JPEG marker and segment analyzer
Parse JPEG markers, read APP segments and comments, and extract data appended after the end-of-image marker.
Open in ctfpalJPEG is a stream of markers, each beginning with FF. The ones that matter for forensics are the application segments (APP0 through APP15), the comment segment (COM), and the end-of-image marker FFD9.
Unlike PNG’s rigid chunk list, JPEG’s segments are optional and variable, which means there is no clean audit - you cannot say that every byte belongs somewhere. What you can do is find the boundaries the format guarantees, and treat anything outside them as deliberate.
Where things hide
- After `FFD9`. The image ends there. Everything after is appended data - most often a ZIP, since
unzipsearches from the end of a file and will happily find it. - In `COM`. An arbitrary comment string, invisible in any viewer.
- In `APP1`. Where EXIF lives, including the comment fields covered by the EXIF viewer.
- In an unusual `APPn`. Segments the decoder does not recognise are skipped without complaint.
Go deeper
- A workflow for image steganography, from magic bytes to bit planesStego challenges reward order, not inspiration. The sequence that finds the payload: container checks before pixel checks, structure before statistics, and the specific tells that separate a PNG trick from a JPEG one.
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