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
termsrv.dll file, typically stored in
%SystemRoot%\System32\, is the default
ServiceDll value for Terminal Services in
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. On a non-server Windows OS this increased functionality may also enable an adversary to avoid Terminal Services prompts that warn/log out users of a system when a new RDP session is created.
Regularly check component software on critical services that adversaries may target for persistence to verify the integrity of the systems and identify if unexpected changes have been made.
|M1024||Restrict Registry Permissions||
Consider using Group Policy to configure and block modifications to Terminal Services parameters in the Registry.
|ID||Data Source||Data Component||Detects|
Monitor executed commands and arguments for potential adversary actions to modify Registry values (ex:
Monitor unexpected changes and/or interactions with
Monitor module loads by the Terminal Services process (ex:
Monitor processes with arguments that may potentially highlight adversary actions to modify Registry values (ex:
|DS0024||Windows Registry||Windows Registry Key Modification||
Monitor for changes to Registry keys associated with