Table of Contents

pngcheck in CTF: How to Analyze and Repair PNG Files

author:

rudy_candy

category:

tools

✅ What is pngcheck?

pngcheck is a command-line tool for validating and analyzing PNG image files. It can:

Verify PNG file integrity
Detect corrupted or missing chunks
Display detailed chunk information
Help repair PNG structure issues

In CTF forensics challenges, pngcheck is extremely useful because many problems involve:

Corrupted PNG files
Hidden data in PNG chunks
Manually fixing PNG headers
Inspecting custom or malformed chunks

🛠️ Basic pngcheck Commands

1. Validate a PNG file

pngcheck file.png

Shows if the PNG is valid or corrupted.
Reports missing or broken chunks.

2. Verbose output with chunk details

pngcheck -v file.png

Prints full information on:
- Chunk type
- Length
- CRC
- Offsets
Very useful for debugging CTF files.

3. Display chunk names only

pngcheck -c file.png

Shows only chunk types without full information.

4. Display even more detail (e.g., zlib info)

pngcheck -fvp file.png

-f: force checking even on errors
-v: verbose
-p: check pixel data

🎯 How pngcheck is Used in CTF Challenges

1. Fixing corrupted PNG files

Many CTF challenges provide PNGs that:

Have missing headers
Contain broken chunks
Have incorrect CRC values
Have truncated or extra data

pngcheck helps identify exactly what’s wrong.

Example:

pngcheck: CRC error in chunk IHDR (expected 0xAE426082, got 0xDEADBEEF)

2. Detecting hidden or custom chunks

PNG supports custom chunk types, and CTF creators often hide data in them.

Examples:

tEXt
iTXt
zTXt
Custom chunks like abCD, flAg, etc.

Using:

pngcheck -v file.png

…you can see every chunk and its size.

3. Finding appended hidden data

Sometimes extra data is added after the IEND chunk.

pngcheck file.png

If output includes:

extra data after IEND chunk

→ This usually means hidden content or a secondary file is appended.

4. Working with multi-layer stego images

pngcheck is often the first step before tools like:

zsteg
binwalk
stegsolve

It confirms the PNG is intact or identifies what must be fixed before deeper analysis.

🔍 Common CTF Patterns Using pngcheck

Pattern	Description	How pngcheck Helps
Corrupted IHDR	Wrong image header or size	Shows header errors and expected values
Broken CRC	Modified chunk CRCs	Tells which chunks have incorrect CRC
Missing chunks	Removed IHDR, IDAT, or IEND	Detects which chunk is missing
Hidden chunks	Custom or oversized chunks hide data	Lists all chunk names + sizes
Appended data	Extra data after IEND	Detects trailing content
Multi-layer PNG stego	PNG + zsteg + appended file	Helps identify each layer

📌 Tips for Using pngcheck in CTFs

Run pngcheck -v as your first step when a PNG “doesn’t open.”
Look carefully at CRC errors—they often reveal tampering.
If a chunk is corrupted, you may need to manually fix it with a hex editor.
Check for hidden tEXt chunks—flags are often stored there.
If pngcheck reports “extra data after IEND,” try extracting it: tail -c +offset file.png > hidden.bin

🧩 Recommended Workflow in CTF

Run: pngcheck -v file.png
Note any:
- Missing chunks
- Broken CRCs
- Extra data after IEND
If corrupted, fix using hexedit or reconstruct with correct magic numbers.
Scan for hidden chunks.
After fixing structural issues, proceed with deeper stego tools:
- zsteg file.png
- binwalk -e file.png
- strings file.png
Extract hidden data or analyze appended content.

pngcheck