From d8d98caa784f4dee791bdfef8ef338abd582b933 Mon Sep 17 00:00:00 2001
From: Ocean Bennett <204957658+undergroundrap@users.noreply.github.com>
Date: Sat, 4 Jul 2026 17:21:18 -0400
Subject: [PATCH] fix(security): sanitize email rich body render path (#5212)

---
 static/js/document.js                  | 19 +++++++++++++------
 static/js/markdown.js                  |  3 ++-
 tests/test_markdown_dom_xss_helpers.py | 13 +++++++++++++
 3 files changed, 28 insertions(+), 7 deletions(-)

diff --git a/static/js/document.js b/static/js/document.js
index b245ec852..0aadaa503 100644
--- a/static/js/document.js
+++ b/static/js/document.js
@@ -2378,20 +2378,27 @@ import { bindMenuDismiss, dismissOrRemove } from './escMenuStack.js';
   }
 
   // ── WYSIWYG email body helpers ──
+  function _emailPlainTextToHtml(text) {
+    const d = document.createElement('div');
+    d.textContent = text == null ? '' : String(text);
+    return d.innerHTML.replace(/\n/g, '<br>');
+  }
+
   function _emailBodyToHtml(text) {
     const t = (text || '').trim();
     if (!t) return '';
     // If it already contains a formatting/structural HTML tag, it's a saved
-    // WYSIWYG body — use it verbatim. (Checking a leading '<' isn't enough: a
+    // WYSIWYG body — sanitize it before rendering. (Checking a leading '<' isn't enough: a
     // rich body often starts with plain text, e.g. "Hi <b>there</b>".)
-    if (/<\/?(b|i|u|s|strong|em|del|strike|a|p|div|br|ul|ol|li|h[1-3]|blockquote|span|code|pre)\b[^>]*>/i.test(t)) return t;
+    if (/<\/?(b|i|u|s|strong|em|del|strike|a|p|div|br|ul|ol|li|h[1-3]|blockquote|span|code|pre)\b[^>]*>/i.test(t)) {
+      return markdownModule.sanitizeAllowedHtml
+        ? markdownModule.sanitizeAllowedHtml(t)
+        : _emailPlainTextToHtml(t);
+    }
     // Email body: keep author-typed `:shortcode:` text literal. Issue #345
     // (shortcode → emoji) is scoped to chat; do not rewrite colons in mail.
     try { return markdownModule.mdToHtml(text, { shortcodes: false }); }
-    catch (_) {
-      const d = document.createElement('div'); d.textContent = text;
-      return d.innerHTML.replace(/\n/g, '<br>');
-    }
+    catch (_) { return _emailPlainTextToHtml(text); }
   }
   // Mirror the rich body's plain text into the hidden textarea so the existing
   // send / draft / change-detection plumbing (which reads the textarea) stays
diff --git a/static/js/markdown.js b/static/js/markdown.js
index f2bb7f85f..a3b27fbc8 100644
--- a/static/js/markdown.js
+++ b/static/js/markdown.js
@@ -133,7 +133,7 @@ function _cleanAllowedHtmlOnce(htmlString) {
   return tpl.innerHTML;
 }
 
-function sanitizeAllowedHtml(html) {
+export function sanitizeAllowedHtml(html) {
   const raw = String(html == null ? '' : html);
   // Non-browser context (e.g. a future SSR/Node import): fail closed by
   // escaping rather than trusting the markup.
@@ -824,6 +824,7 @@ export function renderMermaid(container) {
 const markdownModule = {
   escapeHtml,
   mdToHtml,
+  sanitizeAllowedHtml,
   squashOutsideCode,
   renderContent,
   processWithThinking,
diff --git a/tests/test_markdown_dom_xss_helpers.py b/tests/test_markdown_dom_xss_helpers.py
index 25b18417d..db9ab9c9b 100644
--- a/tests/test_markdown_dom_xss_helpers.py
+++ b/tests/test_markdown_dom_xss_helpers.py
@@ -23,3 +23,16 @@ def test_markdown_raw_html_sanitizer_strips_scriptable_css():
     assert "if (name === 'style')" in src
     assert r"javascript:|vbscript:|data:|expression\(" in src
     assert "el.removeAttribute(attr.name);" in src
+
+
+def test_email_rich_body_render_path_reuses_raw_html_sanitizer():
+    markdown_src = (_REPO / "static" / "js" / "markdown.js").read_text(encoding="utf-8")
+    document_src = (_REPO / "static" / "js" / "document.js").read_text(encoding="utf-8")
+    email_body_helper = document_src.split("function _emailBodyToHtml(text)", 1)[1].split(
+        "  // Mirror the rich body's plain text", 1
+    )[0]
+
+    assert "export function sanitizeAllowedHtml(html)" in markdown_src
+    assert "sanitizeAllowedHtml," in markdown_src
+    assert "markdownModule.sanitizeAllowedHtml(t)" in email_body_helper
+    assert "return t;" not in email_body_helper