.ISO file signature
A ISO 9660 CD-ROM filesystem data file begins with the bytes 43 44 30 30 31 (CD001 in ASCII) at offset 32769 (0x8001). The signature is the same whatever the file is named, which is why a renamed or extensionless ISO file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 32769 | 43 44 30 30 31ISO 9660 primary volume descriptor, 1 byte into sector 16. | CD001 |
MIME type: application/x-iso9660-image · usual extensions: .iso
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.iso
file file.isoWindows (PowerShell):
Format-Hex -Path file.iso -Count 32This signature sits at offset 32769, so look there rather than at the first bytes: xxd -s 32769 -l 16 file.iso.
If the bytes do not match
A file named .iso whose first bytes are something else is not a ISO 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 ISO guide explains what a genuine file should contain.