CVE-2026-76894: XSS

Summary

The MCP Apps UI resource (an interactive DataStore table rendered in the host's webview/iframe) communicates with its parent via postMessage. Its inbound message listener performs no event.origin validation, and its outbound messages are sent with target origin ''. Any window/frame able to reach the document can inject data into the table and interfere with the MCP UI handshake, and the UI's own JSON-RPC messages are broadcast without an origin restriction.

Affected code

src/resources/datastore-table-ui.ts (inline HTML):

js window.addEventListener('message', function (event) { var msg = event.data; // no check of event.origin if (!msg || typeof msg !== 'object') return; if (msg.id !== undefined && pending[msg.id]) { ... resolve/reject pending MCP request ... } ... if (data && (data.fields || data.records)) initTable(data); // renders attacker-supplied data });

// outbound: window.parent.postMessage({ jsonrpc: '2.0', id: id, method: method, params: params || {} }, ''); mcpNot: window.parent.postMessage({ jsonrpc:'2.0', method:method, params:params||{} }, '');

There is no allowlist of accepted origins for inbound messages and no fixed target origin for outbound messages.

Impact

- Content spoofing. A frame that can postMessage to the UI can supply arbitrary fields/records, causing the table to render fabricated rows and links (e.g. a phishing linkurl) that appear to come from the queried portal. (Values are HTML-escaped, so this is display spoofing, not script injection — no XSS.) - Handshake/UX interference. Because inbound messages matching a pending id resolve/reject the UI's MCP requests, an attacker frame can spoof responses to the ui/initialize handshake or otherwise disrupt the panel. - Outbound leakage. Posting to '' means the UI's JSON-RPC messages are delivered to whatever the parent origin is; if the resource is ever embedded by a context other than the trusted host, those messages leak to it.

Exploitability depends on the host embedding model (in a correct MCP Apps host the parent is the trusted host, which limits who can message the frame), so this is a hardening / defense-in-depth issue rather than a directly remotely-triggerable one — hence Low.

Proof of concept

- poc/verify-no-origin-check.mjs — static confirmation that the listener has no origin check and outbound uses '':

[x] validates event.origin : NO [x] outgoing postMessage targetOrigin : '' (any) RESULT: VULNERABLE

- poc/parent-spoof-poc.html — a page that embeds the UI and injects fabricated rows cross-origin with no handshake; the UI renders them.

Remediation

- Validate event.origin against the expected host origin on every inbound message; drop messages from other origins. - Send outbound postMessage with an explicit target origin (the host origin established at initialize time) instead of ''. - Optionally, bind to a nonce/channel established during ui/initialize so only the negotiated host can drive the panel.