.ICO file signature
A MS Windows icon resource file begins with the bytes 00 00 01 00 at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless ICO file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 00 00 01 00Bytes 4 and 5 hold the image count. Cursor files (.cur) use 00 00 02 00. |
MIME type: image/x-icon · usual extensions: .ico
How to check a file's signature
Without installing anything: drop the file on our detector. It reads the bytes in your browser and shows the hex dump alongside the verdict; nothing is uploaded.
macOS and Linux (Terminal):
xxd -l 32 file.ico
file file.icoWindows (PowerShell):
Format-Hex -Path file.ico -Count 32If the bytes do not match
A file named .ico whose first bytes are something else is not a ICO file. The common explanations are a wrong or deliberately misleading extension, a download that saved an HTML error page instead of the file, or a container such as ZIP wrapping the real content. The detector reports what the bytes say; the ICO guide explains what a genuine file should contain.