fix(security): block plaintext WebSocket connections to non-loopback addresses (#20803)

* fix(security): block plaintext WebSocket connections to non-loopback addresses

Addresses CWE-319 (Cleartext Transmission of Sensitive Information).

Previously, ws:// connections to remote hosts were allowed, exposing
both credentials and chat data to network interception. This change
blocks ALL plaintext ws:// connections to non-loopback addresses,
regardless of whether explicit credentials are configured (device
tokens may be loaded dynamically).

Security policy:
- wss:// allowed to any host
- ws:// allowed only to loopback (127.x.x.x, localhost, ::1)
- ws:// to LAN/tailnet/remote hosts now requires TLS

Changes:
- Add isSecureWebSocketUrl() validation in net.ts
- Block insecure connections in GatewayClient.start()
- Block insecure URLs in buildGatewayConnectionDetails()
- Handle malformed URLs gracefully without crashing
- Update tests to use wss:// for non-loopback URLs

Fixes #12519

* fix(test): update gateway-chat mock to preserve net.js exports

Use importOriginal to spread actual module exports and mock only
the functions needed for testing. This ensures isSecureWebSocketUrl
and other exports remain available to the code under test.

src/gateway/client.ts

@@ -21,6 +21,7 @@ import {
   type GatewayClientName,
 } from "../utils/message-channel.js";
 import { buildDeviceAuthPayload } from "./device-auth.js";
+import { isSecureWebSocketUrl } from "./net.js";
 import {
   type ConnectParams,
   type EventFrame,
@@ -109,6 +110,26 @@ export class GatewayClient {
       this.opts.onConnectError?.(new Error("gateway tls fingerprint requires wss:// gateway url"));
       return;
     }
+
+    // Security check: block ALL plaintext ws:// to non-loopback addresses (CWE-319, CVSS 9.8)
+    // This protects both credentials AND chat/conversation data from MITM attacks.
+    // Device tokens may be loaded later in sendConnect(), so we block regardless of hasCredentials.
+    if (!isSecureWebSocketUrl(url)) {
+      // Safe hostname extraction - avoid throwing on malformed URLs in error path
+      let displayHost = url;
+      try {
+        displayHost = new URL(url).hostname || url;
+      } catch {
+        // Use raw URL if parsing fails
+      }
+      const error = new Error(
+        `SECURITY ERROR: Cannot connect to "${displayHost}" over plaintext ws://. ` +
+          "Both credentials and chat data would be exposed to network interception. " +
+          "Use wss:// for the gateway URL, or connect via SSH tunnel to localhost.",
+      );
+      this.opts.onConnectError?.(error);
+      return;
+    }
     // Allow node screen snapshots and other large responses.
     const wsOptions: ClientOptions = {
       maxPayload: 25 * 1024 * 1024,