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fix: restrict HTML preview network access #574

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fix: restrict HTML preview network access #574

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15 changes: 13 additions & 2 deletions src/ui/file-preview/src/components/html-renderer.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -4,12 +4,23 @@
*
* The rendered preview runs inside a nested sandboxed iframe, which is itself inside
* the MCP app's sandboxed iframe chain. Scripts and external resources (CDNs) are
* allowed since the sandbox isolation prevents any escape.
* allowed for inline interactivity, while CSP blocks outbound requests.
*/
import { renderCodeViewer } from './code-viewer.js';
import { escapeHtml } from './highlighting.js';
import type { HtmlPreviewMode } from '../types.js';

const HTML_PREVIEW_CSP = [
"default-src 'none'",
"script-src 'unsafe-inline'",
"style-src 'unsafe-inline'",
'img-src data: blob:',
'media-src data: blob:',
"connect-src 'none'",
"form-action 'none'",
"base-uri 'none'",
].join('; ');

function resolveThemeFrameStyles(): { background: string; text: string; fontFamily: string } {
if (typeof window === 'undefined' || typeof document === 'undefined') {
return {
Expand All @@ -27,7 +38,7 @@ function resolveThemeFrameStyles(): { background: string; text: string; fontFami

function renderSandboxedHtmlFrame(content: string): string {
const palette = resolveThemeFrameStyles();
const frameDocument = `<!doctype html><html><head><meta charset="utf-8" /><style>html,body{margin:0;padding:0;background:${palette.background};color:${palette.text};}body{font-family:${palette.fontFamily};padding:16px;line-height:1.5;}img{max-width:100%;height:auto;}</style></head><body>${content}</body></html>`;
const frameDocument = `<!doctype html><html><head><meta charset="utf-8" /><meta http-equiv="Content-Security-Policy" content="${HTML_PREVIEW_CSP}" /><style>html,body{margin:0;padding:0;background:${palette.background};color:${palette.text};}body{font-family:${palette.fontFamily};padding:16px;line-height:1.5;}img{max-width:100%;height:auto;}</style></head><body>${content}</body></html>`;
return `<iframe class="html-rendered-frame" title="Rendered HTML preview" sandbox="allow-scripts allow-forms allow-popups" referrerpolicy="no-referrer" srcdoc="${escapeHtml(frameDocument)}"></iframe>`;
}

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12 changes: 12 additions & 0 deletions test/test-html-preview-csp.js
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Original file line number Diff line number Diff line change
@@ -0,0 +1,12 @@
import assert from 'assert';
import { renderHtmlPreview } from '../dist/ui/file-preview/src/components/html-renderer.js';

const rendered = renderHtmlPreview('<script>fetch("https://example.com")</script>', 'rendered');

assert.ok(rendered.html.includes('Content-Security-Policy'));
assert.ok(rendered.html.includes('connect-src &#39;none&#39;'));
assert.ok(rendered.html.includes('form-action &#39;none&#39;'));
assert.ok(rendered.html.includes('sandbox="allow-scripts allow-forms allow-popups"'));
assert.ok(rendered.html.includes('&lt;script&gt;fetch(&quot;https://example.com&quot;)&lt;/script&gt;'));

console.log('PASS: rendered HTML previews carry a restrictive CSP');