feat(provenance): nh3 HTML sanitizer for scraped post descriptions

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/utils/html_sanitize.py

@@ -0,0 +1,39 @@
+"""Backend HTML sanitization for scraped Post.description.
+
+Provenance descriptions come from gallery-dl (untrusted third-party HTML).
+We render them via v-html in the UI sub-slice, so they MUST be sanitized
+server-side to a tight allowlist. nh3 (Rust/ammonia bindings) is used;
+bleach is deprecated upstream and intentionally not used.
+"""
+
+import nh3
+
+ALLOWED_TAGS = {
+    "p", "a", "br", "em", "strong", "b", "i", "ul", "ol", "li", "blockquote",
+}
+ALLOWED_ATTRIBUTES = {"a": {"href", "title"}}
+ALLOWED_URL_SCHEMES = {"http", "https"}
+
+
+def sanitize_post_html(raw: str | None) -> str | None:
+    """Return sanitized HTML, or None for null/empty/whitespace input.
+
+    - keeps only ALLOWED_TAGS / ALLOWED_ATTRIBUTES
+    - only http/https hrefs survive (javascript: etc. dropped)
+    - <script> content removed entirely
+    - all <a> get rel="noopener noreferrer" (nh3 default link_rel)
+    - never raises
+    """
+    if raw is None:
+        return None
+    stripped = raw.strip()
+    if not stripped:
+        return None
+    return nh3.clean(
+        stripped,
+        tags=ALLOWED_TAGS,
+        attributes=ALLOWED_ATTRIBUTES,
+        url_schemes=ALLOWED_URL_SCHEMES,
+        link_rel="noopener noreferrer",
+        clean_content_tags={"script", "style"},
+    )

requirements.txt

@@ -26,3 +26,6 @@ gallery-dl>=1.32,<1.33
 # Utilities
 python-dotenv>=1.2,<2.0
 structlog>=25.5,<26.0
+
+# HTML sanitization for scraped post descriptions (FC-2d provenance)
+nh3>=0.2,<0.3

tests/test_html_sanitize.py

@@ -0,0 +1,38 @@
+from backend.app.utils.html_sanitize import sanitize_post_html
+
+
+def test_none_and_blank_return_none():
+    assert sanitize_post_html(None) is None
+    assert sanitize_post_html("") is None
+    assert sanitize_post_html("   \n  ") is None
+
+
+def test_allowed_tags_survive():
+    out = sanitize_post_html("<p>hi <strong>there</strong><br><em>x</em></p>")
+    assert "<p>" in out and "<strong>" in out and "<br>" in out and "<em>" in out
+
+
+def test_disallowed_tags_and_attrs_stripped_text_kept():
+    out = sanitize_post_html('<script>alert(1)</script><p onclick="x">body</p>')
+    assert "<script>" not in out
+    assert "alert(1)" not in out  # clean_content removes script *content* too
+    assert "onclick" not in out
+    assert "body" in out
+
+
+def test_div_stripped_but_text_preserved():
+    out = sanitize_post_html("<div><span>keep</span> me</div>")
+    assert "<div>" not in out and "<span>" not in out
+    assert "keep" in out and "me" in out
+
+
+def test_links_get_rel_and_keep_http():
+    out = sanitize_post_html('<a href="https://example.com/p">link</a>')
+    assert 'href="https://example.com/p"' in out
+    assert 'rel="noopener noreferrer"' in out
+
+
+def test_javascript_href_dropped():
+    out = sanitize_post_html('<a href="javascript:alert(1)">x</a>')
+    assert "javascript:" not in out
+    assert "x" in out  # text preserved, dangerous href removed