Didier Stevens

Monday 26 April 2021

Quickpost: Decrypting Cobalt Strike Traffic

Filed under: Encryption,Malware,My Software,Quickpost — Didier Stevens @ 0:00

I have been looking at several samples of Cobalt Strike beacons used in malware attacks. Although work is still ongoing, I already want to share my findings.

Cobalt Strike beacons communicating over HTTP encrypt their data with AES (unless a trial version is used). I found code to decrypt/encrypt such data in the PyBeacon and Geacon Github repositories.

This code works if you know the AES key: which is not a problem in the use cases of the code above, as it is developed to simulate a beacon. Beacons generate their own AES key, and thus these beacon simulations also generate their own AES key.

But what if you’re analyzing real beacons used in malware attacks? How do you obtain the AES key?

I found a way to extract the keys (AES and HMAC) from process memory of a running beacon.

I use the following procdump command to prepare process memory dumps:

procdump -mp -w -s 1 -n 5 malware.exe

Then I start the beacon malware.exe in a malware analysis virtual machine while capturing traffic with Wireshark.

My new tool cs-extract-key.py looks in the dumped process memory for the unencrypted (RSA encryption) metadata that a beacon sends to the C2. This metadata contains the AES en HMAC keys.


This method does not always work: the metadata is overwritten after some time, so the process dump needs to be taken quickly after the beacon is started. And there are also cases were this metadata can not be found (I suspect this is version bound).

For those cases, my tool has another way of obtaining the keys. I extract the encrypted data of the first post of the beacon to the C2 (this is called a callback in the PyBeacon code):

And then I provide this to my tool, together with the process dump. My tool will then proceed with a dictionary attack: extract all possible AES and HMAC keys from the process dump, and try do authenticate and decrypt the callback. If this works, the keys have been found:

And once I have obtained the keys, I can pass them to my traffic decoding program that I have updated to include decryption (and that I have renamed to cs-parse-http-traffic.py):

Quickpost info

Sunday 25 April 2021


Filed under: My Software,video — Didier Stevens @ 10:13

This is a new tool (beta) to analyze ISO files. I made this for a webinar I presented: a demo on how to use my templates to create your own tools.

isodump.py is in my Github beta repository.

The complete webinar is here, if you want to jump directly to the demo where I explain how to make a tool like isodump.py, go here.

Monday 19 April 2021

Lua CSV Wireshark Dissector

Filed under: My Software,Networking,Wireshark — Didier Stevens @ 0:00

In December 2020 I provided online Wireshark training to one of our NVISO clients. During the second day, when we cover the development of custom dissectors written in Lua, a question about CSV data came up. When the data exchanged over TCP, for example, has the CSV format (fields separated by a separator), how can I write a dissector for that?

While answering the question, I realized that this is a case that could be solved with a generic dissector. And the same night, I developed the first version.

Say you have a packet capture with a TCP connection. And the data exchanged over TCP consists of different fields, separated by a separator character.

Like this example:

Because Wireshark does not recognize the protocol used in this TCP connection, the content is just displayed as data.

With Lua dissector csv-dissector.lua, the data is dissected into different fields:

The separator character (pipe character | in this example) is something that can be configured:

Other changes can be made, but these have to be made in the code of the dissector itself:

  • Changing the port
  • Changing the number of fields
  • Change the name of the fields



csv_dissector_V0_0_2.zip (https)

MD5: E8CCE089FB0574775AB39DADED3B7AA2

SHA256: 5C8DC0F2BB97AA660E2576B23379B6F12FB88126F0EFC7A2F69E76EBA8E782BD

Sunday 18 April 2021


Filed under: Announcement,My Software — Didier Stevens @ 17:56

metatool.py is a tool to help with the analysis of Metasploit or Cobalt Strike URLs.

More info can be found in my SANS Internet Storm Center diary entry “Finding Metasploit & Cobalt Strike URLs“.

It is still in my Github beta repository here.

Thursday 1 April 2021

Overview of Content Published in March

Filed under: Announcement — Didier Stevens @ 0:00

Here is an overview of content I published in March:

Blog posts:

YouTube videos:

Videoblog posts:

SANS ISC Diary entries:

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