h1

Kits, Profiles, and Scripts… Oh my!

October 3, 2017

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

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.

Applet Kit

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.

Artifact Kit

Introduced in January 2014, the Artifact Kit controls Cobalt Strike’s process to generate executable and DLL files.

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.

Resource Kit

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.

Elevate Kit

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.

Custom Reports

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.

The Aggressor Script documentation covers Custom Reports and hosts the source code for the built-in reports 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.

External C2

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.

Closing Thoughts

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.

h1

Cobalt Strike 3.9 – Livin’ in a Stager’s Paradise

September 20, 2017

Cobalt Strike 3.9 is now available. This release brings several additions to Malleable C2 with an emphasis on staging flexibility.

Malleable HTTP/S Staging

Stagers are tiny programs that download the Beacon payload and pass control to it. Stagers are a way to use a size-constrained attack to deliver a large payload like Beacon. While I recommend working stageless, stagers are helpful in some situations. Wouldn’t it be nice if you could disguise staging to look like something else? That’s possible now.

This release introduces Malleable C2 flexibility into Beacon’s HTTP and HTTPS stagers. Cobalt Strike 3.9 profiles may modify the HTTP staging URI, add client headers, add URI parameters, and place arbitrary data before or after the encoded payload stage. Here’s the Malleable C2 profile for the above screenshot:

http-stager {
	set uri_x86 "/_init.gif";
	set uri_x64 "/__init.gif";

	client {
		parameter "key1" "value1";
		parameter "key2" "value2";
		header "Host" "yeah this works too";
	}

	server {
		header "Content-Type" "image/gif";

		output {
			prepend "\x01\x00\x01\x00\x00\x02\x01\x44\x00\x3b";
			prepend "\xff\xff\xff\x21\xf9\x04\x01\x00\x00\x00\x2c\x00\x00\x00\x00";
			prepend "\x47\x49\x46\x38\x39\x61\x01\x00\x01\x00\x80\x00\x00\x00\x00";
			print;
		}
	}
}

And, here’s a recorded demonstration:

More Malleable C2 Features

While the HTTP staging gains the most flexibility in this release, 3.9 enhances Malleable C2 in other ways too.

The dns_stager_prepend option places a string before the encoded payload stage delivered via DNS TXT records. This offsets the content in this process and pushes back on signatures that target fixed TXT records in Cobalt Strike’s DNS staging process.

set dns_stager_prepend "v=spf1 mx include:_spf.google.com -all other:";

This release adds an obfuscate setting to the Malleable PE directives. This option masks the Beacon DLL’s import table. Together, the obfuscate setting and strrep (introduced in 3.7), give you a lot of control over which strings are visible in the Beacon stage.

stage {
	set obfuscate "true";

And, Malleable C2 gains a mask statement for its data transform blocks. The mask statement generates a random 4-byte value, masks your data with this value, and prepends this 4-byte value to the masked data. This last step makes it possible to reverse the mask step. The mask statement is interpreted and applied with each Beacon transaction. The mask statement makes it possible to randomize parts of your profile.

Authorization Files

The licensed version of Cobalt Strike 3.9 and later now requires an authorization file to start. The update program, distributed with the Cobalt Strike trial, downloads this authorization file.

The authorization file includes your license expiration date and a unique customer ID. Cobalt Strike 3.9 and later embeds this ID into the Beacon payload stage and any stagers generated by Cobalt Strike. This value is the last 4-bytes of the Beacon payload stager.

Check out the release notes to see a full list of what’s new in Cobalt Strike 3.9. Licensed users will need to download the 3.9 trial and use the updated update program to get the latest. A 21-day Cobalt Strike trial is also available.

h1

Living the Ghost Life: Announcing Specter Ops, Inc.

July 10, 2017

Have you seen this cute ghost inside of a hexagon? The logo is for Specter Ops, Inc., a new cyber-security consulting firm. Today’s the company’s launch day. The press release is here. The website is here.

Today, Specter Ops, Inc. is 13 people who have given me technical guidance on Cobalt Strike, trained Cobalt Strike users, and made amazing contributions to the security community (to include open source projects and content Cobalt Strike users benefit directly from).

