PNGFixer - CTF-Oriented PNG Resizer Tool

pngfixer is a lightweight CTF-focused Python script that allows you to resize PNG images by directly patching the IHDR chunk (width and height) and recalculating the CRC32 checksum. It validates the PNG signature and IHDR block before modification.

Perfect for CTF challenges involving:

• Polyglot files (e.g., PNG + ZIP, PNG + PHP)
• Image dimension spoofing (bypassing filters)
• Metadata manipulation
• Steganography or file inclusion bypass

No heavy dependencies — just pure binary patching.

Features

• Validates PNG signature (\x89PNG\r\n\x1a\n)
• Checks IHDR chunk (length = 13, type = IHDR)
• Modifies width/height in IHDR
• Recalculates correct CRC32 for the chunk
• Zero image processing libraries (no PIL, no OpenCV)
• Installable as a global command on Kali Linux

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Requirements

• Kali Linux (or any Debian-based distro)
• Python 3 (default on Kali)
• zlib → built-in
• sudo access for installation

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Installation (Step-by-Step)

1. clone this repository https://github.com/CR0M80/PNGFixer.git:

2. Grant execution permission to the installer:

   cd PNGFixer
   chmod +x setup.sh

3. Run the installer with sudo:

   sudo ./setup.sh

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Usage in CTF

pngfixer <input.png> --width <w in px> --height <h in px> -o <output.png>

Syntax : pngfixer png_path --width new_width_px --height new_height_px -o new_file_path

Argument	Required	Description
png_path	Yes	Input PNG file
--width	Yes	New width (> 0)
--height	Yes	New height (> 0)
-o	Yes	Output file path

Limitations & Warnings

• Does NOT re-encode pixel data → image may appear corrupted in viewers
• Only modifies IHDR → other chunks (IDAT, etc.) remain unchanged
• Not for real image editing → use convert, Pillow, or GIMP
• Use in CTF only → may break legitimate PNGs

Pro Tip: Combine with exiftool, binwalk, or steghide for full CTF workflow.

Uninstallation

Remove the command globally:

sudo rm /usr/local/bin/pngfixer

Authors

Built for DFIR & STEGANOGRAPHY CTF players