Decoding Encoded Cobalt Strike Payloads

When analyzing intrusions or working on threat hunting scenarios, one common challenge is dealing with obfuscated or encoded payloads from adversaries. Cobalt Strike—often used by red teams and, unfortunately, threat actors—frequently employs encoding to mask its true nature.

Understanding the Encoding

Payloads may be encoded using Base64, custom XOR schemes, or even layered encodings. Our goal is to identify these layers and systematically decode them until we reveal the core payload.

Tools & Techniques

To decode these payloads, you'll want a toolkit of methods:

• Strings & Hex Dumps: Start by inspecting the payload with strings or a hex editor. Look for suspicious markers like Base64 padding (= signs) or repeating patterns.
• Base64 Decoding: If you detect Base64 patterns, run base64 -d or use a Python one-liner to decode: echo 'encodedstring' | base64 -d
• XOR or Custom Encoding: Try common XOR keys, often single-byte, or attempt frequency analysis. A simple Python script can guess XOR keys if you suspect simple XOR:

  for key in range(256):
      decoded = ''.join(chr(ord(c) ^ key) for c in encoded_data)
      if 'MZ' in decoded: # Checks for PE header
          print("Possible key:", key)
  

  Layered Decoding

  Cobalt Strike payloads may have multiple layers. You might decode Base64, reveal a blob that is then XOR'ed, and once decoded, reveal another chunk that’s compressed. Each layer you peel back brings you closer to the original shellcode or PE file.

  Practical Example

  Consider a payload that after strings inspection shows a large Base64 block. Decode that Base64 and you get a binary blob that makes little sense. Attempt XOR with common keys (0x50, 0x00, etc.) until you see 'MZ'. Once you see that MZ signature, you know you've uncovered a Windows PE file—very likely the Cobalt Strike beacon DLL.

  Conclusion

  By systematically applying decoding steps—Base64 decoding, XOR key guessing, layer stripping—you can uncover the original Cobalt Strike payload. Understanding these techniques is crucial for digital forensics, threat hunting, and incident response, allowing you to reveal the adversary’s toolkit and respond effectively.