Abusing progressive JPEG scans to embed animations in one file

Sources: maurycyz.com, HN discussion
Summary: Maurycy published a write-up on 2026-07-17 that exploits the multi-scan structure of progressive JPEG to play a short animation from a single standards-compliant image file. Progressive JPEG renders successive scans at increasing detail, and by concatenating minimal DC-only frames (each a valid JPEG at 1/16 resolution) the author packs roughly 90 distinct frames that browsers render in sequence before they stop decoding at around nine scans. There is no timing metadata, so playback speed depends entirely on network delay.
Why it matters: The technique documents a decoder-behavior edge case in a ubiquitous image format, relevant to anyone parsing or sandboxing untrusted image input where scan count and frame concatenation are not bounded.