Vite: First 4 bytes of files outside of `server.fs` setting is exposed by `?vite-wasm-instance`

Summary

The first 4 bytes of files outside of the server.fs setting can be returned to the browser by a request containing ?vite-wasm-instance.

Impact

Only applications that match the following conditions are affected:

• The Vite development server is explicitly exposed to the network using --host or the server.host option.
• A file contains sensitive information within its first 4 bytes.
• Vite is run using Node.js or Deno (applications running on Bun are not affected).

Details

Requests containing the ?vite-wasm-instance query are handled by the internal Wasm plugin's load hook. This hook removes the query and reads the resulting file path directly from disk, but server.fs checks were not applied.

After reading the file, Vite attempts to parse it as WebAssembly. When this parsing fails, Node.js and Deno throw an error that includes the first 4 bytes of the file.

PoC

Create a denied file and start an affected Vite development server:

echo 'SECRET_KEY=abc123\n' > .env
npm run dev
curl 'http://localhost:5173/.env?vite-wasm-instance'

This will show:

Failed to parse WASM file "<root>/.env": WebAssembly.compile(): expected magic word 00 61 73 6d, found 53 45 43 52 @+0