In a concerning development, the North Korean-backed hacking group Kimsuky has intensified its use of custom-built tools to exploit Remote Desktop Protocol (RDP) for controlling compromised systems.
AhnLab Security Intelligence Center (ASEC) reports that the group has developed a proprietary version of the open-source RDP Wrapper to enable remote desktop access on machines where this feature is otherwise unavailable.
This tactic is part of a broader campaign involving spear-phishing attacks and malware deployment.
Malware Delivery
Kimsuky’s operations begin with targeted spear-phishing emails containing malicious shortcut files (*.LNK).
These files are disguised as legitimate documents, often mimicking formats like PDFs or Excel spreadsheets.
Once executed, the shortcuts trigger PowerShell or Mshta commands to download additional payloads from external servers.
Among the deployed malware are two key tools: PebbleDash, a backdoor, and the custom-made RDP Wrapper.
The RDP Wrapper, typically an open-source utility for enabling remote desktop features on non-supported Windows versions, has been modified by Kimsuky to evade detection.
The group reportedly uses custom export functions within the wrapper to bypass file-based antivirus scans.
Once installed, this tool allows attackers to activate RDP services on infected systems, even in restricted environments.
Network Access
To overcome challenges posed by private network configurations, Kimsuky employs proxy malware that facilitates external access to compromised machines.
These tools act as intermediaries between the infected system and external networks, enabling seamless RDP connections.
Recent investigations identified three types of proxy tools used by the group, including one based on publicly available Go-language revsock code.
In addition to remote control capabilities, Kimsuky integrates other malware into its operations.
Keyloggers are deployed via PowerShell scripts or executable files, recording user keystrokes in concealed directories.
Furthermore, information-stealing malware targets credentials stored in web browsers.
A recently identified variant extracts encryption keys from browser configuration files to bypass security measures.
Kimsuky’s reliance on RDP has grown significantly over time.In earlier campaigns, the group primarily used backdoors for system control.
However, recent attacks show a shift toward leveraging tools like RDP Wrapper and proxy malware for direct remote access.
According to the AhnLab Security Intelligence Center (ASEC), this approach allows them to maintain persistence while minimizing detection risks.
The group also employs additional techniques to enhance their control over infected systems.
For example, they use custom malware to patch Windows’ RDP service (termsrv.dll), enabling multiple concurrent remote sessions without alerting active users.
Cybersecurity experts recommend several steps to mitigate risks associated with such attacks:
- Avoid opening email attachments from unknown sources.
- Regularly update operating systems and software to patch vulnerabilities.
- Use strong passwords and enable multi-factor authentication (MFA) for all accounts.
- Employ endpoint protection solutions capable of detecting advanced threats like those used by Kimsuky.
The continued evolution of Kimsuky’s tactics underscores the growing sophistication of state-sponsored cyber threats.
Organizations must remain vigilant against these persistent adversaries by implementing robust security measures and maintaining awareness of emerging attack vectors.
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