ASF (WMV / WMA) file signature
Magic number at the start of the file
Windows Media files (WMV video and WMA audio) use the Advanced Systems Format. Every ASF file begins with the same 16-byte GUID that starts 30 26 B2 75, so WMV, WMA and generic .asf files share this signature and are told apart by their internal streams.
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 30 26 B2 75 8E 66 CF 11, it is a ASF (WMV / WMA) file.
Formats that use the 30 26 B2 75 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -l 16 example.wmv
- hexdump -C -n 16 example.wmv
- file example.wmv
Windows (PowerShell)
- Format-Hex -Path example.wmv -Count 16
Python
- open("example.wmv","rb").read(8).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the ASF (WMV / WMA) file signature?
ASF (WMV / WMA) files start with the hex bytes 30 26 B2 75 8E 66 CF 11. 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.wmv on Linux or macOS, or Format-Hex -Path example.wmv -Count 16 in Windows PowerShell, and read the first bytes.
Can a file fake the ASF (WMV / WMA) signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine ASF (WMV / WMA) file has these exact bytes; a file with the wrong bytes is not really ASF (WMV / WMA), whatever its name says.