227. Ransomware Gangs Patch System DLL to Enable Multiple Simultaneous RDP Connections

Hello everyone!

I'm sure you know that adversaries LOVE RDP. In most cases they use it for lateral movement. Today we'll look at the following technique: Server Software Component: Terminal Services DLL (T1505.005).

According to this report on Crypto24 ransomware group, the adversary patched termsrv.dll to enable multiple simultaneous RDP connections. The threat actor also executed the following commands related to the DLL:

takeown.exe /F C:\Windows\System32\termsrv.dll /A

icacls.exe C:\Windows\System32\termsrv.dll /grant Administrators

We can use it to build detection logic:

event_type: "processcreatewin"

AND

proc_file_path: ("takeown.exe" OR "icacls.exe")

AND

cmdline: "termsrv.dll"

We can also hunt for file modification events related to termsrv.dll:

event_type: "filewrite"

AND

file_path: "termsrv.dll"

See you tomorrow!


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