diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5ecc8bb0b1..b0a7c4d1de 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -39,6 +39,12 @@ file is the human-readable summary of what changed and why.
   only on macOS, where the VM backend can honour them: Windows sandboxes
   follow the VM-global limits in the user's `.wslconfig`, and the Linux
   backend has no per-sandbox limits.
+- **The sandbox egress broker now requires a per-sandbox secret** — each
+  running sandbox gets its own random token, injected into the guest and
+  required on every brokered request. Without it the broker on Windows (which
+  must listen on TCP so the guest can reach the host) would have been usable
+  by any other process able to reach that port, spending the real gateway
+  credential the broker attaches on the host side.
 - **Named egress rules for sandboxes** — administrators can grant sandboxes
   extra brokered destinations beyond the model gateway by creating
   `sandbox-egress.json` in the app's data directory: an object of name →
diff --git a/packages/codev-sandbox/src/broker.test.ts b/packages/codev-sandbox/src/broker.test.ts
index c246953a18..27d1dc7746 100644
--- a/packages/codev-sandbox/src/broker.test.ts
+++ b/packages/codev-sandbox/src/broker.test.ts
@@ -140,6 +140,31 @@ test("route headers replace the guest's placeholder credential", async () => {
   upstream.stop(true)
 })
 
+test("rejects callers without the shared secret", async () => {
+  const upstream = Bun.serve({ port: 0, fetch: () => Response.json({ ok: true }) })
+  const broker = createBrokerServer({
+    routes: [{ prefix: "/llm", upstream: `http://127.0.0.1:${upstream.port}/v1` }],
+    auth: "secret-token",
+  })
+  const port = await broker.listen()
+
+  const anonymous = await fetch(`http://127.0.0.1:${port}/llm/models`)
+  expect(anonymous.status).toBe(401)
+  const wrong = await fetch(`http://127.0.0.1:${port}/llm/models`, { headers: { authorization: "Bearer nope" } })
+  expect(wrong.status).toBe(401)
+  // An unauthorized caller must not be able to probe which routes exist.
+  const denied = await fetch(`http://127.0.0.1:${port}/not-a-route`)
+  expect(denied.status).toBe(401)
+
+  const allowed = await fetch(`http://127.0.0.1:${port}/llm/models`, {
+    headers: { authorization: "Bearer secret-token" },
+  })
+  expect(allowed.status).toBe(200)
+
+  await broker.close()
+  upstream.stop(true)
+})
+
 test("reports upstream failures as 502 instead of hanging", async () => {
   const broker = createBrokerServer({
     routes: [{ prefix: "/llm", upstream: "http://127.0.0.1:1/nowhere" }],
diff --git a/packages/codev-sandbox/src/broker.ts b/packages/codev-sandbox/src/broker.ts
index 1f75c1d94f..5d7c628aaa 100644
--- a/packages/codev-sandbox/src/broker.ts
+++ b/packages/codev-sandbox/src/broker.ts
@@ -22,6 +22,11 @@ export type SandboxEgress = {
   routes: BrokerRoute[]
   guestConfig?: string
   fetchImpl?: typeof fetch
+  // Shared secret the guest must present. Required in practice: the WSL
+  // backend can only reach the host over TCP, so without it any local (or
+  // LAN-reachable) process could use the broker as an open proxy to the
+  // gateway, spending the real token the broker attaches host-side.
+  auth?: string
 }
 
 // Named egress rules: the admin-editable shape for extra routes beyond the
@@ -99,9 +104,16 @@ export function forwardableResponseHeaders(headers: Headers) {
   return Object.fromEntries([...headers.entries()].filter(([name]) => !RESPONSE_HEADER_DROP.has(name.toLowerCase())))
 }
 
