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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 May 2023, we identified DcRAT, a clone of AsyncRAT, at a consumer services customer. DcRAT is a remote access tool with info-stealing and ransomware capabilities. The malware is actively distributed using explicit lures for OnlyFans pages and other adult content.
The Lures
In observed instances, victims were lured into downloading Zip files containing a VBScript loader which is executed manually. File naming convention suggest the victims were lured using explicit photos or OnlyFans content for various adult film actresses.
Due to lack of telemetry, we were unable to identify how victims were served the Zip file in the May case. Analysis of samples submitted to VirusTotal dates this activity back to January 2023, with new samples submitted as recently as June 4th, 2023.
VBScript Loader
The loader (MD5 43876a44cc7736ff6432cb5d14c844fe) is a slightly
modified version of this
VBScript file analyzed by Splunk in 2021. The script is a legitimate printer-related Windows script modified to include the loader.
The script contains the same overall functionality of earlier versions; thus, this will only be a summary:
The payload, dynwrapx.dll, and shellcode are embedded within the file and are hex encoded, reversed, and padded with junk characters. The strings are reversed, and the extra characters replaced during runtime.
A small change from other version is the use of 3-character length junk strings such as (“X_x”) in the image below. Since the string is reversed prior to the replace() function, the actual string to replace should be “x_X”. Fortunately for whomever modified the script, this VBScript function is case-insensitive.
Figure 1 Snippet showing shellcode extraction and decoding.
Checks the OS (Operating Systems) architecture using WMI (Windows Management Instrumentation) and spawns a new 32-bit process if necessary.
Extracts the embedded dynwrapx.dll file, decodes it and registers it using Regsvr32 to gain access to DynamicWrapperX object.
Uses the object to load CallWindowProcW from user32.dll and VirtualAlloc from kernel32.dll.
Figure 2 Shellcode execution and payload injection.
Loads the payload (BinaryData) into memory then calls CallWindowProcW to execute the shellcode, ultimately injecting the payload into \Microsoft.NET\Framework\v4.0.30319\RegAsm.exe.
The Payload
In observed cases, the payload was DcRAT, which was injected into RegAsm.exe. Not to be confused with Dark Crystal RAT, this Remote Access Tool is a modified version of the popular AsyncRAT. The code for DcRAT is available on GitHub, although the author decided to archive it in February 2022 due to “abuse”.
Comparing the repos, DcRAT includes multiple plugins not offered in the base AsyncRAT repository.
Figure 3 Comparing plugins for DcRAT and AsyncRAT
Beyond baseline capabilities such as keylogging, remote access, webcam monitoring, and file manipulation, DcRAT offers a browser
credential and cookie stealer, a Discord token stealer and ransomware plugin (among other features).
The ransomware plugin encrypts non-system files and appends “.DcRat” to the filename:
Figure 4 Snippet of DcRAT's ransomware plugin.
When encryption is completed, an extortion note is left on the Desktop containing a list of encrypted files:
Figure 5 Creating the extortion note following encryption.
Distinguishing DcRAT from AsyncRAT
DcRAT is likely to be flagged by AV or malware sandboxes as AsyncRAT given most of the codebase is the same. The easiest method of quickly identifying DcRAT is by examining the PBKDF2 salt value using a tool such as
dnSpy:
Figure 6 PBKDF2 salt value as viewed in dnSpy.
DcRAT can also be identified by examining the decrypted configuration, where the mutex contains DC*string*RatMutexqwqdan3chun:
Figure 7 Decrypted DcRAT configuration. Mutex is highlighted.
Or by examining the X509Certificate:
Figure 8 Certificate extracted from config, parsed using CyberChef with DcRAT identifiers highlighted
How did we find it?
eSentire MDR for Endpoint
identified and blocked the VBscript loader when executed by the user.
What did we do?
Our team of 24/7 SOC Cyber Analysts isolated the system and performed additional investigation before alerting the customer.
What can you learn from this TRU positive?
Open-source security tools and malware are commonly adopted by both low-skilled and sophisticated actors.
The use of lures related to adult content decreases likelihood of self-reporting by victims.
DcRAT offers multiple methods of monetizing infected systems, file stealing, credential theft, and ransomware.
While RATs such as DcRAT and AsyncRAT are freely available, they are still capable tools for fraud, initial access or ransomware when paired with a crypter or loader that can circumvent defenses.
Public samples linked to the activity mentioned above date back to January 2023 and show no significant changes. This suggests the operators, using a combination of free malware and a publicly known VBS loader, have likely sustained some success over this six-month period.
Recommendations from our Threat Response Unit (TRU) Team:
Restrict click-to-run execution of script files such as .vbs.
Create new “Open With” parameters for script files (.js, .jse, .hta, .vbs) so they open with notepad.exe. This setting is found in the Group Policy Management Console under User Configuration > Preferences > Control Panel Settings > Folder Options.
Alternatively, configure Attack Surface Reduction rules
to block “Block JavaScript or VBScript from launching downloaded executable content”
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 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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