diff --git a/electron/external-login.cjs b/electron/external-login.cjs
new file mode 100644
index 000000000..9b4d18d70
--- /dev/null
+++ b/electron/external-login.cjs
@@ -0,0 +1,20 @@
+/**
+ * The login URL a server hands the desktop app goes to shell.openExternal,
+ * so only web pages pass: any other scheme (file:, smb:, ms-msdt:, ...)
+ * would launch whatever OS handler is registered for it.
+ */
+function isExternalLoginUrl(value) {
+  try {
+    const { protocol } = new URL(value);
+    return protocol === "https:" || protocol === "http:";
+  } catch {
+    return false;
+  }
+}
+
+/** A non-privileged port for the loopback callback listener. */
+function isCallbackPort(value) {
+  return Number.isInteger(value) && value >= 1024 && value <= 65535;
+}
+
+module.exports = { isExternalLoginUrl, isCallbackPort };
diff --git a/electron/main.cjs b/electron/main.cjs
index bf781b319..1a20f6bc2 100644
--- a/electron/main.cjs
+++ b/electron/main.cjs
@@ -36,6 +36,7 @@ const { quitApp } = require("./app-quit.cjs");
 const { selectLinuxPasswordStore } = require("./linux-password-store.cjs");
 const { resolveLocalShell, repairSpawnHelpers } = require("./local-shell.cjs");
 const { registerLocalFileHandlers } = require("./local-files.cjs");
+const { isExternalLoginUrl, isCallbackPort } = require("./external-login.cjs");
 
 const localTerminalSessions = new Map();
 
@@ -1663,6 +1664,12 @@ ipcMain.handle("get-embedded-server-status", () => {
 ipcMain.handle(
   "external-browser-login",
   async (_event, authUrl, callbackPort) => {
+    if (!isExternalLoginUrl(authUrl)) {
+      return { success: false, error: "Invalid authentication URL" };
+    }
+    if (!isCallbackPort(callbackPort)) {
+      return { success: false, error: "Invalid callback port" };
+    }
     const http = require("http");
 
     return new Promise((resolve) => {
diff --git a/scripts/electron-external-login.test.ts b/scripts/electron-external-login.test.ts
new file mode 100644
index 000000000..c143efe3d
--- /dev/null
+++ b/scripts/electron-external-login.test.ts
@@ -0,0 +1,41 @@
+import { createRequire } from "node:module";
+import { describe, expect, it } from "vitest";
+
+const require = createRequire(import.meta.url);
+const { isExternalLoginUrl, isCallbackPort } =
+  require("../electron/external-login.cjs") as {
+    isExternalLoginUrl: (value: unknown) => boolean;
+    isCallbackPort: (value: unknown) => boolean;
+  };
+
+describe("Electron external login", () => {
+  it.each([
+    "https://idp.example.com/authorize?client_id=termix",
+    "http://keycloak.lan:8080/realms/home/protocol/openid-connect/auth",
+  ])("opens web login URLs: %s", (url) => {
+    expect(isExternalLoginUrl(url)).toBe(true);
+  });
+
+  it.each([
+    "file:///C:/Windows/System32/calc.exe",
+    "smb://attacker/share",
+    "ms-msdt:/id PCWDiagnostic",
+    "javascript:alert(1)",
+    "not a url",
+    undefined,
+  ])("refuses other schemes: %s", (url) => {
+    expect(isExternalLoginUrl(url)).toBe(false);
+  });
+
+  it("accepts the ports the login screen picks", () => {
+    expect(isCallbackPort(17832)).toBe(true);
+    expect(isCallbackPort(17931)).toBe(true);
+  });
+
+  it.each([0, 80, 65536, 17832.5, "17832", NaN, undefined])(
+    "refuses callback port %s",
+    (port) => {
+      expect(isCallbackPort(port)).toBe(false);
+    },
+  );
+});