In terms of services: Specter Ops, Inc. has three focus areas: We offer adversary simulation services (red team ops), breach assessments (hunt), and adversary resilience assessments. The last one is particularly interesting. Resilience assessments help harden large Active Directory environments by identifying lateral movement vectors and measuring an organization’s adherence to the principle of least privilege. It’s good stuff. We offer assessments, staff augmentation, training, and program development in each of these areas.

The firm is new, but if you check out the team page, you’ll see there are several familiar faces. You’ll also note I said “we”. I have responsibilities at Specter Ops, Inc. as well. I’m the company’s President and my duties include: strategic guidance, mentoring, and starting conversations with “I remember when I was technical…”.

Day-to-day, I’m still in a Strategic Cyber LLC office space. I continue to argue with procurement people about contract details (yes, really), provide support to my users, and develop Cobalt Strike. This continues to happen under the Strategic Cyber LLC banner. Nothing’s changed here.

For Cobalt Strike users, this is exciting. Specter Ops, Inc. is an official “go to” for Cobalt Strike training and offers services that Strategic Cyber LLC never had the man-power to do. If you’d like engage Specter Ops, Inc., direct your queries to info@specterops.io.

I want to say thank you to each member of the Specter Ops, Inc. team. Thank you for taking the next steps of your career with this firm. I look forward to building an amazing company with you and I’m very excited for the things that will come next. I’m also very proud of all of you for pulling together this launch.

Onwards!

h1

OPSEC Considerations for Beacon Commands

June 23, 2017

A good operator knows their tools and has an idea of how the tool is accomplishing its objectives on their behalf. This blog post surveys Beacons commands and provides background on which commands inject into remote processes, which commands spawn jobs, and which commands rely on cmd.exe or powershell.exe.

API-only

These commands are built-into Beacon and rely on Win32 APIs to meet their objectives.

cd
cp
download
drives
exit
getuid
kerberos_ccache_use
kerberos_ticket_purge
kerberos_ticket_use
jobkill
kill
link
ls
make_token
mkdir
mv
ppid
ps
pwd
rev2self
rm
rportfwd
socks
steal_token
timestomp
unlink
upload

House-keeping Commands

The following commands are built into Beacon and exist to configure Beacon or perform house-keeping actions. Some of these commands (e.g., clear, downloads, help, mode, note) do not generate a task for Beacon to execute.

cancel
checkin
clear
downloads
help
jobs
mode dns
mode dns-txt
mode dns6
mode http
note
powershell-import
sleep
socks stop
spawnto

Post-Exploitation Jobs (Process Execution + Remote Process Injection)

Many Beacon post-exploitation features spawn a process and inject a capability into that process. Beacon does this for a number of reasons: (i) this protects the agent if the capability crashes, (ii) this scheme makes it seamless for an x86 Beacon to launch x64 post-exploitation tasks. The following commands run as post-exploitation jobs:

browserpivot
bypassuac
covertvpn
dcsync
desktop
elevate
hashdump
keylogger
logonpasswords
mimikatz
net
portscan
powerpick
psinject
pth
screenshot
shspawn
spawn
ssh
ssh-key
wdigest

OPSEC Advice: Use the spawnto command to change the process Beacon will launch for its post-exploitation jobs. The default is rundll32.exe (you probably don’t want that). The ppid command will change the parent process these jobs are run under as well.

Process Execution

These commands spawn a new process:

execute
runas
runu

OPSEC Advice: The ppid command will change the parent process of commands run by execute. The ppid command does not affect runas or spawnu.

Process Execution: Cmd.exe

The shell command depends on cmd.exe.

The pth and getsystem commands get honorable mention here. These commands rely on cmd.exe to pass a token to Beacon via a named pipe.

OPSEC Advice: the shell command uses the COMSPEC environment variable to find the preferred command-line interpreter on Windows. Use Aggressor Script’s &bsetenv function to point COMSPEC to a different cmd.exe location, if needed. Use the ppid command to change the parent process the command-line interpreter is run under. To pth without cmd.exe, execute the pth steps by hand.

