Terminal Services DLL
Where this stands
One published detection covers this technique. Every one is unverified — no rule on Siemphony has been executed against real telemetry.
What the technique is
Adversaries may abuse components of Terminal Services to enable persistent access to systems. Microsoft Terminal Services, renamed to Remote Desktop Services in some Windows Server OSs as of 2022, enable remote terminal connections to hosts. Terminal Services allows servers to transmit a full, interactive, graphical user interface to clients via RDP. Windows Services that are run as a "generic" process (ex: svchost.exe) load the service's DLL file, the location of which is stored in a Registry entry named ServiceDll. The termsrv.dll file, typically stored in %SystemRoot%\System32\, is the default ServiceDll value for Terminal Services in HKLM\System\CurrentControlSet\services\TermService\Parameters\. Adversaries may modify and/or replace the Terminal Services DLL to enable persistent access to victimized hosts. Modifications to this DLL could be done to execute arbitrary payloads (while also potentially preserving normal termsrv.dll functionality) as well as to simply enable abusable features of Terminal Services. For example, an adversary may enable features such as concurrent Remote Desktop Protocol sessions by either patching the termsrv.dll file or modifying the ServiceDll value to point to a DLL that provides increased RDP functionality. […]
Read it on attack.mitre.orgWhat MITRE says you would watch
- AN0595
Adversary modifies or replaces the Terminal Services DLL (termsrv.dll) or changes the associated ServiceDll Registry value to load an arbitrary or patched DLL that enables persistent and enhanced RDP access. This may include binary replacement, registry tampering, and unexpected module loads by the svchost.exe -k termsvcs process.
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