Chameleon
Chameleon is yet another PowerShell obfuscation tool designed to bypass AMSI and commercial antivirus solutions.
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Overview
The tool has been developed as a Python port of the Chimera project, by tokioneon_. As such, it uses mostly the same techniques to evade common detection signatures, such as:
- comment deletion/substitution
- string substitution (variables, functions, data-types)
- variable concatenation
- indentation randomization
- semi-random backticks insertion
- case randomization
- encoding
Why porting it
Chimera was indeed a shiny project, so why did I decided to port it to Python and why you should use chameleon? Well, there are several reasons why I decided to build Chameleon. I wrote a more detailed post about them here. I've also listed below the most important ones.
Reliability
As the author of Chimera states in the readme, the chimera script can successfully obfuscate scripts that the author tested personally, which are contained in the shells directory. However, the tool is not very reliable with other, untested, scripts. Quoting the author:
there's no telling how untested scripts will reproduce with Chimera...
This alone was a good reason to attempt to make the tool a bit more reliable, and also capable to obfuscate more complex scripts.
Speed
Chimera attempts several obfuscation steps, which usually requires the input to be read from a file, and stored back in a file again. While this is a safe approach, because each step is saved to disk (let's say there is an error at step n, we would still have the result of the obfuscation till n - 1), this is not really efficient. The overhead of writing and reading from a file at each time make the tool really slow when operating on large scripts (up to several minutes with the -a option).
Chameleon, instead, performs all obfuscation steps in memory, meaning it is extremely faster.
Portability
Chimera has been developed as a Bash Script, and heavily relies on common Linux utilities to accomplish the obfuscation.
Chameleon, on the other hand, is built with Python, meaning that you can use it wherever Python is installed.
Smart evasion checking
Chimera offers a function to submit scripts to VirusTotal directly. While this might be considered a useful utility, it will expose the obfuscated script to third party threat-intelligence, weakening the obfuscation engine.
To address this issue, Chameleon uses the utility AMSITrigger by RhytmStick, to check if the obfuscated result will indeed bypass AMSI.
Improvements
So far, we've talked about the efficiency and reliability issues of chimera, but what are the real improvements from an obfuscation standpoint? The techniques used by Chameleon are for the most the same as Chimera, with some improvements:
- "Smart" variable scope identification (function local variables will be replaced "carefully" or left untouched)
- Random backticks insertion (not just limited to a set of strings)
- Random case switch (not just limited to a set of strings)
- Supports an external obfuscation mapping for functions
and parameters(TODO) - Additional Base64 Encoding wrapping
Chameleon manages to handle function and local parameters by implementing a very minimalist PowerShell "reader", which is capable of distinguish three contexts:
- Global/Main Scope
- In-Function Scope
- Param() Blocks
The reader is still not a real parser, and relies on Dick Language to find relevant areas limits.
Usage
Using the tool is pretty straightforward, as observable from the help:
usage: chameleon.py [-h] [-l {0,1,2,3,4,5}] -o OUTPUT [-v] [-s] [-d] [-n] [-c] [-f] [-b] [--random-backticks] [-r] [-i] [-x] [-j] [-a] [--decimal] [--base64] [-z] [-F FUNCTION_MAPPING] [-K KEYWORDS] [-B BACKTICKS] [-t {r,d,h}] [--safe] [--verbose] [--about]
target
Chameleon - PowerShell script obfuscator (Improved Python port of Chimera)
positional arguments:
target Script to obfuscate
optional arguments:
-h, --help show this help message and exit
-l {0,1,2,3,4,5}, --level {0,1,2,3,4,5}
String manipulation Level (1: MIN, 5: MAX, 0: RANDOM)
-o OUTPUT, --output OUTPUT
Store the payload in a file
-v, --variables Enable variable obfuscation
-s, --strings Enable string obfuscation
-d, --data-types Enable data types obfuscation
-n, --nishang Enable Nishang scripts obfuscation
-c, --comments Enable comments obfuscation
-f, --functions Enable functions obfuscation
-b, --use-backticks Enable use of backticks with generated strings
--random-backticks Enable use of backticks randomization
-r, --random-cases Enable upper/lower randomization
-i, --random-spaces Enable indentation randomization
-x, --hex-ip Enable indentation randomization
-j, --true-false-null
Try and obfuscate $true, $false and $null (experimental)
-a, --enable-all Enable all obfuscation types
--decimal Convert obfuscated payload to decimal format
--base64 Convert obfuscated payload to base64 format
-z, --check Check the script against AMSI Trigger (@RythmStick, @rasta-mouse)
-F FUNCTION_MAPPING, --function-mapping FUNCTION_MAPPING
Add custom keywords to obfuscate
-K KEYWORDS, --keywords KEYWORDS
Add custom keywords to obfuscate
-B BACKTICKS, --backticks BACKTICKS
Add a list of words to backtick
-t {r,d,h}, --randomization-type {r,d,h}
Type of randomization (r: Random, d: Dictionary, h: Hybrid)
--safe Reduce obfuscation of certain variables
--verbose Enable verbose output
--about Shows additional information about the tool
Notes
Worth saying that, even if now Chameleon is capable of obfuscate also complex scripts, it's still not comparable with
Invoke-Obfuscation, which actually is way more mature and is also backed-up by a fully fledged parser Management.Automation.Language.Parser.
Next steps
Moreover, Chameleon is still not perfect and still needs further development to increase both its accuracy and improve its obfuscation techniques. A non-exhaustive list of planned improvements are below:
- Upgrade the PowerShell reader
- Include other encoding schemes
- Add more obfuscation methods
Contribute
If you want to contribute, just fork the repository. Any PR is well accepted.
Credits
Worth saying that Chameleon would not be a thing without the work of tokioneon_ on Chimera, as the most of the obfuscation process was ported from Bash to Python (of course with some mods).