diff --git a/src/backend/database/routes/users.ts b/src/backend/database/routes/users.ts
index c1b273798..972c53e6e 100644
--- a/src/backend/database/routes/users.ts
+++ b/src/backend/database/routes/users.ts
@@ -11,6 +11,7 @@ import { authLogger } from "../../utils/logger.js";
 import { AuthManager } from "../../utils/auth-manager.js";
 import { DatabaseSaveTrigger } from "../../utils/database-save-trigger.js";
 import { parseUserAgent } from "../../utils/user-agent-parser.js";
+import { isDesktopAppOrigin } from "../../utils/cors-config.js";
 import { deleteUserAndRelatedData } from "./delete-user-data.js";
 import {
   allowsDesktopAutoSession,
@@ -744,7 +745,11 @@ router.get("/setup-required", async (req, res) => {
  */
 router.post("/internal/auto-session", async (req, res) => {
   try {
-    if (!allowsDesktopAutoSession() || !isLoopbackRequest(req)) {
+    if (
+      !allowsDesktopAutoSession() ||
+      !isLoopbackRequest(req) ||
+      !isDesktopAppOrigin(req.headers.origin)
+    ) {
       authLogger.warn(
         "Rejected non-loopback attempt to access auto-session endpoint",
         { source: req.ip },
diff --git a/src/backend/tests/utils/cors-config.test.ts b/src/backend/tests/utils/cors-config.test.ts
index e511120c9..1812ca66e 100644
--- a/src/backend/tests/utils/cors-config.test.ts
+++ b/src/backend/tests/utils/cors-config.test.ts
@@ -1,6 +1,9 @@
 import { afterEach, describe, expect, it } from "vitest";
 import type { Request } from "express";
-import { isCorsOriginAllowed } from "../../utils/cors-config.js";
+import {
+  isCorsOriginAllowed,
+  isDesktopAppOrigin,
+} from "../../utils/cors-config.js";
 
 function request(headers: Record<string, string> = {}): Request {
   return {
@@ -11,6 +14,7 @@ function request(headers: Record<string, string> = {}): Request {
 
 afterEach(() => {
   delete process.env.CORS_ALLOWED_ORIGINS;
+  delete process.env.ELECTRON_EMBEDDED;
 });
 
 describe("isCorsOriginAllowed", () => {
@@ -41,4 +45,41 @@ describe("isCorsOriginAllowed", () => {
     const req = request({ host: "termix.example" });
     expect(isCorsOriginAllowed(req, "https://attacker.example")).toBe(false);
   });
+
+  describe("embedded desktop backend", () => {
+    it("rejects other websites even with no allowlist", () => {
+      process.env.ELECTRON_EMBEDDED = "true";
+      const req = request({ host: "127.0.0.1:30001" });
+      expect(isCorsOriginAllowed(req, "https://evil.example")).toBe(false);
+    });
+
+    it("still allows the desktop app and the dev server", () => {
+      process.env.ELECTRON_EMBEDDED = "true";
+      const req = request({ host: "127.0.0.1:30001" });
+      expect(isCorsOriginAllowed(req, "file://")).toBe(true);
+      expect(isCorsOriginAllowed(req, "http://localhost:5173")).toBe(true);
+      expect(isCorsOriginAllowed(req, undefined)).toBe(true);
+    });
+
+    it("allows an explicitly configured origin", () => {
+      process.env.ELECTRON_EMBEDDED = "true";
+      process.env.CORS_ALLOWED_ORIGINS = "https://portal.example";
+      expect(isCorsOriginAllowed(request(), "https://portal.example")).toBe(
+        true,
+      );
+    });
+  });
+});
+
+describe("isDesktopAppOrigin", () => {
+  it("accepts no origin, file:// and the dev server", () => {
+    expect(isDesktopAppOrigin(undefined)).toBe(true);
+    expect(isDesktopAppOrigin("file://")).toBe(true);
+    expect(isDesktopAppOrigin("http://127.0.0.1:5173")).toBe(true);
+  });
+
+  it("rejects websites", () => {
+    expect(isDesktopAppOrigin("https://evil.example")).toBe(false);
+    expect(isDesktopAppOrigin("null")).toBe(false);
+  });
 });
diff --git a/src/backend/utils/cors-config.ts b/src/backend/utils/cors-config.ts
index a5c7a1d78..979991cf4 100644
--- a/src/backend/utils/cors-config.ts
+++ b/src/backend/utils/cors-config.ts
@@ -14,15 +14,28 @@ function getAllowedOrigins(): string[] {
     .filter(Boolean);
 }
 
+/**
+ * Origins allowed to drive the embedded desktop backend: the desktop app
+ * itself and the dev server. Any website open in a browser on the same
+ * machine can reach loopback too, so nothing else gets in.
+ */
+export function isDesktopAppOrigin(origin: string | undefined): boolean {
+  if (!origin) return true;
+  return (
+    DEV_ORIGINS.includes(origin) || origin.startsWith(ELECTRON_FILE_ORIGIN)
+  );
+}
+
 export function isCorsOriginAllowed(
   req: Request,
   origin: string | undefined,
 ): boolean {
-  if (!origin) return true;
-  if (DEV_ORIGINS.includes(origin)) return true;
-  if (origin.startsWith(ELECTRON_FILE_ORIGIN)) return true;
+  if (isDesktopAppOrigin(origin)) return true;
 
   const configured = getAllowedOrigins();
+  if (process.env.ELECTRON_EMBEDDED === "true") {
+    return configured.includes(origin) || origin === getRequestOrigin(req);
+  }
   if (configured.length === 0) return true;
   if (configured.includes(origin)) return true;