute the malware via phishing emails with the password included in the content. Users who are tricked into opening the encrypted Excel file with the provided password end up downloading malware on their systems.

In the latest campaign, threat actors are using Excel to distribute LimeRAT, a well-known Trojan that can be used to download additional malware on compromised systems. But instead of encrypting the malware-laden Excel files, the malware authors are making them "read-only," says Matthew Gardiner, director of enterprise security campaigns at Mimecast.

"The attacker embeds the malicious code in the Excel document and saves the file as read-only," Gardiner says. This causes the Excel file to be automatically encrypted using VelvetSweatshop, the default password that is embedded in Excel, he notes.

When the file arrives via phishing email, the social engineering in the email encourages the recipient to open it, Gardiner says. If the user follows through, Excel first attempts to open the encrypted file using the default password.

"If that works, it knows the file is intended to be read-only and opens it as such," Gardiner says. At that point, the embedded malicious code in the Excel file will execute if the endpoint security software on the infected devices doesn't get in the way first. With the LimeRAT campaign, the criminals behind it have also been encrypting the actual content of the spreadsheet to try and hide the exploit and payload from malware detection systems, Mimecast said.

For the latest attack to succeed, a user would still need to click on the malicious Excel attachment, Gardiner says. But by using the read-only technique, the attackers have eliminated the need for victims to be social-engineered into entering a password to open the encrypted Excel file.

"Any extra step allows more time for the person to think and creates friction for the attacker," Gardiner says. "It is similar to limiting the steps for using an online shopping cart on an e-commerce site."  

Gardiner says this is not the first time adversaries have employed read-only Excel files to deploy malware. The technique has surfaced periodically but usually in campaigns associated with advanced attackers.

"This technique continues to be leveraged and used across increasingly sophisticated malware-centric attacks," Gardiner notes. Its use in the LimeRAT campaign demonstrates how cybercriminals often continue to exploit and build on techniques that might be considered old, he says.

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