Enforce Security Boundaries Early

Apply security controls at the controlling layer and before triggering any side effects.

• Iframe/sandbox: Treat sandbox on ancestor iframes as the effective policy for descendants. Ensure the default sandbox includes any permissions required by the hosted app (otherwise functionality may break or be silently over-restricted).
• Allowlists/URLs: For deep links or user-provided URIs, validate and apply allowlist decisions before calling discovery or any backend operation that could fetch manifests, probe endpoints, or initialize services. Re-check after resolution for defense in depth.

Example pattern:

// (1) Correct effective sandbox policy at the controlling layer
iframe.setAttribute(
  'sandbox',
  sandbox.permissions || 'allow-scripts allow-same-origin allow-forms'
);

// (2) Early allowlist enforcement before backend side effects
const uri = new URL(link).searchParams.get('uri');
if (!uri) throw new Error('mcp deep link is missing the uri parameter');

// allowlist check MUST happen BEFORE discoverExtension
if (!isAllowedMcpUri(uri)) {
  throw new Error('Blocked MCP URI by allowlist');
}

const { data, error } = await discoverExtension({ body: { uri } });
// (defense in depth) optionally re-check resolved endpoint/host here

Use this as a review checklist for any code that: 1) constructs sandboxed browsing contexts, or 2) accepts URIs/deep links and performs discovery/network calls.