CSSHide

CSSHide is a tool for encoding a payload inside a mock-CSS file using CSS colour values with the aim of blending into common network traffic - Hiding in plain style.

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Installation

C:\> git clone https://github.com/inb1ts/CSSHide.git
C:\> cd CSSHide
C:\> pip install rcssmin
C:\> python .\csshide.py --help

Usage

Supports two different formats of CSS colour attribute:

• rgb - rgb(1 2 3)
• hex - #ffffff

The payload is split into 3-byte chunks and encoded into these values before being placed amongst other randomly generated CSS attributes. There is an initial section of variables, as it allows us to encode a more concentrated portion of the payload straight away, before moving onto randomly-selected CSS selectors. The output can be minified to reduce size - as is occasionally done with legitimate CSS files - or left as the default format.

C:\>python.exe csshide.py -h
usage: csshide.py PAYLOAD_FILE [-h] [-f {rgb, hex}] [--no-minify] [-o OUT_FILE]

CSSHide encodes a payload in CSS colour values. Supports either rbg or hex CSS colour formats, and
minified/normal output.

positional arguments:
  payload               the payload file payload you want to encode

options:
  -h, --help            show this help message and exit
  -s SELECTORS, --selectors SELECTORS
                        CSS selector wordlist
  -f {rgb,hex}, --format {rgb,hex}
                        format of CSS color
  --minify, --no-minify
                        minify output to save space (default: True)
  -o OUTFILE, --outfile OUTFILE
                        name of output file to create

An example of fetching the CSS file from a server and decoding it in C (and using the WinAPI) can be found in the example_decode directory.

Example output

Minified rgb

• csshide.py calc.bin

Non-minified rbg

• csshide.py calc.bin --no-minify

Minified hex

• csshide.py calc.bin -f hex

Non-minified hex

• csshide.py calc.bin -f hex --no-minify

Limitations

• Size - Legitimate CSS files usually aren't too large, so whilst the tool will still work with a large payload (tested with Havoc agent shellcode of 95kb), the outputted file will look comically large for a CSS file and would definitely turn some heads.
• Limited selection of CSS properties and values - The randomised CSS properties and values are pulled from a hardcoded list that does not encompass all possible CSS properties.
• Can't currently blend between rgb and hex formats - It's one or the other at the moment.
• Doesn't currently utilise the alpha component of the rgb colour format
• Predictably starts with the root: variable section, but would preferably randomise more elements at the start of the file such as font inclusions

Resources/Credits

• Shout out to Maldev Academy - the modules on payload encoding gave me the idea to try this.