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fix(web): guard the app-origin port and the trusted-origin grant

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Copilot review round 3 on #2100.

- MCP_APP_ORIGIN_PORT could equal CLIENT_PORT or MCP_SANDBOX_PORT. The
  app-origin listener starts BEFORE the web server binds, so that is not a
  race it loses — it wins the port and the Inspector dies with EADDRINUSE.
  `CLIENT_PORT=6278` was a valid config before this feature existed. It now
  resolves to a dynamic port with a warning: of the three listeners this is
  the only one that can move, and refusing to boot would cost the user the
  whole Inspector over a feature they may not be using.
- The proxy's same-origin refusal covered only its own origin. An app served
  from the INSPECTOR's origin is no safer: same-origin with the top document
  means direct access to window.top, its DOM and its storage, and the
  cross-origin frame in between buys nothing. Both trusted origins are now
  refused before allow-same-origin is granted.
- Replaced the `as unknown as number` on `server.address()` with a real
  narrowing of the string/null members of its return type.
- MCP_APP_ORIGIN_PORT added to the v1→v2 migration guide's env table, its
  Docker recipe, and a troubleshooting entry for the symptom this actually
  produces (the app renders, but its own backend rejects `Origin: null`).
- clients/web/README.md now states which failure is outside the fallback
  guarantee — a published document the browser cannot reach — and why there
  is no honest signal to fall back on. Docs and code no longer disagree.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>
This commit is contained in:
cliffhall 2026-08-24 00:35:21 -04:00 • committed by GitHub
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30
clients/web/server/web-server-config.ts
View file

@ -377,7 +377,35 @@ export function buildWebServerConfig(
"");
const sandboxPort = resolveSandboxPort();
const appOriginPort = resolveAppOriginPort();
// The app-origin listener starts BEFORE the web server binds (both in prod
// `startHonoServer` and in the Vite plugin), so a configured collision is not
// a harmless race — the app origin wins the port and the Inspector itself
// then dies with EADDRINUSE. `CLIENT_PORT=6278` is now such a config, and it
// was perfectly valid before this feature existed.
//
// Resolved to a dynamic port rather than by refusing to boot: of the three
// listeners this is the only one that can move (the sandbox URL and the web
// port are both advertised to the browser as fixed), and its own EADDRINUSE
// path already degrades this way. Losing a predictable app origin costs an
// app declaring `_meta.ui.domain` its forwarded port; refusing to boot costs
// the user the whole Inspector over a feature they may not be using.
const requestedAppOriginPort = resolveAppOriginPort();
const appOriginCollidesWith =
requestedAppOriginPort !== 0 &&
(requestedAppOriginPort === port
? "the web server (CLIENT_PORT)"
: requestedAppOriginPort === sandboxPort
? "the MCP Apps sandbox (MCP_SANDBOX_PORT)"
: null);
if (appOriginCollidesWith) {
console.warn(
`App origin: port ${requestedAppOriginPort} is already ${appOriginCollidesWith}; ` +
`using an OS-assigned port instead. Apps declaring _meta.ui.domain will still ` +
`render, but the origin they are served from is no longer predictable — set ` +
`MCP_APP_ORIGIN_PORT to a free one if your app's backend allowlists it.`,
);
}
const appOriginPort = appOriginCollidesWith ? 0 : requestedAppOriginPort;
let logger: Logger | undefined;
if (process.env.MCP_LOG_FILE) {