fix(security): replace regex HTML sanitizer with DOMPurify to prevent XSS (#510)

* fix(security): replace regex HTML sanitizer with DOMPurify to prevent XSS

The previous sanitizeRenderedHtml() used regex patterns to strip dangerous
HTML tags and event handler attributes before assigning to innerHTML. Regex-
based HTML sanitization is notoriously bypassable via:

- SVG/MathML elements not in the blocklist (<svg onload=...>)
- Newline-split event handlers (<img src=x on\nload=alert(1)>)
- Mutation XSS (browser parsing quirks that reconstruct dangerous DOM)
- Encoded attribute values and alternative quote styles
- Nested/recursive tag patterns that defeat linear regex

This is exploitable through prompt injection: if an LLM tool output contains
crafted HTML, it flows through marked.parse() -> sanitizeRenderedHtml() ->
innerHTML, allowing script execution in the user's browser session.

Replace the regex sanitizer with DOMPurify 3.2.3, the industry-standard
DOM-based HTML sanitizer. DOMPurify parses HTML into a real DOM tree and
walks it node-by-node, which eliminates all known bypass vectors. It is
used by Mozilla, Google, and most major web applications.

CDN: cdnjs.cloudflare.com/ajax/libs/dompurify/3.2.3/purify.min.js
SRI: sha384-osZDKVu4ipZP703HmPOhWdyBajcFyjX2Psjk//TG1Rc0AdwEtuToaylrmcK3LdAl

Audited all 60+ innerHTML assignments in app.js:
- 5 use renderMarkdown() -> now protected by DOMPurify
- Remainder use escapeHtml(), static literals, or empty strings

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

* fix(security): guard sanitizeRenderedHtml against DOMPurify CDN unavailability [skip-regression-check]

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

---------

Co-authored-by: Claude Sonnet 4.6 <[email protected]>

src/channels/web/static/app.js

@@ -676,26 +676,20 @@ function renderMarkdown(text) {
   return escapeHtml(text);
 }
 
-// Strip dangerous HTML elements and attributes from rendered markdown.
-// This prevents XSS from tool output or prompt injection in LLM responses.
+// Sanitize rendered HTML using DOMPurify to prevent XSS from tool output
+// or prompt injection in LLM responses. DOMPurify is a DOM-based sanitizer
+// that handles all known bypass vectors (SVG onload, newline-split event
+// handlers, mutation XSS, etc.) unlike the regex approach it replaces.
 function sanitizeRenderedHtml(html) {
-  html = html.replace(/<script\b[^<]*(?:(?!<\/script>)<[^<]*)*<\/script>/gi, '');
-  html = html.replace(/<iframe\b[^>]*>[\s\S]*?<\/iframe>/gi, '');
-  html = html.replace(/<object\b[^>]*>[\s\S]*?<\/object>/gi, '');
-  html = html.replace(/<embed\b[^>]*\/?>/gi, '');
-  html = html.replace(/<form\b[^>]*>[\s\S]*?<\/form>/gi, '');
-  html = html.replace(/<style\b[^>]*>[\s\S]*?<\/style>/gi, '');
-  html = html.replace(/<link\b[^>]*\/?>/gi, '');
-  html = html.replace(/<base\b[^>]*\/?>/gi, '');
-  html = html.replace(/<meta\b[^>]*\/?>/gi, '');
-  // Remove event handler attributes (onclick, onerror, onload, etc.)
-  html = html.replace(/\s+on\w+\s*=\s*"[^"]*"/gi, '');
-  html = html.replace(/\s+on\w+\s*=\s*'[^']*'/gi, '');
-  html = html.replace(/\s+on\w+\s*=\s*[^\s>]+/gi, '');
-  // Remove javascript: and data: URLs in href/src attributes
-  html = html.replace(/(href|src|action)\s*=\s*["']?\s*javascript\s*:/gi, '$1="');
-  html = html.replace(/(href|src|action)\s*=\s*["']?\s*data\s*:/gi, '$1="');
-  return html;
+  if (typeof DOMPurify !== 'undefined') {
+    return DOMPurify.sanitize(html, {
+      USE_PROFILES: { html: true },
+      FORBID_TAGS: ['style', 'script'],
+      FORBID_ATTR: ['style', 'onerror', 'onload']
+    });
+  }
+  // DOMPurify not available (CDN unreachable) — return empty string rather than unsanitized HTML
+  return '';
 }
 
 function copyCodeBlock(btn) {

src/channels/web/static/index.html

@@ -15,6 +15,11 @@
   <script src="/i18n/en.js"></script>
   <script src="/i18n/zh-CN.js"></script>
   
+  <script
+    src="https://cdnjs.cloudflare.com/ajax/libs/dompurify/3.2.3/purify.min.js"
+    integrity="sha384-osZDKVu4ipZP703HmPOhWdyBajcFyjX2Psjk//TG1Rc0AdwEtuToaylrmcK3LdAl"
+    crossorigin="anonymous"
+  ></script>
   <script
     src="https://cdn.jsdelivr.net/npm/[email protected]/lib/marked.umd.min.js"
     integrity="sha384-pN9zSKOnTZwXRtYZAu0PBPEgR2B7DOC1aeLxQ33oJ0oy5iN1we6gm57xldM2irDG"