CVE-2026-39363

Summary

server.fs check was not enforced to the fetchModule method that is exposed in Vite dev server's WebSocket. 

Impact

Only apps that match the following conditions are affected:

• explicitly exposes the Vite dev server to the network (using --host or server.host config option)
• WebSocket is not disabled by server.ws: false

Arbitrary files on the server (development machine, CI environment, container, etc.) can be exposed.

Details

If it is possible to connect to the Vite dev server’s WebSocket without an Origin header, an attacker can invoke fetchModule via the custom WebSocket event vite:invoke and combine file://... with ?raw (or ?inline) to retrieve the contents of arbitrary files on the server as a JavaScript string (e.g., export default "...").

The access control enforced in the HTTP request path (such as server.fs.allow) is not applied to this WebSocket-based execution path.

PoC

1. Start the dev server on the target 

   Example (used during validation with this repository):

   ```bash

   pnpm -C playground/alias exec vite --host 0.0.0.0 --port 5173

   ```

1. Confirm that access is blocked via the HTTP path (example: arbitrary file)

   ```bash

   curl -i 'http://localhost:5173/@fs/etc/passwd?raw'

   ```

   Result: 403 Restricted (outside the allow list)

   <img width="3898" height="1014" alt="image" src="https://github.com/user-attachments/assets/f6593377-549c-45d7-b562-5c19833438af" />

1. Confirm that the same file can be retrieved via the WebSocket path

   By connecting to the HMR WebSocket without an Origin header and sending a vite:invoke request that calls fetchModule with a file://... URL and ?raw, the file contents are returned as a JavaScript module.

  <img width="1049" height="296" alt="image" src="https://github.com/user-attachments/assets/af969f7b-d34e-4af4-8adb-5e2b83b31972" />

  <img width="1382" height="955" alt="image" src="https://github.com/user-attachments/assets/6a230d2e-197a-4c9c-b373-d0129756d5d7" />