.WASM file signature
A Web Assembly file begins with the bytes 00 61 73 6D 01 00 00 00 (.asm.... 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 WASM file is still recognisable.
Signature
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 00 61 73 6D 01 00 00 00Magic then version 1. | .asm.... |
MIME type: application/wasm · usual extensions: .wasm
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.wasm
file file.wasmWindows (PowerShell):
Format-Hex -Path file.wasm -Count 32If the bytes do not match
A file named .wasm whose first bytes are something else is not a WASM 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 WASM guide explains what a genuine file should contain.