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SharpRDPHijack

Sharp RDP Hijack is a proof-of-concept .NET/C# Remote Desktop Protocol (RDP) session hijack utility for disconnected sessions

Background

RDP session hijacking is a post-exploitation technique for taking control of (forcefully) disconnected interactive login sessions. The technique is described in Mitre ATT&CK T1563 - Remote Service Session Hijacking: RDP Hijacking.

Notes

  • SharpRDPHijack.cs compiles in Visual Studio 2019 under .NET Framework v.4.
  • TS/ RDP Session query may require privileges depending on the target machine.
  • Session hijacking requires an elevated (administrator) context to connect to another session.
  • NT AUTHORITY\SYSTEM context is required to take control of a session unless a target session user's password is known. Without a supplied password, SharpRDPHijack will (attempt to) impersonate NT AUTHORITY\SYSTEM.
  • Windows 2019 Server session hijacking exhibits interesting behavior vs prior OS versions. Upon hijacking a session that is redirected to an active RDP session, the Windows login screen prompts for the user's password/credential. If redirected to the console session, this redirection is successful and seamless. This presents an interesting research opportunity (IMO).
  • Several folks have inquired about the function/necessity of this utility when you can do the same thing with tscon.exe or Mimikatz TS. The goal of writing this POC was to gain a better understanding of what was happening at the Win32 API level (more specifically - Wtsapi32) and to have a simpler option for connecting to other sessions (preferably in C#). In this implementation, the two functions/methods that do the heavy lifting are WTSConnectSession and WTSDisconnectSession.
  • Potentially, there is an advantage such that this utility could evade specific detection analytics for tscon.exe + supporting command usage. Defensive guidance in the linked resources page are useful for addressing abuse of this technique (e.g. logging off disconnected sessions after a timeout period in Group Policy) as well as implementing domain admin login resiliency best practices to minimize domain exposure where non-DA accounts have admin rights on machines also used by DAs.

Usage

[*] Parameters:
    --tsquery=<host> : Query a host to identify RDP/TS session information (not required for other switches)
    --session=<ID> : Target session identifier
    --password=<User's Password> : Session password if known (otherwise optional - not required for disconnect switch)
    --console : Redirect session to console session instead of current (active) session
    --disconnect : Disconnect an active (remote) session

[*] Example Usage 1: Impersonate NT AUTHORITY\SYSTEM to hijack session #6 and redirect to the current session
    SharpRDPHijack.exe --session=6

[*] Example Usage 2: Impersonate NT AUTHORITY\SYSTEM to hijack session #2 and redirect to the console session
    SharpRDPHijack.exe --session=2 --console

[*] Example Usage 3: Hijack Remote Desktop session #4 with knowledge of the logged-on user's password
    SharpRDPHijack.exe --session=4 --password=P@ssw0rd

[*] Example Usage 4: Disconnect active session #3
    SharpRDPHijack.exe --session=3 --disconnect

[*] Example Usage 5: Query the local host for RDP/TS session information
    SharpRDPHijack.exe --tsquery=localhost

To Do

  • Clean up session validation

Other Notable Implementations

  • TScon - Microsoft Terminal Services connection utility (tscon.exe)
  • Mimikatz - TS Module

Ethics

Sharp RDP Hijack is designed to help security professionals perform ethical and legal security assessments and penetration tests. Do not use for nefarious purposes.

Resources with Defensive Considerations

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A POC Remote Desktop (RDP) session hijack utility for disconnected sessions

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