diff --git a/PROJECT_INSTRUCTIONS.md b/PROJECT_INSTRUCTIONS.md
index a19eb2f..ceb010b 100644
--- a/PROJECT_INSTRUCTIONS.md
+++ b/PROJECT_INSTRUCTIONS.md
@@ -24,7 +24,7 @@ If anything conflicts, follow **this** doc.
   - `docker compose -f docker-compose.dev.yml up -d --build backend`
 - After backend env/CORS changes, recreate the backend service so `backend/.env` is reloaded:
   - `docker compose -f docker-compose.dev.yml up -d --force-recreate --no-deps backend`
-- For the Docker frontend on port `3010`, `ALLOWED_ORIGINS` must include the exact browser origin, for example `http://localhost:3010` and `http://127.0.0.1:3010`.
+- For the Docker frontend on port `3010`, `ALLOWED_ORIGINS` must include exact loopback browser origins such as `http://localhost:3010` and `http://127.0.0.1:3010`; private LAN origins such as `http://192.168.7.45:3010` are allowed by the backend CORS helper for local device testing.
 - Verify the restarted API with `GET http://127.0.0.1:5000/` and `GET http://127.0.0.1:5000/config`.
 - Do not print or commit real `.env` values while checking or updating local Docker env.
 
diff --git a/backend/app.js b/backend/app.js
index c6a97f1..cea98ba 100644
--- a/backend/app.js
+++ b/backend/app.js
@@ -4,6 +4,7 @@ const path = require("path");
 const User = require("./models/user.model");
 const requestIdMiddleware = require("./middleware/request-id");
 const { sendError } = require("./utils/http");
+const { isAllowedCorsOrigin, parseAllowedOrigins } = require("./utils/cors-origins");
 
 const app = express();
 app.use(requestIdMiddleware);
@@ -14,17 +15,11 @@ if (process.env.NODE_ENV !== "production") {
   app.use("/test", express.static(path.join(__dirname, "public")));
 }
 
-const allowedOrigins = (process.env.ALLOWED_ORIGINS || "")
-  .split(",")
-  .map((origin) => origin.trim())
-  .filter(Boolean);
+const allowedOrigins = parseAllowedOrigins(process.env.ALLOWED_ORIGINS || "");
 app.use(
   cors({
     origin: function (origin, callback) {
-      if (!origin) return callback(null, true);
-      if (allowedOrigins.includes(origin)) return callback(null, true);
-      if (/^http:\/\/192\.168\.\d+\.\d+/.test(origin)) return callback(null, true);
-      if (/^https:\/\/192\.168\.\d+\.\d+/.test(origin)) return callback(null, true);
+      if (isAllowedCorsOrigin(origin, allowedOrigins)) return callback(null, true);
       console.error(`CORS blocked origin: ${origin}`);
       callback(new Error(`CORS blocked: ${origin}. Add this origin to ALLOWED_ORIGINS environment variable.`));
     },
diff --git a/backend/tests/cors-origins.test.js b/backend/tests/cors-origins.test.js
new file mode 100644
index 0000000..9ad4bbc
--- /dev/null
+++ b/backend/tests/cors-origins.test.js
@@ -0,0 +1,37 @@
+const {
+  isAllowedCorsOrigin,
+  isPrivateLanHostname,
+  parseAllowedOrigins,
+} = require("../utils/cors-origins");
+
+describe("CORS origin helpers", () => {
+  test("parses configured allowed origins", () => {
+    expect(parseAllowedOrigins(" http://localhost:3010, http://127.0.0.1:3010 ,,")).toEqual([
+      "http://localhost:3010",
+      "http://127.0.0.1:3010",
+    ]);
+  });
+
+  test("allows exact configured origins", () => {
+    const allowedOrigins = ["http://localhost:3010"];
+
+    expect(isAllowedCorsOrigin("http://localhost:3010", allowedOrigins)).toBe(true);
+  });
+
+  test("allows browser access from the local private network", () => {
+    expect(isAllowedCorsOrigin("http://192.168.7.45:3010", [])).toBe(true);
+  });
+
+  test("recognizes RFC1918 private IPv4 hostnames", () => {
+    expect(isPrivateLanHostname("10.1.2.3")).toBe(true);
+    expect(isPrivateLanHostname("172.16.0.1")).toBe(true);
+    expect(isPrivateLanHostname("172.31.255.254")).toBe(true);
+    expect(isPrivateLanHostname("192.168.7.45")).toBe(true);
+  });
+
+  test("rejects public and lookalike hostnames", () => {
+    expect(isAllowedCorsOrigin("http://192.168.7.45.example.test:3010", [])).toBe(false);
+    expect(isAllowedCorsOrigin("http://203.0.113.10:3010", [])).toBe(false);
+    expect(isAllowedCorsOrigin("ftp://192.168.7.45:3010", [])).toBe(false);
+  });
+});
diff --git a/backend/utils/cors-origins.js b/backend/utils/cors-origins.js
new file mode 100644
index 0000000..42c4a74
--- /dev/null
+++ b/backend/utils/cors-origins.js
@@ -0,0 +1,54 @@
+function parseAllowedOrigins(value = "") {
+  return String(value)
+    .split(",")
+    .map((origin) => origin.trim())
+    .filter(Boolean);
+}
+
+function parseIpv4(hostname) {
+  const parts = hostname.split(".");
+  if (parts.length !== 4) return null;
+
+  const octets = parts.map((part) => {
+    if (!/^\d{1,3}$/.test(part)) return null;
+
+    const value = Number(part);
+    return value >= 0 && value <= 255 ? value : null;
+  });
+
+  return octets.some((octet) => octet === null) ? null : octets;
+}
+
+function isPrivateLanHostname(hostname) {
+  const octets = parseIpv4(hostname);
+  if (!octets) return false;
+
+  const [first, second] = octets;
+  return (
+    first === 10 ||
+    (first === 172 && second >= 16 && second <= 31) ||
+    (first === 192 && second === 168)
+  );
+}
+
+function isAllowedCorsOrigin(origin, allowedOrigins = []) {
+  if (!origin) return true;
+  if (allowedOrigins.includes(origin)) return true;
+
+  try {
+    const url = new URL(origin);
+    if (url.protocol !== "http:" && url.protocol !== "https:") {
+      return false;
+    }
+
+    return isPrivateLanHostname(url.hostname);
+  } catch {
+    return false;
+  }
+}
+
+module.exports = {
+  isAllowedCorsOrigin,
+  isPrivateLanHostname,
+  parseAllowedOrigins,
+};