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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.exeThen 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.
Example:
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
My new version of plugin_biff.py has a new option: –hexrecord.
Here I’ll show how I use this to remove the sheet protection from malicious spreadsheets.
If you want to open a malicious spreadsheet (for example with Excel 4 macros) in a sandbox, to inspect its content with Excel, chances are that it is protected.
I’m not talking about encryption (this is something that can be handled with my tool msoffcrypto-crack.py), but about sheet protection.
Enabling sheet protection can be done in Excel as follows:
Although you have to provide a password, that password is not used to derive an encryption key. An .xls file with sheet protection is not encrypted.
If you use my tool oledump.py together with plugin_biff.py, you can select all BIFF records that have the string “protect” in their name or description (-O protect). This will give you different records that govern sheet protection.
First, let’s take a look at an empty, unprotected (and unencrypted) .xls spreadsheet. With option -O protect I select the appropriate records, and with option -a I get an hex/ascii dump of the record data:
We can see that there are several records, and that their data is all NULL (0x00) bytes.
When we do the same for a spreadsheet with sheet protection, we get a different view:
First of all we have 4 extra records, and their data isn’t zero: the flags are set to 1 (01 00 little-endian) and the Protection Password data is AB94. That is the hash of the password (P@ssw0rd) we typed to create this sheet.
To remove this sheet protection, we just need to set all data to 0x00. This is something that can be done with an hex editor.
First use option -R instead of option -a:
This will give you the complete records (type, length and data) in hexadecimal. Next you can search for each record using this hexadecimal data with an hex editor and set the data bytes to 0x00.
Searching for the first record 120002000100:
Setting the data to 0x00: 0100 -> 0000
Do this for the 4 records, and then save the spreadsheet under a different name (keep the original intact).
Now you can open the spreadsheet, and the sheet protection is gone. You can now unhide hidden sheets for example.
Quickpost info