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TRU Positives: Weekly investigation summaries and recommendations from eSentire's Threat Response Unit (TRU)

Pulse Check on OneNote for Malware Delivery

BY eSentire Threat Response Unit (TRU)

March 30, 2023 | 6 MINS READ


Threat Intelligence

Threat Response Unit

TRU Positive/Bulletin

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Adversaries don’t work 9-5 and neither do we. At eSentire, our 24/7 SOCs are staffed with Elite Threat Hunters and Cyber Analysts who hunt, investigate, contain and respond to threats within minutes.

We have discovered some of the most dangerous threats and nation state attacks in our space – including the Kaseya MSP breach and the more_eggs malware.

Our Security Operations Centers are supported with Threat Intelligence, Tactical Threat Response and Advanced Threat Analytics driven by our Threat Response Unit – the TRU team.

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).

Figure 1 OneNote samples containing embedded executable content submitted to VirusTotal in 2023.

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.

Figure 2 OneNote samples submitted to VirusTotal in March 2023.

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.

Figure 3 Emotet OneNote file, March 17th, 2023.

Click.wsf contains nearly a thousand lines of obfuscated VBScript code:

Figure 4 Snippet of Click.wsf

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.

Figure 5 Snippet of cleartext code in Click.wsf

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.

Figure 6 Emotet OneNote sample, retrieved March 21, 2023
Figure 7 Snippet of obfuscated code in "output1.js"

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).

Figure 8 Amazon order details lure.

When clicked, the embedded VBScript attempts to load Luca Stealer, Eternity Stealer, a clipboard hijacker, and Cobalt Strike (Figure 9).

Figure 9 Embedded "Order_Details.vbs" script contains code to retrieve payloads hosted on Discord.

What can you learn from this TRU positive?

Figure 10 Percentage of code delivery and execution techniques employed by email-borne threats observed in eSentire's telemetry in 2023 (up to March 20).

Recommendations from our Threat Response Unit (TRU) Team:

Indicators of Compromise

Suspected BatLoader Domains Registered in February 2023:

Indicator Note
erkaradyator[.]com[.]tr Emotet Payload Host
de3310abc7103503a589bbf857197d6d Luca Stealer
d6df00c71c455a6d7ec8193ee54ddfe9 Eternity Stealer
4d6bdd67cdabadd18e2210fc6ad2c094 Cobalt Strike

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.

If you are not currently engaged with an MDR provider, eSentire MDR can help you reclaim the advantage and put your business ahead of disruption.

Learn what it means to have an elite team of Threat Hunters and Researchers that works for you. Connect with an eSentire Security Specialist.

eSentire Unit
eSentire Threat Response Unit (TRU)

The eSentire Threat Response Unit (TRU) is an industry-leading threat research team committed to helping your organization become more resilient. TRU is an elite team of threat hunters and researchers that supports our 24/7 Security Operations Centers (SOCs), builds threat detection models across the eSentire XDR Cloud Platform, and works as an extension of your security team to continuously improve our Managed Detection and Response service. By providing complete visibility across your attack surface and performing global threat sweeps and proactive hypothesis-driven threat hunts augmented by original threat research, we are laser-focused on defending your organization against known and unknown threats.

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