Process Execution: PowerShell.exe

The following commands launch powershell.exe to perform some task on your behalf.

powershell
spawnas
spawnu
winrm
wmi

OPSEC Advice: Use the ppid command to change the parent process powershell.exe is run under. Be aware, there are alternatives to each of these commands that do not use powershell.exe:

  • spawnu has runu which runs an arbitrary command under another process.
  • spawnas has runas which runs an arbitrary command as another user.
  • powershell has powerpick, this command runs powershell scripts without powershell.exe.
  • It’s also possible to laterally spread without the winrm and wmi commands.

Remote Process Injection

The post-exploitation job commands (previously mentioned) rely on process injection too. The other commands that inject into a remote process are:

dllinject
inject
shinject

Service Creation

The following internal Beacon commands create a service (either on the current host or a remote target) to run a command. These commands use Win32 APIs to create and manipulate services.

getsystem
psexec
psexec_psh

h1

Cobalt Strike 3.8 – Who’s Your Daddy?

May 23, 2017

Cobalt Strike 3.8 is now available. This release adds features to spawn processes with an alternate parent process. This release also gives the operator control over the script templates Cobalt Strike uses in its attacks and workflows.

Processes with Alternate Parents

A favorite hunt technique is to instrument a host to report all new processes, their arguments, and the parent process. Hunt operators (and automated solutions) separate the noise from the interesting by looking for odd parent/child process relationships.

This release of Cobalt Strike pushes back on this technique with the ppid command. The PPID command tasks Beacon to launch cmd.exe, powershell.exe, and other processes with an alternate parent. This feature takes advantage of an API, introduced with Windows Vista, to enable consent.exe to launch elevated processes with the non-elevated requester as the parent.

This opens a lot of possibilities. For example, if I’m in a user context, I might set explorer.exe as my parent with something plausible (e.g, iexplore.exe) for my temporary processes. If I’m in a SYSTEM context, I might use services.exe as my parent process and ask Beacon to use svchost.exe for its temporary processes.

To benefit from the ppid command, your session must have rights to access the parent process. I also recommend that you specify a parent process that exists in the same desktop session. If you don’t, random commands and workflows may fail.

Another way to hop Desktop Sessions

It’s possible, with a few extra steps, to run commands under a parent that lives in another desktop session. Programs run this way will take on the rights and identity of their parent.

Beacon’s runu command runs an arbitrary command as a child of another parent. This command takes the necessary extra steps to do this across session boundaries.
The spawnu command builds on this primitive to spawn a session with powershell.exe.

These commands offer means to spawn a payload, in another desktop session, without remote process injection. As detection of remote process injection becomes more common, it’s important to have other ways to achieve our goals without this offensive technique.

The Resource Kit

Cobalt Strike 3.8’s Resource Kit finally gives you a way to change Cobalt Strike’s built-in script templates! The Resource Kit is a collection of Cobalt Strike’s default script templates and a sample Aggressor Script to bring these into Cobalt Strike. Go to Help -> Arsenal from a licensed copy of Cobalt Strike to download the Resource Kit.

The Resource Kit benefits from new Aggressor Script hooks to provide the PowerShell, Python, and VBA script templates Cobalt Strike uses in its workflows.

Check out the release notes to see a full list of what’s new in Cobalt Strike 3.8. Licensed users may use the update program to get the latest. A 21-day Cobalt Strike trial is also available.

h1

Java Startup Bug in Java 1.8u131

April 26, 2017

If you recently updated your penetration testing environment, it’s possible you were greeted with a special surprise. Cobalt Strike and its team server will no longer start.

Instead of Cobalt Strike, you’re now greeted with this very intuitive and helpful error: The Parallel GC can not be combined with -XX:ParallelGCThreads=0.

I’ve had a few emails about this. My first answer: I have no idea what that means. Now, I have an answer! This is a known bug in Java 1.8u131. This recent update to Oracle’s Java introduces a change that breaks the -XX:+AggressiveHeap command line option Cobalt Strike uses. This command line option is not uncommon in the Java world and other applications are affected.

The Java team is aware of this bug and it has a priority level 2. This is the level reserved for Crashes, losses of data, and severe memory leaks. They’re taking it seriously and I expect this problem to go away in a coming Java update.

On Linux, one way to work around this Oracle Java bug is to update the cobaltstrike and teamserver scripts to specify the -XX:ParallelGCThreads=8 option after the java command.

