PNG with a bad CRC checksum

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

Walk the chunks and verify each CRC. Browsers tolerate damage that strict decoders reject, so check before you store or process the image.

import struct, zlib

with open("png-bad-crc.png", "rb") as f:
    data = f.read()
if data[:8] != b"\x89PNG\r\n\x1a\n":
    raise SystemExit("not a PNG")

pos = 8
while True:
    if pos + 8 > len(data):
        raise SystemExit("truncated: the file ends before an IEND chunk")
    length, ctype = struct.unpack(">I4s", data[pos:pos + 8])
    end = pos + 12 + length
    if end > len(data):
        raise SystemExit(f"truncated: the {ctype.decode()} chunk needs {length} bytes")
    if zlib.crc32(data[pos + 4:end - 4]) != struct.unpack(">I", data[end - 4:end])[0]:
        raise SystemExit(f"bad CRC in the {ctype.decode()} chunk")
    if ctype == b"IEND":
        break
    pos = end
print("chunk structure is valid")

Run against this file, it prints: bad CRC in the IDAT chunk