If I had to describe Cobalt Strike in one word, I’d say ‘flexible’. There are a lot of options to control Cobalt Strike’s features and indicators. In this post, I’ll introduce these options, explain the rationale for each, and point you to resources to explore them further.
Aggressor Script is Cobalt Strike’s built-in scripting language. It is the preferred way to add features to Cobalt Strike, override existing behaviors (kits take advantage of this), and automate your engagements.
Several public scripts add new workflows and features to Cobalt Strike. For example, CACTUS TORCH adds user-driven attack options to Cobalt Strike with x64 and stageless variations too. Tyler Rosonke wrote a script to add persistence options for Beacon.
Cobalt Strike also ships with a headless client, agscript, that connects to a team server and hosts an Aggressor Script for you. This client is designed for long-running bots. Common uses of headless Aggressor Scripts is to force DNS beacons to “check in” or notify an operator, via a text or email, that they have a new session.
If you’d like to dig deeper into Aggressor Script, jump over to the Aggressor Script documentation. I also regularly post Aggressor Script snippets as Github gists. Finally, Lee Kagan has created an Aggressor Scripts Collection that aggregates many of the publicly available scripts into one place.
A kit is source code to a Cobalt Strike feature coupled with a script that forces Cobalt Strike to use your implementation over the built-in one. Kits give you control over the artifacts and processes that deliver the beacon payload.
The concept of kits in Cobalt Strike came out of necessity.
For a long time, Java Signed Applets and Java Applet exploits were a staple client-side attack option. In January 2013, I implemented Cobalt Strike-native versions of these attacks. The Smart Applet attack bundled several Java exploits into one package. The Java Signed Applet attack hosted a self-signed Java applet on Cobalt Strike’s web server. If a vistor let the applet run, it’d result in code execution for the attacker.
While the above options were great, at the time, my users needed an option to modify these attacks to evade detection. This is where the Applet Kit came in. The Applet Kit is the source code to Cobalt Strike’s Java Applet attacks. Included with the Applet Kit is an applet.cna script. When you load this script, Cobalt Strike uses your applet attacks instead of its built-in options.
The Applet Kit is available from the Cobalt Strike arsenal. This is a one-page site available to licensed Cobalt Strike users. Go to Help -> Arsenal from Cobalt Strike to reach it.
As Java in the browser became more constrained, Cobalt Strike users would often sign the built-in Java Signed Applet attack with their code-signing certificate. The use of a valid code-signing certificate kept this attack alive past its expected expiration date.
The contract of the Artifact Kit is simple. Cobalt Strike provides shellcode and meta-information to a scripted function. The scripted function is responsible for returning an executable or DLL that runs that shellcode.
The Artifact Kit is also available from the Cobalt Strike arsenal. The arsenal hosts my implementation of the default artifacts in Cobalt Strike. A few variations are available in the Artifact Kit distribution as well.
To use the Artifact Kit: download the default implementation, make changes, build it, and load the artifact.cna script that registers itself to handle executable and DLL file requests in your Cobalt Strike.
Many Cobalt Strike attacks and workflows take advantage of PowerShell, Python, and VBA scripts to get the job done. The Resource Kit controls the PowerShell, Python, and VBA script templates in Cobalt Strike.
Again, the contract here is simple. Cobalt Strike provides a registered script with shellcode, meta-information, and a description of what it wants. The registered script returns a script that executes the shellcode.
The Resource Kit is also available in the Cobalt Strike Arsenal.
Many Cobalt Strike users combine the Resource Kit with Invoke-Obfuscation to make Cobalt Strike’s PowerShell scripts much less obvious.
It’s a goal of Cobalt Strike to make it easy to combine your team’s “secret sauce” with the toolset. One spot where this comes together well is privilege escalation. Aggressor Script exposes APIs that allow scripts to register privilege escalation exploits with Beacon’s elevate command and Elevate Privileges dialog.
