WebAssembly file signature
Magic number at the start of the file · .asm
A WebAssembly binary module starts with 00 61 73 6D, the NUL byte followed by the ASCII text asm, then a four-byte version, currently 01 00 00 00.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file starts with the bytes 00 61 73 6D (the text .asm), it is a WebAssembly file.
What each byte of 00 61 73 6D means
Here is the signature byte by byte, the way a hex editor shows it: the position in the file, the value in hex and in decimal, and the character that value stands for in ASCII. Bytes with no printable character show a dot.
| Byte offset | Hex | Decimal | ASCII |
|---|---|---|---|
| 0 | 00 | 0 | . |
| 1 | 61 | 97 | a |
| 2 | 73 | 115 | s |
| 3 | 6D | 109 | m |
Formats that use the 00 61 73 6D signature
No extension in our database is documented under this signature: the bytes belong to the WebAssembly format itself, which usually appears as a wrapper around something else. Files that carry it are normally named WASM.
How to check a file's signature
You can read the bytes of any file yourself. They are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.wasm
- hexdump -C -n 16 example.wasm
- file example.wasm
Windows (PowerShell)
- Format-Hex -Path example.wasm -Count 16
Python
- open("example.wasm","rb").read(4).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the WebAssembly file signature?
WebAssembly files start with the hex bytes 00 61 73 6D (.asm in ASCII). This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 16 example.wasm on Linux or macOS, or Format-Hex -Path example.wasm -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the WebAssembly signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine WebAssembly file has these exact bytes; a file with the wrong bytes is not really WebAssembly, whatever its name says.
Related signatures
Sources
Reuse this signature
This entry is part of the WhatFileType file signature reference, published under a CC BY 4.0 licence with the full provenance for every entry, a copy-paste embed and a JSON export. Credit WhatFileType and the data is yours to republish.