Tuesday, January 12, 2016


I don't think there are any members of the InfoSec community who aren't familiar with the TCP/IP Swiss army knife, that is Netcat. So naturally when I learned that someone had written a similar tool in PowerShell, I was intrigued. It was months after learning about Mick Douglas's and Luke Baggett's original version of PowerCat that I actually tried out the tool and reviewed its implementation. What Mick and Luke started was quite ingenious, and I must give credit where it is due. However, I think most people would agree that the code's lack of adherence to PowerShell best practices made it very difficult to follow and contribute to. Contributors did, however, file issues for excessive CPU usage, file transfers not working, unexpected errors, and inoperable functionality; one pull-request also attempted to add SSL encryption. All of this inspired me to dismantle and rebuild their work from the ground up.

During my dismantling I discovered that calls were being made to external binaries. As an amateur PowerShell purist, I was determined to remove these external dependencies and replace them with equivalent .NET implementations. In particular, Netstat was being used to test for available ports and Nslookup was being used to craftily communicate with dnscat2 (very cool). I was able to write a helper function to replace Netstat. I ended up removing the dnscat2 functionality to exclude Nslookup, but I hope you'll agree that I made up for this by adding a covert channel of my own; more on that later.

Further along, I realized that the capture of command output had been over-engineered by redirecting the standard out/error of a process and attempting to asynchronously read from those streams. This required the use of additional functions and is the first contributor to excessive CPU usage. I found a simple remedy to this situation by running all user input in a scriptblock and letting PowerShell handle the output and errors for me.

What started as 40 lines of code...

...was reduced to less than 10 lines, and no extra function definitions.

Moreover, the excessive CPU usage arose from a by-design loop that continuously checks for completed asynchronous operations without any throttle control. You can test a simplified version of this yourself by opening Task Manager and having a look at PowerShell's CPU statistics while running the following one-liners.

With the CPU consumption cured, I was able to start sprinkling in my own improvements. Most notably, making up for the removal of dnscat2 functionality by adding in a covert communication channel that leverages SMB pipes for stealth. The beauty of SMB pipes is two-fold. First, they communicate over a port (445) that has to be open in a Windows domain and is usually open by default in non-domain scenarios. Second, all SMB pipe communications are owned by the System process; i.e. PID 4. This makes it especially difficult for investigators to determine that the connection was actually originating from PowerShell.
There is one caveat to the SMB pipe communications, in that the .NET classes supporting them weren't available until version 3.5.1. While this technically means that SMB pipes aren't fully supported in PowerShell v2, most machines that only have v2 installed also have .NET 3.5.1 installed. In my testing, the SMB pipes work flawlessly on Windows 7 with v2; but I have not tested back to Windows Vista or XP. With a covert comms-channel back in the mix, we can move on to weaponizing this code for field use.

Mick and Luke did a great job of packaging the code for execution and I used their implementation as the basis for my own. I've received several requests (and will likely oblige them in the near future) to package all of my version into a single script so that it will be more portable. My standard response to this request is that the packaging feature already exists via New-PowerCatPayload. This function was designed for weaponizing and outputs a customized script payload perfect for distribution to remote machines. Lets look at an example of how to do that, using WMI.
With just two lines, the payload is created and running by abusing the Create method of WMI's Win32_Process class. As shown above, Invoke-WmiMethod reports that it successfully created (ReturnValue = 0) a new process with PID 6800. The host PowerShell's PID is 2740 and the working directory is C:\Users, connecting to the PowerCat listener changes the working directory to C:\Windows\System32 and is now executing from PID 6800. A TCP-callback payload, pictured below, works a little differently. Create a new payload that specifies your machine as the -RemoteIp. Start your local TCP listener first, then use WMI to execute the TCP payload on a remote machine. In these examples everything is running on the local machine. To run this on a remote machine the -ComputerName parameter must be specified to Invoke-WmiMethod. This is a very simple way to move laterally through a network.
The final piece of this puzzle was to hide PowerCat's traffic from network detection mechanisms using encryption. In a pull-request to Mick and Luke's project there was an implementation of self-signed X509 certificate creation and SSL encryption, adding 595 lines of code to the project. I was quite pleased to discover that there is actually a COM interface for creating X509 certificates and was able to create my own helper function with only 50 lines of code. SSL, check. You can find the entire project here.

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