Pulse Check on OneNote for Malware Delivery

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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…

What did we find?

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.

Now, nearly a month later where do we stand?

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).

Emotet Returns, Pivots to OneNote

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.

Emotet's OneNote Documents

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.

Another Emotet sample, first seen on March 21st contains a “Connect to Cloud” lure with an embedded JavaScript file (Figure 6). Like the previous sample, it attempts to retrieve the DLL payload from several hardcoded domains (see the table at the end of this blog) then executes it with regsvr32.exe.

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).

What can you learn from this TRU positive?

• Microsoft OneNote files are simple to weaponize with click-to-run scripts hidden behind image lures.
  • Email-centric threats like Qakbot and Emotet have both shifted to OneNote in lieu of Office macros or disk image files (.iso) execution techniques. However, Qakbot has since reverted to previous techniques.
• Between January 1 and March 20, 2023, OneNote accounted for 57% of email-borne malware analyzed by our Threat Intelligence team (Figure 10). However, only a single malicious OneNote file has been observed since February 17th in our telemetry. In that case, the file was an Emotet OneNote file similar to Figure 3. Execution was identified by MDR for Endpoint and quickly remedied by our 24/7 SOC Cyber Analysts.

• It’s unlikely the OneNote filetype will see extended use in the future. The current weaponization technique relies on using a built-in file insertion feature coupled with the ability to execute embedded files when placed behind an image layer.
  • Both features could be easily remedied by Microsoft in future versions of OneNote and would significantly curtail this attack method.
• Finally, submissions to public sandboxes and observations in our telemetry have severely diminished, suggesting actors have moved on or are finding less success in penetrating perimeter content filtering.

Recommendations from our Threat Response Unit (TRU) Team:

• If not required for legitimate business purposes, consider blocking OneNote attachments at your perimeter gateways.
• Train users to identify and report potentially malicious content using Phishing and Security Awareness Training (PSAT) programs.
  • Users should specifically be aware of the threat of OneNote documents for malware delivery.
• Protect endpoints against malware by:
  • Ensuring antivirus signatures are up-to-date.
  • Using a Next-Gen AV (NGAV) or Endpoint Detection and Response (EDR) tool to detect and contain threats.

Indicators of Compromise

Suspected BatLoader Domains Registered in February 2023:

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