-export function createBrokerServer(options: { routes: BrokerRoute[]; fetchImpl?: typeof fetch }) {
+export function createBrokerServer(options: { routes: BrokerRoute[]; fetchImpl?: typeof fetch; auth?: string }) {
   const fetchImpl = options.fetchImpl ?? fetch
   const server = createServer(async (request, response) => {
+    // Checked before the route table is consulted, so an unauthorized caller
+    // learns nothing about which upstreams exist.
+    if (options.auth && request.headers.authorization !== `Bearer ${options.auth}`) {
+      response.writeHead(401, { "content-type": "application/json" })
+      response.end(JSON.stringify({ error: "sandbox egress unauthorized" }))
+      return
+    }
     const resolved = resolveBrokerRoute(options.routes, request.url ?? "/")
     if (!resolved) {
       response.writeHead(403, { "content-type": "application/json" })
diff --git a/packages/codev-sandbox/src/bwrap/provider.ts b/packages/codev-sandbox/src/bwrap/provider.ts
index 0dcec97c1a..9b063b48b7 100644
--- a/packages/codev-sandbox/src/bwrap/provider.ts
+++ b/packages/codev-sandbox/src/bwrap/provider.ts
@@ -119,7 +119,7 @@ export function createBwrapProvider(options: BwrapProviderOptions): SandboxProvi
       // No routes means no egress at all — the config injection below can
       // still apply without a listener.
       const broker = egress?.routes.length
-        ? createBrokerServer({ routes: egress.routes, fetchImpl: egress.fetchImpl ?? fetchImpl })
+        ? createBrokerServer({ routes: egress.routes, fetchImpl: egress.fetchImpl ?? fetchImpl, auth: egress.auth })
         : null
       if (broker) await broker.listenUnix(join(dirs.runtime, "broker.sock"))
       if (egress?.guestConfig) await writeFile(join(dirs.bridge, "codev.json"), egress.guestConfig)
diff --git a/packages/codev-sandbox/src/vz/provider.ts b/packages/codev-sandbox/src/vz/provider.ts
index 26687aef41..a432940477 100644
--- a/packages/codev-sandbox/src/vz/provider.ts
+++ b/packages/codev-sandbox/src/vz/provider.ts
@@ -131,7 +131,7 @@ export function createVzProvider(options: VzProviderOptions): SandboxProvider {
       // No routes means no egress at all — the config injection below can
       // still apply without a listener.
       const broker = egress?.routes.length
-        ? createBrokerServer({ routes: egress.routes, fetchImpl: egress.fetchImpl ?? fetchImpl })
+        ? createBrokerServer({ routes: egress.routes, fetchImpl: egress.fetchImpl ?? fetchImpl, auth: egress.auth })
         : null
       if (broker) await broker.listenUnix(join(dirs.runtime, "broker.sock"))
       if (egress?.guestConfig) await writeFile(join(dirs.guestConfig, "codev.json"), egress.guestConfig)
diff --git a/packages/codev-sandbox/src/wsl/provider.ts b/packages/codev-sandbox/src/wsl/provider.ts
index 8c64f38105..e9914e760d 100644
--- a/packages/codev-sandbox/src/wsl/provider.ts
+++ b/packages/codev-sandbox/src/wsl/provider.ts
@@ -184,7 +184,7 @@ export function createWslProvider(options: WslProviderOptions): SandboxProvider
       // No routes means no egress at all — the config injection below can
       // still apply without a listener.
       const broker = egress?.routes.length
-        ? createBrokerServer({ routes: egress.routes, fetchImpl: egress.fetchImpl ?? fetchImpl })
+        ? createBrokerServer({ routes: egress.routes, fetchImpl: egress.fetchImpl ?? fetchImpl, auth: egress.auth })
         : null
       // The guest dials the host at the NAT default gateway address, so the
       // broker cannot bind loopback here.
diff --git a/packages/desktop/src/main/sandbox/egress.ts b/packages/desktop/src/main/sandbox/egress.ts
index ad7829a791..231f142332 100644
--- a/packages/desktop/src/main/sandbox/egress.ts
+++ b/packages/desktop/src/main/sandbox/egress.ts
@@ -1,3 +1,4 @@
+import { randomUUID } from "node:crypto"
 import { readFile } from "node:fs/promises"
 import { join } from "node:path"
 import { app, net } from "electron"
@@ -22,6 +23,10 @@ export async function resolveSandboxEgress(): Promise<SandboxEgress> {
   // back keeps the common case (hub login, no base_url) from silently
   // producing a sandbox with no egress.
   const baseUrl = creds?.baseUrl ?? loadGatewayUrl()
+  // Per-start secret the guest presents on every brokered request. The WSL
+  // backend can only reach the host over TCP, so without this the broker would
+  // be an open proxy to the gateway for anything that can reach the port.
+  const auth = randomUUID()
   const gateway =
     creds?.apiKey && baseUrl && creds.model ? { apiKey: creds.apiKey, baseUrl, model: creds.model } : null
   return {
@@ -40,10 +45,11 @@ export async function resolveSandboxEgress(): Promise<SandboxEgress> {
         : []),
       ...(await namedEgressRules()),
     ],
+    auth,
     guestConfig: JSON.stringify(
       {
         ...(gateway
-          ? buildGatewayConfig({ apiKey: "sandbox-broker", baseUrl: GUEST_BROKER_BASE, model: gateway.model })
+          ? buildGatewayConfig({ apiKey: auth, baseUrl: GUEST_BROKER_BASE, model: gateway.model })
           : {}),
         // Everything the agent does is confined to the VM and the one mounted
         // project folder, so permission prompts buy nothing here — auto-allow