I advise that you stay away from Oracle Java 1.8u131. If you already updated to Java 1.8u131, then downgrade to Java 1.8u121.

What about OpenJDK? I continue to recommend Oracle’s Java distribution for use with Cobalt Strike. Oracle’s Java distributions go through a series of acceptance tests to make sure the build is sane. This isn’t always the case with OpenJDK builds/packages. This has led to serious issues in the past.

Update May 23, 2017: Cobalt Strike 3.8 includes a work-around for this issue in its Linux, MacOS X, and Windows launchers. Download the 3.8 trial package to get the latest version of these launchers. This will address the issue.

h1

Cobalt Strike 3.7 – Cat, Meet Mouse

March 15, 2017

The 8th release of the Cobalt Strike 3.0 series is now available. The release extends Malleable C2 to influence how Beacon lives in memory, adds code-signing for executables, and gives operators control over which proxy server Beacon uses. There’s a lot of good stuff here. Let’s dig into it.

Malleable PE

A key goal of Cobalt Strike is to challenge analysts and keep the toolset interesting as they and their capabilities evolve. Many forward leaning security programs rely on memory forensics to detect and respond to actors with capabilities similar to and beyond Cobalt Strike. This release adds some flexibility in this area.

Cobalt Strike 3.7’s Malleable C2 stage block specifies how Beacon lives in memory through changes to Beacon’s Reflective DLL stage. Here’s what this looks like:

stage {
	set userwx "false";
	set compile_time "14 Jul 2009 8:14:00";
	set image_size_x86 "512000";
	set image_size_x64 "512000";
	
	transform-x86 {
		prepend "\x90\x90"; # NOP, NOP!
		strrep "ReflectiveLoader" "DoLegitStuff";
	}

	transform-x64 {
		# transform the x64 rDLL stage
	}
}

Let’s start with permissions: Something magical happens when an analyst sweeps processes for RWX pages. Payloads fall out of the memory. Stagers fall out too. This is because many offensive tools use these liberal permissions, even when they’re not needed. The userwx option gives you control over this. Set userwx to false and Beacon’s Reflective DLL Loader will avoid these permissions.

Veteran analysts with multiple rounds of Cobalt Strike experience may know the size of Beacon’s reflective DLL in memory. I’ve heard 0x42000 thrown around many times. This is the SizeOfImage value in Beacon’s PE header. The image_size_x86 and image_size_x64 options control this value. If you emulate a specific threat actor, consider a SizeOfImage value that matches their malware.

Finally, in the cyber, attribution matters. Nothing feeds fantastical pet attribution theories quite like when the actor compiled their malware. The compile_time option changes this value in Beacon’s Reflective DLL header.

The transform-x86 and transform-x64 blocks pad and transform Beacon’s stage. If you choose to prepend data, make sure it’s valid code for the stage’s architecture. There’s no check for this.

The Malleable C2 documentation has more information on these new options.

Code Signing

This release adds a code signing capability to Cobalt Strike. This feature requires a valid code-signing certificate stored in a Java Keystore file. Those of you who’ve signed Cobalt Strike’s Java Applet already have one of these keystores available. Use Malleable C2’s code-signer block to tell Cobalt Strike about your code-signing certificate.

# setup code-signing certificate for our EXEs and DLLs
code-signer {
	set keystore "keystore.jks";
	set password "password";
	set alias    "server";
}

After you specify a certificate, the Sign executable file box in Attacks -> Packages -> Windows EXE and Attacks -> Packages -> Windows EXE (S) becomes available. Check this box and Cobalt Strike will sign your artifact. It’s as easy as that!

Proxy Override

I’ve had many requests for a way to specify alternate proxy settings for Beacon. This release adds user-specified proxy settings to Beacon’s stageless artifacts. Go to Attacks -> Packages -> Windows EXE (S) and press next to the proxy field to configure this option.

You may leave these proxy settings as-is (default), instruct Beacon to connect directly, or make Beacon use the proxy configuration and credentials that you specify. These options should help in a few situations.

Check out the release notes to see a full list of what’s new in Cobalt Strike 3.7. Licensed users may use the update program to get the latest. A 21-day Cobalt Strike trial is also available.