With Nitro 2 (the default server engine in Nuxt 3 and Nuxt 4), the Nitro development server bundle written by nuxt dev can be fetched through the Vite dev server. Nitro inlines the resolved runtimeConfig into that bundle, so private runtime config values that were resolved at build time can be read by anyone with HTTP access to the dev server.
When no appSecret is configured (via NUXT_APP_SECRET, NITRO_APP_SECRET or runtimeConfig.appSecret), nuxt dev generates a development secret and stores it in <buildDir>/app-secret (by default .nuxt/app-secret). The build directory is inside the directories that Nuxt passes to Vite's server.fs.allow, so the file can be fetched from the dev server: curl http://<dev-server>/_nuxt/@fs/<absolute-path-to-project>/.nuxt/app-secret Anyone who can read it can compute every secret derived from it with deriveSecret(), including the default password used by useSession, …
A client that can connect to the dev server's HMR socket can send a nuxt:dev:client-error event whose message ends in an absolute file path. The dev server reads that file while building the error report, and its contents are returned in the error overlay, on the dev error channel, and in the report shown in the terminal.
Nuxt allows any websites to send any requests to the development server and read the response due to default CORS settings.