Hidden desktops are a legitimate Windows capability, often used by specialized software, and occasionally used by malware.
MedusaHVNC is a remote access trojan (RAT) being sold as malware-as-a-service (MaaS). It is promoted through its own website and a Telegram channel. It was found and analyzed by BlackFog, with the analysis finding a hidden virtual network computing (HVNC) module that opens a legitimate browser on a separate hidden Windows desktop,
Since it operates from a hidden desktop, its operation is invisible to the user.
The malware uses a 5-stage infection chain. It starts when the legitimate wscript.exe executes a JScript launcher. The script waits for just over 7.5 seconds and then builds its embedded files under %TEMP%Nx2981Okkr2.
Several files are written to disc, including an encrypted payload and a .bat in the Startup folder to maintain persistence.
Windows AutoIT is used to decrypt the payload and start charmap.exe (the Windows character map utility. The loader, now inside charmap.exe, contains two further layers of encryption. “The first applies a 16-byte repeating XOR operation to 1,009,152 bytes from the .data section. The second uses ChaCha20 to decrypt 998,912 bytes of ciphertext with a 32-byte key, a 12-byte nonce, and an initial counter value of 1,” write the researchers.
That installed final payload ‘is an unsigned PE32+ x86-64 console executable containing a .pay section and the family string MedusaHVNC.’ It communicates with the operator’s C2 at a hardcoded address: 51.89.204.28:4444.
The operator can create a browser of choice within the hidden desktop from Chrome, Edge, and Firefox. Legitimate Windows functions, including BitBlt, EnumWindows, and PrintWindow support screen and window capture, while SendInput and SetWindowsHookExW are associated with synthetic input and interaction.
“Clipboard functions, including OpenClipboard, GetClipboardData, and SetClipboardData, provide another way to move information into or out of the session,” comment the researchers.
The hidden desktop allows the attacker to take full advantage of legitimate Windows tools without being observed by the user. The C2 is hardcoded into the malware but is relatively safe from observation. The result is a stealthy and persistent RAT.
The only obvious mitigation is detection of unexpected data exfiltration. Even if the RAT’s operation is out of view in the unknown and hidden desktop, the data must still be exfiltrated from the network. Detection of unexplained data leaving the network is always an indication that something is wrong somewhere.
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