usdAG / SharpLink

Create file system symbolic links from low privileged user accounts within PowerShell

SharpLink

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SharpLink is a C# namespace containing classes that allow low privileged user accounts to create file system and registry symbolic links on Windows operating systems. It uses a technique that was first identified by James Forshaw and implemented within the symboliclink-testing-tools. SharpLink provides a C# implementation and allows to create symbolic links directly from PowerShell in an object oriented fashion.

Usage

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The following listings show short usage examples of SharpLink. More details can be found within the docs folder.

File System

PS C:\> $code = (iwr https://raw.githubusercontent.com/usdAG/SharpLink/main/SharpLink.cs).content
PS C:\> Add-Type $code
                                                                                                 
PS C:\> $s = New-Object de.usd.SharpLink.Symlink("C:\Users\Public\Example\link", "C:\ProgramData\target.txt")
PS C:\> $s.Open()
[+] Creating Junction: C:\Users\Public\Example -> \RPC CONTROL
[+] Creating DosDevice: Global\GLOBALROOT\RPC CONTROL\link -> \??\C:\ProgramData\target.txt
[+] Symlink setup successfully.

PS C:\> echo "Hello World :D" > C:\Users\Public\Example\link
PS C:\> type C:\ProgramData\target.txt
Hello World :D

PS C:\> $s.Close()
[+] Removing Junction: C:\Users\Public\Example
[+] Deleting DosDevice: Global\GLOBALROOT\RPC CONTROL\link -> \??\C:\ProgramData\target.txt
[+] Symlink deleted.

Windows Registry

PS C:\> $code = (iwr https://raw.githubusercontent.com/usdAG/SharpLink/main/SharpLink.cs).content
PS C:\> Add-Type $code

PS C:\> $r = New-Object de.usd.SharpLink.RegistryLink("HKCU\link", "HKCU\Volatile Environment")
PS C:\> $r.Open()
[+] Creating registry key: \Registry\User\S-1-5-[...]-1001\link
[+] Assigning symlink property pointing to: \Registry\User\S-1-5-[...]-1001\Volatile Environment
[+] Symlink setup successful!

PS C:\> reg query HKCU\link /v HOMEDRIVE

HKEY_CURRENT_USER\link
    HOMEDRIVE    REG_SZ    C:

PS C:\> $r.Close()
[+] Registry key \Registry\User\S-1-5-[...]-1001\link was successfully removed.

LinkGroups

PS C:\> $g = New-Object de.usd.SharpLink.LinkGroup
PS C:\> $g.AddSymlink("C:\Users\Public\Example\link", "C:\ProgramData\target.txt")
PS C:\> $g.AddSymlink("C:\Users\Public\Example\link2", "C:\ProgramData\target2.txt")
PS C:\> $g.AddRegistryLink("HKCU\LINK", "HKCU\TARGET")
PS C:\> $g.Open()
[+] Creating Junction: C:\Users\Public\Example -> \RPC CONTROL
[+] Creating DosDevice: Global\GLOBALROOT\RPC CONTROL\link -> \??\C:\ProgramData\target.txt
[+] Symlink setup successfully.
[+] Junction C:\Users\Public\Example -> \RPC CONTROL does already exist.
[+] Creating DosDevice: Global\GLOBALROOT\RPC CONTROL\link2 -> \??\C:\ProgramData\target2.txt
[+] Symlink setup successfully.
[+] Creating registry key: \Registry\User\S-1-5-[...]-1001\LINK
[+] Assigning symlink property poitning to: \Registry\User\S-1-5-[...]-1001\TARGET
[+] RegistryLink setup successful!

PS C:\> $g.Status()
[+] LinkGroup contains 3 link(s):
[+]
[+] Link type: File system symbolic link
[+]     Link path: C:\Users\Public\Example\link
[+]     Target path: C:\ProgramData\target.txt
[+]     Associated Junction: C:\Users\Public\Example
[+]     Associated DosDevice: Global\GLOBALROOT\RPC CONTROL\link
[+]
[+] Link type: File system symbolic link
[+]     Link path: C:\Users\Public\Example\link2
[+]     Target path: C:\ProgramData\target2.txt
[+]     Associated Junction: none
[+]     Associated DosDevice: Global\GLOBALROOT\RPC CONTROL\link2
[+]
[+] Link Type: Registry symbolic link
[+]     Link key: \Registry\User\S-1-5-[...]-1001\LINK
[+]     Target key: \Registry\User\S-1-5-[...]-1001\TARGET
[+]     Created: True

PS C:\> $g.Close()
[+] Removing Junction: C:\Users\Public\Example
[+] Deleting DosDevice: Global\GLOBALROOT\RPC CONTROL\link -> \??\C:\ProgramData\target.txt
[+] Symlink deleted.
[+] Deleting DosDevice: Global\GLOBALROOT\RPC CONTROL\link2 -> \??\C:\ProgramData\target2.txt
[+] Symlink deleted.
[+] Registry key \Registry\User\S-1-5-[...]-1001\LINK was successfully removed.

Acknowledgements and References

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SharpLink ports some of the symlink functionalities from the symboliclink-testing-tools to C#. The general idea as well as the required Win32 API calls were taken from the symboliclink-testing-tools repository. Furthermore, the C# code for creating and deleting junctions and for working with the Windows registry was basically copied from the below mentioned reference.

• Abusing Symlinks on Windows
• Create, Delete and Examine Junction Points in C#
• symboliclink-testing-tools
• Windows Registry Stuff