.CHM file signature
A MS Windows HtmlHelp Data file begins with the bytes 49 54 53 46 03 00 00 00 (ITSF in ASCII) at the start of the file. The signature is the same whatever the file is named, which is why a renamed or extensionless CHM file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 49 54 53 46 03 00 00 00 | ITSF |
MIME type: application/vnd.ms-htmlhelp · usual extensions: .chm
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.chm
file file.chmWindows (PowerShell):
Format-Hex -Path file.chm -Count 32If the bytes do not match
A file named .chm whose first bytes are something else is not a CHM 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 CHM guide explains what a genuine file should contain.