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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…
In December, we published a summary of BatLoader activity whereby Google Search Ads were used to impersonate software such as WinRAR to deliver malicious Windows Installer files. The installer files contained custom action commands which used PowerShell to download and execute payloads (Redline Stealer, Ursnif, etc.) hosted on legitimate websites.
Throughout February 2023, TRU has observed a series of newly registered websites impersonating various applications and brands. Included among these are:
In addition to comparable domain registration attributes, these websites tend to follow a similar naming convention where one or more characters are appended to the impersonated brand name (e.g., adobe-l[.]com vs adobe.com). These sites were used to host imposter download pages and all likely stem from malicious advertisements on Google Search Ads. A more complete list can be found at the end of this post.
BatLoader continues to see changes and improvement since it first emerged in 2022. Recent samples analyzed by TRU utilize Windows Installer files masquerading as the above applications to launch embedded Python scripts.
In mid-February 2023, eSentire MDR for Endpoint blocked an attempt to execute Ursnif via code injection on a manufacturing customer’s endpoint. A subsequent investigation traced the infection to a Google search result for Adobe Reader by the victim user.
The user had clicked on a top-of-page ad on the search results page where they were directed via an intermediary website (adolbe[.]website) to adobe-e[.]com, a webpage masquerading as Adobe Acrobat Reader (Figure 1). As a result, the user unknowingly downloaded and executed AdobeSetup.msi(9ebbe0a1b79e6f13bfca014f878ddeec), BatLoader’s Windows Installer file.
Like previous versions of BatLoader, the MSI file contains Custom Actions to execute commands. In this case, the command executed an embedded batch file (seen here as InstallPython.bat, also observed as PythonFramework.bat) with admin privileges in a hidden window.
A decoy application was written to C:\Program Files (x86)\Chat Mapper along with BatLoader scripts and supporting files (Figure 2).
The batch file (figure 3, insert) performs the following actions:
In this case, two Python files (framework.py and frameworkb.py) were included in the package. These were protected using PyArmor and require unpacking with tools such as PyArmor-Unpacker (Figure 4). The files use a script copied from a Stack Overflow question as a template for executing Python code with elevated privileges.
BatLoader’s instruction set is inserted into the main function of the Stack Overflow Python script. The code retrieves an encrypted payload as control.exe.enc then executes a series of Windows commands. The instructions executed by both Python files were nearly identical except for a change in the payload URL.
The commands shown in Figure 5 are summarized as follows:
In the above-mentioned case, the payload intercepted by MDR for Endpoint was Ursnif (md5hash 0cb75b1192b23b8e03d955f1156ad19e), specifically isfb_v2.14+ variant configured to connect to the following C2 domains:
Ursnif’s persistence was achieved using a registry run key (VirtualStop) under HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run. This value executed a shortcut (LineType.lnk) which in turn launches a PowerShell script (CharReturn.ps1), as seen in Figure 6.
CharReturn.ps1 executed a staged PowerShell loader from registry at HKEY_USERS\Software\AppDataLow\Software\Microsoft\[randomstring]. The loader contained the embedded Ursnif binary which is injected into the Explorer process.
In this incident, MDR for Endpoint identified and blocked PowerShell execution of the Ursnif loader stored in registry.
Observed URL structures would suggest multiple payloads are available for download:
BatLoader has historically been linked to payloads such as Redline Stealer, SystemBC RAT, Syncro RMM, Vidar Stealer, Ursnif and Cobalt Strike. Analysis of more recent samples in March 2023 has yielded both Vidar Stealer and Ursnif trojans.
TRU has reviewed samples from public malware repositories which exhibited slightly different behavior than what was seen in the February incident described above. This sample from mid-February contained a third Python file named ‘networkframework.py’:
Like the others, it is obfuscated with PyArmor and contains an identical series of commands to handle payload retrieval, decryption and execution via WorkFolder.exe. In addition, netframework.py contains checks for curating payloads for domain-joined systems with more than 2 IP neighbors in the system’s ARP table.
This behavior has been previously observed whereby BatLoader executed Cobalt Strike in addition to the standard payloads such as Ursnif or Vidar. We assess this is done to prep systems residing in business networks for further infiltration.
As of time of writing, the payload URLs were no longer available for retrieval, but given that this fits into BatLoader’s known historical target selection patterns, we assess that Cobalt Strike is the probable candidate.
Suspected BatLoader Domains Registered in February 2023:
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Our industry-renowned Threat Response Unit (TRU) is an elite team of threat hunters and researchers, that supports our 24/7 Security Operations Centers (SOCs), builds detection models across our Atlas XDR Cloud Platform, and works as an extension of your security team to continuously improve our Managed Detection and Response service. TRU has been recognized for its threat hunting, original research and content development capabilities. TRU is strategically organized into cross-functional groups to protect you against advanced and emerging threats, allowing your organization to gain leading threat intelligence and incredible cybersecurity acumen.