.SWF file signature
A Small Web File file begins with the bytes 46 57 53 (FWS 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 SWF file is still recognisable.
Known signatures
| Offset | Hex | ASCII |
|---|---|---|
| 0 | 46 57 53Uncompressed. | FWS |
| 0 | 43 57 53zlib-compressed. | CWS |
| 0 | 5A 57 53LZMA-compressed. | ZWS |
MIME type: application/x-shockwave-flash · usual extensions: .swf
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.swf
file file.swfWindows (PowerShell):
Format-Hex -Path file.swf -Count 32If the bytes do not match
A file named .swf whose first bytes are something else is not a SWF 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 SWF guide explains what a genuine file should contain.