Full writeup: https://meashiri.github.io/ctf-writeups/posts/202401-irisctf/#not-just-media
TLDR; We have a video in a MKV container, that also contains two embedded fonts. The default fonts render the subtitle in Chinese. Using MPV to change the subtitle style and switching to the second font (called FakeFont), will give us the flag upon playback
`% mpv --sub-ass-style-overrides=FontName=FakeFont chal.mkv`
![](https://meashiri.github.io/ctf-writeups/posts/202401-irisctf/2024-01-07-23-57-16.png)