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In TRU Positives, eSentire’s Threat Response Unit (TRU) provides a summary of a recent threat investigation. We outline how we responded to the confirmed threat and what recommendations we have going forward.
Here’s the latest from our TRU Team…
In early February 2023, we published a blog about the sudden increase in malicious Microsoft OneNote files observed across our customers. At the time, it seemed OneNote had suddenly become the preferred delivery vector for multiple malware families including Qakbot, Bumblebee Loader, Redline Stealer, ASYNC RAT, Quasar RAT etc.
We assessed that the widespread adoption of this filetype was due to ease of weaponization, lack of support for the MS-ONESTORE filetype, and defensive improvements around disk image files (.iso). We also assessed vendors would be quick to adapt tooling and block OneNote files containing embedded executable content but left open the possibility that actors would improve defense evasion measures.
Reviewing OneNote submissions to VirusTotal, we see a clear spike in activity in early to mid-February, peaking at over 800 submissions on February 3rd. This was followed by several, progressively smaller peaks before nearly vanishing for the final week of February (Figure 1).
In early March 2023, security researchers reported Emotet had resumed mass email delivery operations. Notably, this initial wave did not make use of hijacked email chains (a generic invoice lure was used instead) and leveraged document macros to download and execute Emotet’s payload in the form of a DLL.
In an attempt to evade detection, the Microsoft Office documents were padded to increase their size to over 500MB once unzipped. It’s unlikely this initial wave found much success, given Microsoft’s changes to default security settings for macros. We assess this initial wave was likely to build up a botnet comprised of poorly defended targets whose infrastructure/data could be used for subsequent Emotet campaigns. The following week, we began observing reports of widespread Emotet emails containing malicious OneNote documents.
Focusing in on the March VirusTotal submissions, we can see Emotet OneNote samples as of March 15th (Figure 2). Note: The peak on Friday, March 17 is far smaller than the activity seen in February.
Examining Emotet OneNote samples from March 17th and 20th, the user would be presented with a document protection message which instructs the user to double-click the View button (Figure 3). Clicking this button, then ignoring the explicit pop-up warning, would execute an embedded WSF file positioned behind the image (the WSF file has been repositioned in Figure 3 for demonstration purposes).
This is the standard playbook for weaponized OneNote files, where click-to-run content is inserted into the document and positioned behind an image layer.
Click.wsf contains nearly a thousand lines of obfuscated VBScript code:
Redirecting script execution to a text file reveals the cleartext code (Figure 5). This code attempts to retrieve Emotet’s DLL payload from 12 hardcoded URLs using a user-agent string mimicking a real web browser.
The resulting DLL is saved to the current directory and copied to C:\Users\Username\AppData\Local\randomstring\. The DLL is executed using regsvr32.exe before connecting to several hardcoded C2 IP addresses. A variation of this version uses a “two stage” method whereby the first VBS file (clicked by the victim) writes a second VBS file to C:\Users\Username\AppData\Local\Temp\ which is subsequently executed by the first stage.
Besides Emotet, we have also identified generic loaders, such as this OneNote document uploaded to VirusTotal on March 8th utilizing a shipping order detail lure (Figure 8).
When clicked, the embedded VBScript attempts to load Luca Stealer, Eternity Stealer, a clipboard hijacker, and Cobalt Strike (Figure 9).
Suspected BatLoader Domains Registered in February 2023:
|erkaradyator[.]com[.]tr||Emotet Payload Host|
eSentire’s Threat Response Unit (TRU) is a world-class team of threat researchers who develop new detections enriched by original threat intelligence and leverage new machine learning models that correlate multi-signal data and automate rapid response to advanced threats.
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Our industry-renowned Threat Response Unit (TRU) is an elite team of threat hunters and researchers, that supports our 24/7 Security Operations Centers (SOCs), builds detection models across our Atlas XDR Cloud Platform, and works as an extension of your security team to continuously improve our Managed Detection and Response service. TRU has been recognized for its threat hunting, original research and content development capabilities. TRU is strategically organized into cross-functional groups to protect you against advanced and emerging threats, allowing your organization to gain leading threat intelligence and incredible cybersecurity acumen.