Broken

We opened broken.mp4 using Free Hex Editor Neo to see what’s going on under the hood.

First thing we noticed at the top of the file (0x00 offset):

ctf{this is not the flag}.hehe..

Classic move. Troll detected. But also... nice hint that we’re on the right track.

Here’s the thing about MP4 files, they must start with a proper header, usually beginning with:

00 00 00 ?? 66 74 79 70

That ftyp segment (seen as “ftypisom”, “ftypmp42”) is the file type box and it's mandatory for proper playback.

But our file starts with:

Which is ASCII for ctf{, a red flag. That header is corrupted and definitely not a valid MP4 format.

So now we know, the file is missing or has a scrambled header, which is why media players can’t open it.

we tried manually patching the MP4 header in the hex editor by:

· Replacing the junk at the start with a known-good MP4 header

· Copy-pasting from a working video file’s header

...But no luck. Media players still refused to open the file. Either more was broken deeper in the file, or we just weren’t lucky that day.

Rather than reinvent the wheel, we tried something smarter: Used EaseUS Online MP4 Repair Tool

· Uploaded the broken.mp4 file

· Let the tool do its magic ˙+x

· Downloaded the repaired file

And it worked! The file finally opened in the video player!

When we played the repaired file, we saw this on-screen:

Flag: umcs{h1dd3n_1n_fr4m3}