Detecting Cobalt Strike Beacons

Introduction

Cobalt Strike is a commercial tool for adversary simulation.

Created by Raphael Mudge in 2012, Cobalt Strike was one of the first public red team command and control frameworks.

It’s also used by threat actors and the first activity detected for malicious purposes dates back to 2016.

The main components of Cobalt Strike are a C2 server and a beacon installed on compromised machines.

The Cobalt Strike beacon comes with a number of capabilities, including a command-line interface. The beacon allows the execution of scripts, or commands native to the machine’s operating system.

Cobalt strike for malicious purposes is known to be used by more than 50 threat actor groups. Its use is very common in ransomware attacks.

In terms of beacon types and methods used for connecting to the C2 servers the most common are HTTP (~67%), HTTPS (~29%) and DNS (~3%). Including JITTER in the beacon trying to avoid detection has been detected in roughly 15% of all the beacons analysed.

When it comes to spawning, around 90% of all beacons analysed were spawning rundll32.exe as the main process for lateral movement.

Sophisticated attacks using Cobalt strike beacons, like Nobelium usage of Cobalt strike linked to the SolarWinds campaign, try to evade detection by fine-tuning many of the configurable options.

PREVENTION, DETECTION AND RESPONSE TOOLS

  • Static File Hash analysis: Windows Defender.
  • Endpoint Telemetry: Sysmon (Sysinternals).
  • EDR agent and SIEM: Wazuh.
  • Threat Intel platform: MISP

PREVENTION.

Many of the Cobalt Strike beacons in the wild and additional payloads downloaded as part of the attack chain are going to be flagged and removed by Windows Defender via local file analysis (file hash) or its real time analysis engine.

From a prevention point of view some key points are:

  • Collecting, reviewing and alerting on Windows Defender Settings not matching minimum criteria (Real Time, Enabled, Exclusion extensions, Excluded folders, removable media, etc.). Deploy an inventory collection script (more info here) and alert on inadequate settings.
Windows Defender Settings
  • Ensuring that the antimalware platform is running and in a healthy state
Information level events — Process running and in healthy state
  • Ensuring that Windows Defender is successfully downloading and updating the latest file signatures
Information level events — Signatures Updated
  • Ensuring that periodic scans are run in all agents and completed successfully.
Information level events — Periodic scans

DETECTION.

Network Activity

Communication to C2 Server:

  • Suspicious processes (“winlogon.exe”, “rundll32.exe”, etc.) opening network connections to public IP addresses.

Detection Logic

Keep track and alert on unusual processes (least seen) opening network connections. Special attention to HTTP, HTTPS and DNS outbound connections. Sysmon (event ID 3) provides all the required telemetry:

Network Activity — Least Seen Processes

Visualisations such as network maps linking Process — DST IP — Connection port can help to quickly identify anomalies:

Network Activity — Process Map

Keep track of downloaded payloads (.js files, .gif files, etc.). Sysmon (Event ID 15) provides this telemetry.

Network Activity — Sysmon Event 15

Send relevant observables (DNS requests, Destination IPs, files hashes of downloaded files) to security feeds/threat intel platform to identify IoCs related to these observables. More info here.

Process Activity.

rundll32.exe:

Used in Lateral Movement by Cobalt Strike beacons: rundll32.exe being spawned by another process(es) and its process execution included no arguments.

Detection Logic

Keep track and alert on unusual processes spawning rundll32.exe

Processes Spawning Rundll32.exe

Used in Post Exploitation in Cobalt strike related attacks: rundll32.exe spawning processes like Adfind.exe, Net.exe, or any other Windows processes used for systems, services or network discovery.

Parent Process Spoofing:

Parent process spoofing is a common technique used by Cobalt Strike beacons. With this technique the beacon tries to evade common detection methods such as processes related to the Office suite launching unusual child processes.

Cobalt strike beacons often spoof processes like “word.exe”, or “excel.exe” to “explorer.exe” so that when the child process is launched the telemetry reported by the EDR agent makes the detection of unusual process chains difficult.

Detection Logic

Keep track on unique file hashes (process image) and their mapping to file process image name and location. Spoofed processes will have same process name but different file hash and possibly executed from an unusual location.

UNIQUE RELATIONSHIP PROCESS FILE HASH — PROCESS EXECUTED

Send relevant observables (file hash of the executed file image) to security feeds/threat intel platform to identify IoCs related to these observables. More info here.

Powershell execution (and command line arguments).

Suspicious command line arguments used:

nop, hidden, encodedcommand, nologo, noprofile

The “encodedcommand” (base64) can be extracted and its literal content further analysed, looking for commands like “Net.Webclient”, “Invoke-WebRequest”, etc., commonly used for lateral movement on the source machine.

On the destination machine, a common detection is spotting powershell executions where the parent process = “wsmprovhost.exe” and with a command line = “-Version 5.1 -s -nologo -noprofile”

Detection Logic

Detection rule to spot powershell executions with the encodedcommand command line and decode the content of the base64 string to “clear text”. Include the decoded command as an additional alert in Wazuh manager.

Memory artifacts / Process Injection.

Downloaded payloads try to be executed under the memory space of “Rundll32.exe”.

Detection Logic

Keep track and alert on unsigned DLLs or with no valid certificate loaded into memory. Sysmon (Event ID 7) provides this telemetry:

Unsigned DLLs loaded in memory.

Visualisations such as DLL side loading maps linking Process —DLL — DLL vendor can help to quickly identify anomalies:

Process and DLL side loading map.

Send relevant observables (DLL file hash) to security feeds/threat intel platform to identify IoCs related to these observables. More info here.

Lateral Movement.

PSEXEC is one of the most common processes used by Cobalt strike beacons for lateral movement.

PSEXEC is used to drop a payload in a shared folder (normally ADMIN$) and then to start a new service on the target machine that executes that payload. The payload will spawn another process and finally the remote service is removed.

Detection Logic

Detection rules based on frequency alerting on service creation/modification/deletion activity:

System Services Activity and Telemetry.

Keep track and alert on unusual executables launched by services.exe.

Send relevant observables (process file hash) to security feeds/threat intel platform to identify IoCs related to these observables. More info here.

Need Help?

The functionality discussed in this post, and so much more, are available via the SOCFortress platform. Let SOCFortress help you and your team keep your infrastructure secure.

Website: https://www.socfortress.co/

Platform Demo: https://www.socfortress.co/demo_access.html

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