The Elevate Kit is a collection of public privilege escalation exploits integrated with Cobalt Strike via these APIs. The Elevate Kit demonstrates how to integrate Reflective DLL implementations of privilege escalation attacks from the Metasploit Framework. It also shows how to repurpose attack POCs implemented in PowerShell as well.
The Elevate Kit is hosted on Github. Load the elevate.cna script and you’re ready to go. Don’t be afraid to extend or add to the Elevate Kit. It’s pretty easy. During a recent cyber exercise, I was able to recompile a POC from Github as a Reflective DLL and fire it with Cobalt Strike. The entire process took less than 30 minutes.
Cobalt Strike’s built-in reports are designed to convey red team activities and indicators to a blue team training audience. While the built-in reports are serviceable, it’s not well-known that you can write custom reports for Cobalt Strike too.
I’ve used this feature to generate variations of Cobalt Strike’s built-in reports, split up by IP address ranges, to give tailored information to the blue teams at a large cyber defense exercise.
I’ve had quite a few requests for third-party command and control options with Cobalt Strike’s Beacon payload. The External C2 specification (November 2016) was my answer to these requests.
External C2 documents how to control Beacon over a named pipe and provides a TCP/IP interface to configure an SMB Beacon stage, receive it, and relay traffic between the SMB Beacon and Cobalt Strike. How this traffic is transported and relayed is up to your imagination.
I never announced External C2 as a feature. I wrote the specification, implemented it, and distributed it to customers who requested this feature. I wanted to see what (if anything) these users would do with the specification.
The fine folks at Outflank B.V. were the first, that I know of, to build and use an external C2 with Cobalt Strike. They contacted me to share the success story from one of their engagements. They also asked if (and when), they could publish a blog post to share their code and document the feature. This led to the Cobalt Strike over external C2 – beacon home in the most obscure ways post on their blog. Their External C2 uses a corporate file server as a dead drop for communication between a hard-to-reach target and their Beacon controller. Their external_c2 source code is on Github too.
Shortly after Outflank’s post, MWR Labs posted their thoughts on External C2 and demonstrated a POC to control Beacon via Office 365 Tasks. In both cases, I’m very impressed and I find these first results encouraging. Needless to say, even though it’s not announced, the External C2 specification is public and is implemented as-described in Cobalt Strike today.
Malleable C2 Profiles
Malleable C2 profiles control the indicators and behaviors in the Beacon payload and its stagers. I consider Malleable C2 the most important technology in Cobalt Strike today.
I introduced Malleable C2 as part of Cobalt Strike 2.0 (July 2014). The first release of Malleable C2 controlled the indicators in Beacon’s HTTP communication. Malleable C2 made it possible to use Beacon, but look like the havex trojan or something completely innocuous.
Today, Malleable C2 isn’t just network traffic. Malleable C2 profiles control which SSL certificate Cobalt Strike uses. Profiles also specify the code-signing certificate used to sign executables and DLLs. Malleable C2 profiles have options to influence Beacon’s memory indicators too.
I very much intended Malleable C2 as a threat emulation technology, but it’s much more than that. I didn’t imagine domain fronting when I added Malleable C2 to Cobalt Strike. Yet, when this technique became known, Cobalt Strike was a go-to platform to take advantage of it. Today, I heard about a customer using the string replace feature (for Beacon’s stage) to alter how Beacon runs PowerShell scripts. Again, I wouldn’t have thought of that.
The Malleable C2 Profiles Github repository has several example profiles to start with. You can use one of these, but the barrier to making your own “never seen” profile is very low. I recommend reading the Malleable C2 documentation as well.
Today, there are few things in Cobalt Strike that users don’t have direct control over. Through these tools you may add to Cobalt Strike’s features, modify behaviors that get in your way, change the files that deliver the Beacon payload, and edit the product’s indicators. As red team needs and tradecraft evolve, this flexibility is how Cobalt Strike keeps pace.