T1505.005: Terminal Services DLL

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. 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.

Positive Technologies products that cover the technique

MaxPatrol SIEM knowledge base

— Monitoring of key-modification events for the registry key HKLM\System\CurrentControlSet\Services\TermService\Parameters\ServiceDll. This key should contain the '%SystemRoot%\System32\termsrv.dll' value. If not, check for potentially malicious activities. — Monitoring of reg.exe run events which contain 'termsrv.dll' in their command line. — Monitoring of events related to suspicious DLLs being loaded by Terminal Services (for example, 'svchost.exe -k termsvcs'). By default, it should look like '%SystemRoot%\System32\termsrv.dll'

Expert Required. The technique is detected only with the combination of «PT Product + Expert»

Detection

ID	DS0011	Data source and component	Module: Module Load	Description	Monitor module loads by the Terminal Services process (ex: `svchost.exe -k termsvcs`) for unexpected DLLs (the default is `%SystemRoot%\System32\termsrv.dll`, though an adversary could also use Match Legitimate Name or Location to potentially conceal a malicious payload).

ID	DS0017	Data source and component	Command: Command Execution	Description	Monitor executed commands and arguments for potential adversary actions to modify Registry values (ex: `reg.exe`) or modify/replace the legitimate `termsrv.dll`.

ID	DS0022	Data source and component	File: File Modification	Description	Monitor unexpected changes and/or interactions with `termsrv.dll`, which is typically stored in `%SystemRoot%\System32\`.

ID	DS0024	Data source and component	Windows Registry: Windows Registry Key Modification	Description	Monitor for changes to Registry keys associated with `ServiceDll` and other subkey values under `HKLM\System\CurrentControlSet\services\TermService\Parameters\`.

ID	DS0009	Data source and component	Process: Process Creation	Description	Monitor processes with arguments that may potentially highlight adversary actions to modify Registry values (ex: `reg.exe`) or modify/replace the legitimate `termsrv.dll`.

ID	Data source and component	Description
DS0011	Module: Module Load	Monitor module loads by the Terminal Services process (ex: `svchost.exe -k termsvcs`) for unexpected DLLs (the default is `%SystemRoot%\System32\termsrv.dll`, though an adversary could also use Match Legitimate Name or Location to potentially conceal a malicious payload).
DS0017	Command: Command Execution	Monitor executed commands and arguments for potential adversary actions to modify Registry values (ex: `reg.exe`) or modify/replace the legitimate `termsrv.dll`.
DS0022	File: File Modification	Monitor unexpected changes and/or interactions with `termsrv.dll`, which is typically stored in `%SystemRoot%\System32\`.
DS0024	Windows Registry: Windows Registry Key Modification	Monitor for changes to Registry keys associated with `ServiceDll` and other subkey values under `HKLM\System\CurrentControlSet\services\TermService\Parameters\`.
DS0009	Process: Process Creation	Monitor processes with arguments that may potentially highlight adversary actions to modify Registry values (ex: `reg.exe`) or modify/replace the legitimate `termsrv.dll`.

Mitigation

ID	M1047	Name	Audit	Description	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.

ID	M1024	Name	Restrict Registry Permissions	Description	Consider using Group Policy to configure and block modifications to Terminal Services parameters in the Registry.

ID	Name	Description
M1047	Audit	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.