Infected PowerPoint Files Using Cloud Services to Deliver Multiple Malware

Co-authored by Gustavo Palazolo and Ghanashyam Satpathy

Summary

In 2021, malicious Office documents accounted for 37% of all malware downloads detected by Netskope, showing favoritism for this infection vector among attackers. This is likely due to the ubiquitous usage of Microsoft Office in enterprises across the globe. Throughout 2021 we have analyzed many techniques used by attackers to deliver payloads through infected documents, which included the return of Emotet, a campaign that primarily uses infected documents to spread malware.

Since December 2021, Netskope Threat Labs has observed an increase in the usage of one specific file type from the Microsoft Office suite: PowerPoint. These relatively small files are being delivered through phishing emails, then downloading and executing malicious scripts through LoLBins, a common technique often used to stay under the radar.

We spotted this campaign delivering multiple malware, such as AveMaria (a.k.a. Warzone) and AgentTesla. These files are using Bitly to shorten URLs and different cloud services like MediaFire, Blogger, and GitHub to host the payloads. In this blog post, we will analyze a malicious PowerPoint Add-In file detected by Netskope that delivers multiple malware, including AgentTesla.

Stage 01 – Infected PowerPoint File

The infection flow starts with a phishing email that carries the infected file as an attachment, along with a message that lures the victim to download and open it.

Screenshot of phishing email with a malicious attachment. — Phishing email with a malicious attachment.

The file is fairly small and it doesn’t contain anything but the malicious VBA macro.

Example of infected PowerPoint file. — Infected PowerPoint file.

The macro is obfuscated and it uses an internal function to decrypt important strings at runtime.

Example of obfuscated VBA code within the infected PowerShell file. — Obfuscated VBA code within the infected PowerShell file.

The script deobfuscation is straightforward and leads to the following VBA code.

Screenshot of command executed by the malicious PowerShell file. — Command executed by the malicious PowerShell file.

This technique uses Outlook (COM Object) to execute PowerShell, which bypasses the child process created by PowerPoint.

Screenshot of PowerShell spawned by Outlook’s process. — PowerShell spawned by Outlook’s process.

The script is executed with a combination of PowerShell and mshta, a similar technique employed by BazarLoader.

Screenshot of malicious script being executed through LoLBins. — Malicious script being executed through LoLBins.

Stage 02 – VBS File

The URL contacted by the mshta binary is shortened through the Bitly domain “j.mp”, and the payload is hosted on MediaFire, a cloud service for file storage and sharing.

The next stage is a VBS script that is lightly obfuscated within an HTML page, which is decoded and executed through a simple JavaScript function.