Qakbot Returns to ISO Delivery (For Now)

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 reported that multiple threat groups were leveraging OneNote files for malware delivery. Qakbot was one such threat group and made up the majority of our observations of malicious OneNote files during this period.

Since mid-February, observations of Qakbot delivery via OneNote has faded in our telemetry while use of ISO image files has returned. The absence of OneNote malware in our telemetry suggests the technique is finding less success in circumventing perimeter gateways and content filters. The diminished use of OneNote by Qakbot during the week of February 20^th is corroborated by public mentions (1, 2, 3) of ISO images in lieu of the aforementioned file type.

However, it’s unlikely OneNote has been completely abandoned and it remains probable that threat actors will continue to iterate on the technique to extend its shelf life.

Qakbot’s infection scheme, as observed on February 22, 2023, is summarized below and in Figure 1.

1. A victim is sent an email linking to an externally hosted Zip file.
2. The Zip contains an ISO image file.
3. The victim double clicks the ISO to mount it (1). The mounted file contains a hidden folder (4), a 200+MB padded file and a visible shortcut file disguised as a zip.
4. The shortcut file (2) executes a CMD file (3) when clicked.
5. The CMD file contains commands which use certutil (renamed here as slaughterhouse.exe) to decode Qakbot’s DLL (brainstorms.sql). This file is base64 twice, thus requires two rounds of decoding using certutil. The decoded DLL is then executed using rundll32 and injects Qakbot into wermgr.exe.

How did we find it?

• MDR for Endpoint identified the ISO shortcut and certutil behavior.

What did we do?

• Our team of 24/7 SOC Cyber Analysts triaged the case, isolated the host and alerted the customer.

What can you learn from this TRU positive?

• OneNote quickly gained traction and was adopted by several threat groups (including Qakbot) in January and early February 2023. As mentioned in our previous TRU Positive, it’s probable detection of this filetype has improved.
  • It’s unlikely that the OneNote filetype has been abandoned, and continued vigilance and blocking at perimeter gateways is recommended.
• ISO image files have shown to be a viable vehicle for malware delivery and execution since emerging as a popular alternative to macro-enabled documents in early 2022.
• ISO images can be mounted natively with a click of a button, displaying as a folder to the end user.
  • Victims can be lured into clicking executable content including binaries, scripts or shortcut files masquerading as benign filetypes such as PDFs or folders.
• The use of ISO images has seen some iterative improvements throughout the last year by threats such as Qakbot. This includes encapsulating the image in one or more filetypes (Zips, PDFs, HTML), padding the contents with benign data or chaining together a series of scripts to decode or unpack the final payload.
  • In this case, the Qakbot Zip and ISO image were retrieved by the victim using a link included in an email. It included a standalone file padded with over 200MB of blank spaces, likely to impede analysis of the decompressed ISO file.

Recommendations from our Threat Response Unit (TRU) Team:

• Audit and consider blocking OneNote (.one), ISO images (.iso), Disk Image Files (.img) and Virtual Hard Disk files (.vhd and .vhdx) on perimeter gateways.
• 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.
• Train users to identify and report potentially malicious content using an Phishing and Security Awareness Training (PSAT) program.
  • Users should specifically be aware of the threat of OneNote documents for malware delivery.

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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