OneNote Payload Smuggling: Multiple Threats Leverage OneNote to Deliver Malware

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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?

Since January 31st, 2023, TRU has observed a noted increase in malware delivery using OneNote documents. This follows reports dating back to December 2022 of OneNote documents being used to deliver Formbook malware.

OneNote documents have been adopted to deliver AsyncRAT, Redline Stealer, QuasarRAT, Bumblebee and Qakbot malware. The latter three have been observed in our telemetry and the majority of observations were tied to Qakbot.

How are OneNote Files Used to Deliver Malware?

As explained by Didier Stevens in his recent InfoSec Handler Diary, OneNote documents use a binary format called MS-ONESTORE. Executable files can be embedded within these documents then launched by clicking an icon on the page (Figure 1). “Weaponizing” OneNote documents is as straightforward as inserting executable files (including scripts) into the document body.

Clicking the icon would result in a popup warning that opening the attachment could result in harm (Figure 2). If ignored, OneNote will spawn the appropriate child process (e.g., PowerShell) to execute the code. The popup does not provide any specific contextual information to help the user decide on whether the attachment is safe.

Like other warnings of this nature, it recommends the user judge the risk based on the source of the attachment. This behavior has been long exploited by the likes of Qakbot or Emotet using stolen email threads to masquerade as a trusted source.

In addition to easy insertion of clickable executable content, OneNote offers another desirable trait for malware execution as the attachment can still be clicked even when hidden behind an image (Figure 3).

The image below offers an example of a typical malicious OneNote document. In our observations, these documents masquerade as either OneNote or Office365 with text that lures the victim into clicking on an icon containing the payload (Figure 4).

In the above sample, clicking the icon and accepting the warning prompt would execute an embedded HTA file using the MSHTA process (Figure 5). This HTA file used Curl to retrieve then execute a Qakbot DLL on the machine.

Examining endpoint telemetry shows this execution chain in detail:

In this particular example, Qakbot injected into wermgr.exe then spawned various Windows binaries to orient and assess the value of the target for further exploitation (Figure 7).

Other Observations

Bumblebee

In early February, TRU analyzed a Bumblebee sample intercepted by our MDR for Endpoint service. The execution was blocked, but analysis tied it to an invoice themed email containing a OneNote attachment (Figure 8).

The attachment used the typical OneNote theme to lure the victim in clicking on the “Open” image (Figure 9). This launched the “Open.hta” HTML application file which was blocked by MDR for Endpoint.

Further analysis of the HTA document (Figure 10) revealed several VBScript and JavaScript code sections that write lightly obfuscated code to registry under HKCU\SOFTWARE\rq5w\ and used curl to retrieve and execute the DLL payload (“view.png”). It’s worth noting that this document appears very similar to those delivering Qakbot in late January and early February.

Qakbot PowerShell Variants

Recently observed Qakbot samples in our telemetry employed an embedded PowerShell script using a generic OneNote theme and executed by double clicking the “Open” icon and ignoring the content warning (Figure 11).

We have also seen recent versions with lightly obfuscated PowerShell code (Figure 12). The lack of robust obfuscation may speak to benefits of the OneNote filetype in evading content inspection filters currently.

Quasar RAT

Finally, as explained by a TRU researcher on Twitter, QuasarRAT was observed in early February using OneNote documents. This sample contained an embedded CAB file which dropped and executed a VBS script payload (Figure 13). The VBS script called PowerShell to retrieve a PS script from a remote host. The final payload was a .NET executable that contained Quasar RAT.

How did we find it?

Fortunately, detecting malicious OneNote document execution is relatively straightforward.

In this case, our team of 24/7 SOC Cyber Analysts and Threat Response Unit (TRU) used MDR for Endpoint rules to look for onenote.exe spawning executables or scripts. Existing signatures for Qakbot and other malware abusing this technique were successful in detecting this activity.

What did we do?

• In response, TRU deployed detection content that identifies code execution via OneNote documents.
  • eSentire MDR for Endpoint, Machine-Learning Powered BlueSteel, and eSentire MDR for Network will detect malware and the associated post-infection activity.
• Known attacker infrastructure has been blocked via eSentire’s global deny list.
• We also pushed a security advisory to our customers.

What can you learn from this TRU positive?

• Macro-based code execution via office documents has been largely replaced by alternative user-initiated code execution techniques.
  • Alternative user-execution methods such as shortcut files were widely abused in 2022, using various containers such as password-protected Zips, HTML attachments, ISOs (or all the above combined) to encapsulate and hide executable content from security products in transit.
  • Filtering mechanisms and user-awareness have improved, leading threat actors to pursue alternative techniques as their rate of success declines over time.
• OneNote documents currently offer an attractive alternative, since weaponizing the document is straightforward and embedded content can be executed via an icon which is easily disguised as something harmless.
• Additionally, perhaps due to lack of defensive tooling for the OneNote binary filetype, AV detection is low for an office file with cleartext or mildly obfuscated executable content.
  • This low detection rate may be due to how VirusTotal processes OneNote files currently (Figure 14).
  • It’s worth noting these files have shown significant uptick in our telemetry, implying they are not being stopped by perimeter content filters in transit to victims.

• From a defensive standpoint, it is probable vendors will be quick to detect and block the methods outlined in this post.
• From an offensive standpoint, it is probable adversaries will iterate on this technique to extend its shelf life. It is highly likely that we will see OneNote documents encapsulated in different filetypes (ZIP archives, HTML, etc.) and better obfuscation embedded content to evade detection.

Recommendations from our Threat Response Unit (TRU) Team:

As the adversarial TTPs grow in sophistication, they lead to a certain level of difficulty at which critical business decisions must be made. Preventing the various attack paths utilized by modern threat actors requires actively monitoring the threat landscape, developing, and deploying endpoint detection, and the ability to investigate logs & network data during active intrusions.

To increase your resilience against cyber threats like this, we recommend:

• 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

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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