What Is zbarimg?
zbarimg is a command‑line tool from the ZBar suite used to read and decode QR codes, barcodes, and other machine‑readable symbols from image files.
In CTF challenges—especially forensics and misc categories—QR codes often hide encoded flags, URLs, or hints. zbarimg is one of the quickest ways to extract that data.
It supports:
- QR Code
- Code128
- EAN / UPC
- DataBar
- PDF417 (in some builds)
- and more
Basic Usage
Decode a QR code image
zbarimg qr.png
Typical output:
QR-Code:FLAG{decoded_data_here}
scanned 1 barcode symbols from 1 images in 0.01 seconds
Output only the data (no type label)
zbarimg --raw qr.png
Result:
FLAG{something}
Scan multiple images
zbarimg *.png
Scan from a webcam (if supported)
zbarcam
How zbarimg Is Used in CTF Challenges
1. Extracting encoded flags in QR images
Most common use. A challenge provides a QR code image → you decode it:
zbarimg qr.png
Useful when the QR is visually corrupted but still decodable.
2. Decoding embedded QR codes in other formats
Sometimes QR codes are hidden inside:
- PDFs
- frames of a video
- ZIP archives
- steganographic PNGs
- Metadata in EXIF comments
Workflow example:
binwalk -e suspected.png zbarimg _suspected.extracted/qr_hidden.png
3. Solving multi‑layer or damaged QR challenges
CTFs often distort the QR:
- rotated
- inverted colors
- noise added
- sliced apart and must be reconstructed
- low contrast
zbarimg is surprisingly tolerant, but sometimes you must preprocess:
- invert colors
- increase contrast
- denoise
- reassemble using GIMP or ImageMagick
Example (invert and retry):
convert qr.png -negate fixed.png zbarimg fixed.png
4. Decoding barcodes in forensic challenges
Some challenges hide:
- hex encoded inside Code128
- base64 inside QR
- a URL you must visit
- GPS coordinates
- partial flags that need to be concatenated
Common Patterns in CTF Problems
| Pattern | Description | How zbarimg Helps |
|---|---|---|
| Basic QR with flag | Standard “scan and get flag” | Instantly extract data |
| Multilayer QR | Split among multiple images | Scan each, recombine flag |
| Distorted/noisy QR | Must preprocess first | Retry after cleaning image |
| Inverted QR | Dark/white flipped | Decode after using convert -negate |
| QR inside other files | Hidden via stego or binwalk artifacts | Scan extracted image |
| QR in video frames | Must extract frames | Use ffmpeg, then zbarimg |
| Multiple barcodes in folder | Bulk scanning | zbarimg *.png |
| QR leads to another puzzle | URL → password → next file | First step in a chain |
Recommended Workflow for CTF
- Decode normally
zbarimg qr.png
- If fails, try raw
zbarimg --raw qr.png
- If still fails, preprocess
Common fixes using ImageMagick:
Invert colors:
convert qr.png -negate fixed.png
Increase contrast:
convert qr.png -contrast-stretch 0 fixed.png
Resize (helps with low-resolution QR):
convert qr.png -resize 300% fixed.png
- For videos
ffmpeg -i qr.mp4 frame_%04d.png zbarimg frame_*.png
- For hidden extractions
binwalk -e file.png zbarimg _file.png.extracted/*
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