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.(Citation: Microsoft Remote Desktop Services)

[Windows Service](https://attack.mitre.org/techniques/T1543/003)s that are run as a "generic" process (ex: <code>svchost.exe</code>) load the service's DLL file, the location of which is stored in a Registry entry named <code>ServiceDll</code>.(Citation: Microsoft System Services Fundamentals) The <code>termsrv.dll</code> file, typically stored in `%SystemRoot%\System32\`, is the default <code>ServiceDll</code> 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.(Citation: James TermServ DLL) Modifications to this DLL could be done to execute arbitrary payloads (while also potentially preserving normal <code>termsrv.dll</code> 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](https://attack.mitre.org/techniques/T1021/001) sessions by either patching the <code>termsrv.dll</code> file or modifying the <code>ServiceDll</code> value to point to a DLL that provides increased RDP functionality.(Citation: Windows OS Hub RDP)(Citation: RDPWrap Github) 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.
Terminal Services DLL
source_name: mitre-attack  url: https://attack.mitre.org/techniques/T1505/005  external_id: T1505.005     source_name: James TermServ DLL  description: James. (2019, July 14). @James_inthe_box. Retrieved March 28, 2022.  url: https://twitter.com/james_inthe_box/status/1150495335812177920     source_name: Microsoft System Services Fundamentals  description: Microsoft. (2018, February 17). Windows System Services Fundamentals. Retrieved March 28, 2022.  url: https://social.technet.microsoft.com/wiki/contents/articles/12229.windows-system-services-fundamentals.aspx     source_name: Microsoft Remote Desktop Services  description: Microsoft. (2019, August 23). About Remote Desktop Services. Retrieved March 28, 2022.  url: https://docs.microsoft.com/windows/win32/termserv/about-terminal-services     source_name: RDPWrap Github  description: Stas'M Corp. (2014, October 22). RDP Wrapper Library by Stas'M. Retrieved March 28, 2022.  url: https://github.com/stascorp/rdpwrap     source_name: Windows OS Hub RDP  description: Windows OS Hub. (2021, November 10). How to Allow Multiple RDP Sessions in Windows 10 and 11?. Retrieved March 28, 2022.  url: http://woshub.com/how-to-allow-multiple-rdp-sessions-in-windows-10/     