Fake MP4 with wrong magic bytes

Handle it in code

Each snippet below was run against this exact file with Python 3.14.4 and Node 24.15.0 when this page was built, and the output shown is what it printed.

Python

An MP4 is a sequence of boxes: a 4-byte size, a 4-byte type, then the contents. Walking the top level shows a missing moov box (no index, so nothing to play) or a file that is not an MP4 at all, without a media library.

import struct

kinds = []
with open("fake-mp4-wrong-magic.mp4", "rb") as f:
    while len(head := f.read(8)) == 8:
        size, kind = struct.unpack(">I4s", head)
        header = 8
        if size == 1:  # a 64-bit size follows
            size, header = struct.unpack(">Q", f.read(8))[0], 16
        kinds.append(kind.decode("ascii") if all(32 <= b < 127 for b in kind) else "0x" + kind.hex())
        if size == 0:
            break  # the box runs to the end of the file
        if size < header:
            print(f"rejected: box {kinds[-1]!r} declares {size} bytes")
            break
        f.seek(size - header, 1)
if not kinds or kinds[0] != "ftyp":
    print("not an MP4: the first box is", kinds[0] if kinds else "missing")
elif "moov" not in kinds:
    print("boxes:", ", ".join(kinds) + "; no moov box, so there is nothing to play")
else:
    print("boxes:", ", ".join(kinds))

Run against this file, it prints: not an MP4: the first box is 0x62dda363