☑️Evasion Tools

If we are dealing with advanced security tools, we may not be able to use basic, manual obfuscation techniques. In such cases, it may be best to resort to automated obfuscation tools. This section will discuss a couple of examples of these types of tools, one for Linux and another for Windows.

Linux (Bashfuscator)

git clone https://github.com/Bashfuscator/Bashfuscator
cd Bashfuscator
pip3 install setuptools==65
python3 setup.py install --user

cd ./bashfuscator/bin/
./bashfuscator -h

And then we can add the command we want to obfuscate with -c flag:

anonmak9@htb[/htb]$ ./bashfuscator -c 'cat /etc/passwd'

[+] Mutators used: Token/ForCode -> Command/Reverse
[+] Payload:
 ${*/+27\[X\(} ...SNIP...  ${*~}   
[+] Payload size: 1664 characters

However, running the tool this way will randomly pick an obfuscation technique, which can output a command length ranging from a few hundred characters to over a million characters! So, we can use some of the flags from the help menu to produce a shorter and simpler obfuscated command, as follows:

anonmak9@htb[/htb]$ ./bashfuscator -c 'cat /etc/passwd' -s 1 -t 1 --no-mangling --layers 1

[+] Mutators used: Token/ForCode
[+] Payload:
eval "$(W0=(w \  t e c p s a \/ d);for Ll in 4 7 2 1 8 3 2 4 8 5 7 6 6 0 9;{ printf %s "${W0[$Ll]}";};)"
[+] Payload size: 104 characters

We can now test the outputted command with bash -c '', to see whether it does execute the intended command:

anonmak9@htb[/htb]$ bash -c 'eval "$(W0=(w \  t e c p s a \/ d);for Ll in 4 7 2 1 8 3 2 4 8 5 7 6 6 0 9;{ printf %s "${W0[$Ll]}";};)"'

root:x:0:0:root:/root:/bin/bash
...SNIP...

Windows (DOSfuscation)

There is also a very similar tool that we can use for Windows called DOSfuscation. Unlike Bashfuscator, this is an interactive tool, as we run it once and interact with it to get the desired obfuscated command. We can once again clone the tool from GitHub and then invoke it through PowerShell, as follows:

PS C:\htb> git clone https://github.com/danielbohannon/Invoke-DOSfuscation.git
PS C:\htb> cd Invoke-DOSfuscation
PS C:\htb> Import-Module .\Invoke-DOSfuscation.psd1
PS C:\htb> Invoke-DOSfuscation
Invoke-DOSfuscation> help

We can even use tutorial to see an example of how the tool works. Once we are set, we can start using the tool, as follows:

Invoke-DOSfuscation> SET COMMAND type C:\Users\htb-student\Desktop\flag.txt
Invoke-DOSfuscation> encoding
Invoke-DOSfuscation\Encoding> 1

...SNIP...
Result:
typ%TEMP:~-3,-2% %CommonProgramFiles:~17,-11%:\Users\h%TMP:~-13,-12%b-stu%SystemRoot:~-4,-3%ent%TMP:~-19,-18%%ALLUSERSPROFILE:~-4,-3%esktop\flag.%TMP:~-13,-12%xt

Finally, we can try running the obfuscated command on CMD, and we see that it indeed works as expected:

C:\htb> typ%TEMP:~-3,-2% %CommonProgramFiles:~17,-11%:\Users\h%TMP:~-13,-12%b-stu%SystemRoot:~-4,-3%ent%TMP:~-19,-18%%ALLUSERSPROFILE:~-4,-3%esktop\flag.%TMP:~-13,-12%xt

test_flag

Tip: If we do not have access to a Windows VM, we can run the above code on a Linux VM through pwsh. Run pwsh, and then follow the exact same command from above.