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

Recent FakeBat Activity Observed in December 2022

BY eSentire Threat Response Unit (TRU)

December 28, 2022 | 6 MINS READ


Threat Intelligence

Threat Response Unit

TRU Positive/Bulletin

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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…

This blog was updated in September 2023 based on research from TRU that identifies this as FakeBat. This blog originally identified this threat as BatLoader.

What did we find?

This blog will focus on a recent activity cluster identified in mid-December 2022.

Recent FakeBat Activity

On December 16th, 2022, eSentire’s Security Operations Center responded to several FakeBat infections impacting customers in the financial services and higher education sectors.

In both cases, FakeBat was distributed via a lookalike website for WinRAR:

Figure 1 FakeBat WinRAR lookalike page.

The download link leads to ads-check[.]com, which serves a 302 redirect to down[.]software. The latter responds with a MSI payload file masquerading as the WinRAR utility. Ads-check[.]com is used in multiple stages of FakeBat’s infection chain to track infection sources and progress. The site uses a Russian language login panel:

Figure 2 Ads-check[.]com’s Russian-language admin panel.

The second website in the payload delivery chain, down[.]site contains JavaScript logic for displaying a greeting in either English or Russian based on the user’s browser preferences.

Figure 3 down[.]software JavaScript

According to available data on VirusTotal, the site has been active since late November 2022 and has delivered payloads impersonating WinRAR, Awesome Miner, uTorrent and LightShot.

Figure 4 VirusTotal records for down[.]software.

The initial payload (winrar-x64-611.msi) is a Microsoft Software Installer file created with Advanced Installer. The MSI file is configured to run on systems with at least 4GB of memory and an active Internet connection.

Figure 5 FakeBat PowerShell scripts via Advanced Installer's Custom Actions

The installer file has 5 Custom Actions (above) which use PowerShell to fetch code from Hugging Face, an American AI company whose website offers a code repository for community projects:

Figure 6 PowerShell scripts hosted on Hugging Face.

The DownGPG Custom Action uses PowerShell to retrieve and execute a second PowerShell command. This command downloads and saves two encrypted payloads (Redline and Ursnif).

Figure 7 Custom Action PowerShell command “DownGPG”

The two payloads are hosted on the Bitbucket code repository. The “Downloads” field indicates both files have been downloaded thousands of times since December 1st (how many were legitimate victims is not known).

Figure 8 Secondary FakeBat payloads hosted on BitBucket.

The DownOfficial Custom Action retrieves the Nsudo and WinRAR from their respective download sources. It saves both to the user’s AppData directory.

Figure 9 Custom Action PowerShell command “DownOfficial”

The Stup Custom Action is similar to “DownGPG” – it retrieves an encoded PowerShell command that connects to the ads-check[.]com C2 to likely signal infection start.

Figure 10 Custom Action PowerShell command “Stup"

Stup_1 sleeps then adds .rar, .cmd, .bat, .zip and .exe to Windows Defender exclusions for file extensions.

Figure 11 Custom Action PowerShell command “Stup_1"

Stup_2 excludes several paths and processes from Windows Defender. The process exclusion list includes the two payloads (Redline and Ursnif) and Explorer.exe process, which is likely a target for code injection for Ursnif.

Figure 12 Custom Action PowerShell command “Stup_2"

The Custom Action Final is a PowerShell script that most closely resembles past FakeBat scripts. It contains the function to install GPG4Win as well as encryption and decryption functions. These functions are seemingly identical to past FakeBat scripts and was likely copied from the GitHub repository.

Figure 13 Custom Action PowerShell script “Final"

This section calls the above functions to install GnuPG to AppData, decrypts the payloads using the password ‘putingod’, and executes the two unencrypted payloads with elevated privileges using Nsudo. Finally, it calls home to the ads-check[.]com C2 (likely to record the install).

Figure 14 Bottom section of PowerShell script from "Final" Custom Action.

Decrypted Payloads

Both payloads were gpg-encrypted using the password “putingod”.

ZipCosdaz.exe (4a57cbce13def4a4d9f7bccc49a8af52) is a .NET loader that retrieves Redline Stealer from Redline connects to C2 Address 193.56.146[.]114:44271.

Figure 15 Snippet from .NET Loader "ZipCosdaz.exe"

ZipCosdaz1.exe (c03be50c6fbfd3aec108a7bcd7aaea82) is a loader for Ursnif malware. It stages HTA/PowerShell commands in HKCU\\Software\AppDataLow\Software\Microsoft\{GUID} and injects into Explorer.exe process using PowerShell.

This is the isfb_v2.14+ variant configured to connect to the following C2 domains:

Memory strings include basic living-off-the-land discovery commands common with precursor threats. It should be noted that this variant is not RM3/LDR4, which is suspected to be a precursor for ransomware attacks.

Figure 16 Snippet of Ursnif memory strings

How did we find it?

What did we do?

What can you learn from this TRU Positive?

Recommendations from our Threat Response Unit (TRU):

Indicator Note
winrar[.]software/index-install[.]html Fake WinRAR download for FakeBat
ads-check[.]com FakeBat C2
down[.]software FakeBat Payload Hosting
04d77db9b7c18444b3bd50ee1b99c11c FakeBat Payload “winrar-x64-611.msi”
huggingface[.]co/assop875/news/ Code repo containing FakeBat PowerShell
bitbucket[.]org/ganhack123/load/downloads/ZipCosdaz1.exe.gpg Encrypted Ursnif Download
bitbucket[.]org/ganhack123/load/downloads/ZipCosdaz.exe.gpg Encrypted Redline Download
4a57cbce13def4a4d9f7bccc49a8af52 “ZipCosdaz.exe” Redline
c03be50c6fbfd3aec108a7bcd7aaea82 “ZipCosdaz1.exe” Ursnif
70c8ba4cb07d29019a35a248b5647a14 Unpacked Ursnif Payload
3c24d4cda44e9f3156d62986a4998bdf Redline Payload Redline Payload Hosting Redline C2 Ursnif C2 Ursnif C2 Ursnif C2 Ursnif C2 Ursnif C2 Ursnif C2

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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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