diff --git a/packages/vite/src/node/server/__tests__/environment-transport.spec.ts b/packages/vite/src/node/server/__tests__/environment-transport.spec.ts
new file mode 100644
index 0000000..09eddca
--- /dev/null
+++ b/packages/vite/src/node/server/__tests__/environment-transport.spec.ts
@@ -0,0 +1,203 @@
+import path from 'node:path'
+import { pathToFileURL } from 'node:url'
+import { WebSocket as WebSocketClient } from 'ws'
+import { afterEach, describe, expect, it } from 'vitest'
+import {
+  ModuleRunner,
+  createWebSocketModuleRunnerTransport,
+} from 'vite/module-runner'
+import {
+  DevEnvironment,
+  type HotChannel,
+  type ViteDevServer,
+  createServer,
+} from '../..'
+
+const fixtureRoot = path.resolve(
+  import.meta.dirname,
+  'fixtures/environment-transport',
+)
+const allowedDir = path.resolve(fixtureRoot, 'allowed')
+const insideFile = path.resolve(allowedDir, 'inside.js')
+const outsideFile = path.resolve(fixtureRoot, 'outside.js')
+
+function transport(skipFsCheck?: boolean): HotChannel {
+  return {
+    ...(skipFsCheck === undefined ? {} : { skipFsCheck }),
+    send() {},
+    on() {},
+    off() {},
+  }
+}
+
+describe('environment transport file access', () => {
+  let server: ViteDevServer | undefined
+
+  afterEach(async () => {
+    await server?.close()
+    server = undefined
+  })
+
+  it.each([
+    ['an omitted skipFsCheck flag', undefined],
+    ['skipFsCheck set to false', false],
+  ])('enforces server.fs.allow with %s', async (_description, skipFsCheck) => {
+    server = await createServer({
+      configFile: false,
+      root: fixtureRoot,
+      logLevel: 'silent',
+      server: {
+        middlewareMode: true,
+        watch: null,
+        fs: { allow: [allowedDir] },
+      },
+      environments: {
+        restricted: {
+          dev: {
+            createEnvironment(name, config) {
+              return new DevEnvironment(name, config, {
+                hot: false,
+                transport: transport(skipFsCheck),
+              })
+            },
+          },
+        },
+      },
+    })
+
+    const environment = server.environments.restricted
+    await expect(
+      environment.transformRequest(insideFile),
+    ).resolves.not.toBeNull()
+    await expect(environment.transformRequest(outsideFile)).rejects.toThrow()
+  })
+
+  it('allows trusted custom transports to opt out of the file-system check', async () => {
+    server = await createServer({
+      configFile: false,
+      root: fixtureRoot,
+      logLevel: 'silent',
+      server: {
+        middlewareMode: true,
+        watch: null,
+        fs: { allow: [allowedDir] },
+      },
+      environments: {
+        trusted: {
+          dev: {
+            createEnvironment(name, config) {
+              return new DevEnvironment(name, config, {
+                hot: false,
+                transport: transport(true),
+              })
+            },
+          },
+        },
+      },
+    })
+
+    const transformed =
+      await server.environments.trusted.transformRequest(outsideFile)
+    expect(transformed).not.toBeNull()
+  })
+
+  it('keeps the in-process server module loader trusted', async () => {
+    server = await createServer({
+      configFile: false,
+      root: fixtureRoot,
+      logLevel: 'silent',
+      server: {
+        middlewareMode: true,
+        watch: null,
+        fs: { allow: [allowedDir] },
+      },
+    })
+
+    const module = await server.ssrLoadModule(outsideFile)
+    expect(module.default).toBe('outside')
+  })
+
+  it('keeps module fetching available in custom environments', async () => {
+    let customModuleLoaded = false
+    server = await createServer({
+      configFile: false,
+      root: fixtureRoot,
+      logLevel: 'silent',
+      plugins: [
+        {
+          name: 'observe-custom-module-loads',
+          transform(_code, id) {
+            if (id.startsWith(insideFile)) customModuleLoaded = true
+          },
+        },
+      ],
+      server: {
+        middlewareMode: true,
+        watch: null,
+        fs: { allow: [allowedDir] },
+      },
+      environments: {
+        custom: {
+          dev: {
+            createEnvironment(name, config) {
+              return new DevEnvironment(name, config, {
+                hot: false,
+                transport: transport(),
+              })
+            },
+          },
+        },
+      },
+    })
+
+    await server.environments.custom.fetchModule(insideFile)
+    expect(customModuleLoaded).toBe(true)
+  })
+
+  it('does not expose the default client fetchModule handler over WebSocket', async () => {
+    let clientModuleLoaded = false
+    server = await createServer({
+      configFile: false,
+      plugins: [
+        {
+          name: 'observe-client-module-loads',
+          transform(_code, id) {
+            if (id.startsWith(insideFile)) {
+              clientModuleLoaded = true
+            }
+          },
+        },
+      ],
+      root: fixtureRoot,
+      logLevel: 'silent',
+      server: { watch: null },
+    })
+    await server.listen()
+
+    const address = server.httpServer?.address()
+    if (!address || typeof address === 'string') {
+      throw new Error('Expected the development server to listen on a TCP port')
+    }
+
+    const runner = new ModuleRunner({
+      transport: createWebSocketModuleRunnerTransport({
+        createConnection: () =>
+          new WebSocketClient(
+            `ws://127.0.0.1:${address.port}/?token=${server!.config.webSocketToken}`,
+            ['vite-hmr'],
+          ) as unknown as WebSocket,
+      }),
+      hmr: false,
+      sourcemapInterceptor: false,
+    })
+
+    try {
+      await expect(
+        runner.import(`${pathToFileURL(insideFile).href}?raw`),
+      ).rejects.toThrow()
+    } finally {
+      await runner.close()
+    }
+    expect(clientModuleLoaded).toBe(false)
+  })
+})
diff --git a/packages/vite/src/node/server/__tests__/fixtures/environment-transport/allowed/inside.js b/packages/vite/src/node/server/__tests__/fixtures/environment-transport/allowed/inside.js
new file mode 100644
index 0000000..22e7f15
--- /dev/null
+++ b/packages/vite/src/node/server/__tests__/fixtures/environment-transport/allowed/inside.js
@@ -0,0 +1 @@
+export default 'inside'
diff --git a/packages/vite/src/node/server/__tests__/fixtures/environment-transport/outside.js b/packages/vite/src/node/server/__tests__/fixtures/environment-transport/outside.js
new file mode 100644
index 0000000..c8ebc9f
--- /dev/null
+++ b/packages/vite/src/node/server/__tests__/fixtures/environment-transport/outside.js
@@ -0,0 +1 @@
+export default 'outside'