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Update CSP documentation for widget-declared mode #1064

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44 changes: 31 additions & 13 deletions docs/contributing/mcp-apps-architecture.mdx
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Expand Up @@ -271,19 +271,36 @@ Serves the sandbox proxy HTML that creates the double-iframe architecture.

### Content Security Policy

The sandbox proxy uses a permissive CSP to allow widget content:
The sandbox proxy uses different CSP modes based on widget trust level:

```html
<meta
http-equiv="Content-Security-Policy"
content="
default-src 'self';
script-src 'self' 'unsafe-inline' 'unsafe-eval' blob: data: https://cdn.tailwindcss.com ...;
style-src * blob: data: 'unsafe-inline';
connect-src *;
...
"
/>
**Widget-declared mode** (strict): When a widget declares `connect_domains` and `resource_domains` in its CSP metadata, only those domains are allowed:

```typescript
// connect-src: Only declared domains + localhost
connectDomains = [
"'self'",
...(widgetCsp?.connect_domains || []),
"http://localhost:*",
"http://127.0.0.1:*",
"ws://localhost:*",
"wss://localhost:*"
];

// img-src, media-src: Only declared resource domains
resourceDomains = [
"'self'",
"data:",
"blob:",
...(widgetCsp?.resource_domains || []),
"http://localhost:*"
];
```

**Permissive mode** (fallback): When no CSP metadata is provided, allows all HTTPS connections:

```typescript
connectDomains = ["'self'", "https:", "wss:", "ws:", ...localhostSources];
resourceDomains = ["'self'", "data:", "blob:", "https:", ...localhostSources];
```

### Iframe sandbox attributes
Expand All @@ -297,7 +314,8 @@ The sandbox proxy uses a permissive CSP to allow widget content:
- Double-iframe provides origin isolation
- `allow-same-origin` required for localStorage access
- Widgets should be treated as semi-trusted code
- CSP headers restrict network access
- Widget-declared CSP mode enforces strict domain allowlists
- Permissive mode used only for backward compatibility

## Related files

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