diff --git a/booru_viewer/gui/_source_html.py b/booru_viewer/gui/_source_html.py
new file mode 100644
index 0000000..dca5dff
--- /dev/null
+++ b/booru_viewer/gui/_source_html.py
@@ -0,0 +1,34 @@
+"""Pure helper for the info-panel Source line.
+
+Lives in its own module so the helper can be unit-tested from CI
+without pulling in PySide6. ``info_panel.py`` imports it.
+"""
+
+from __future__ import annotations
+
+from html import escape
+
+
+def build_source_html(source: str | None) -> str:
+    """Build the rich-text fragment for the Source line in the info panel.
+
+    The fragment is inserted into a QLabel set to RichText format with
+    setOpenExternalLinks(True) — that means QTextBrowser parses any HTML
+    in *source* as markup. Without escaping, a hostile booru can break
+    out of the href attribute, inject ``<img>`` tracking pixels, or make
+    the visible text disagree with the click target.
+
+    The href is only emitted for an http(s) URL; everything else is
+    rendered as escaped plain text. Both the href value and the visible
+    display text are HTML-escaped (audit finding #6).
+    """
+    if not source:
+        return "none"
+    # Truncate display text but keep the full URL for the link target.
+    display = source if len(source) <= 60 else source[:57] + "..."
+    if source.startswith(("http://", "https://")):
+        return (
+            f'<a href="{escape(source, quote=True)}" '
+            f'style="color: #4fc3f7;">{escape(display)}</a>'
+        )
+    return escape(display)
diff --git a/tests/gui/test_source_html.py b/tests/gui/test_source_html.py
new file mode 100644
index 0000000..2376678
--- /dev/null
+++ b/tests/gui/test_source_html.py
@@ -0,0 +1,87 @@
+"""Tests for the pure info-panel source HTML builder.
+
+Pure Python. No Qt, no network. Validates audit finding #6 — that the
+helper escapes booru-controlled `post.source` before it's interpolated
+into a QTextBrowser RichText document.
+"""
+
+from __future__ import annotations
+
+from booru_viewer.gui._source_html import build_source_html
+
+
+def test_none_returns_literal_none():
+    assert build_source_html(None) == "none"
+    assert build_source_html("") == "none"
+
+
+def test_plain_https_url_renders_escaped_anchor():
+    out = build_source_html("https://example.test/post/1")
+    assert out.startswith('<a href="https://example.test/post/1"')
+    assert ">https://example.test/post/1</a>" in out
+
+
+def test_long_url_display_text_truncated_but_href_full():
+    long_url = "https://example.test/" + "a" * 200
+    out = build_source_html(long_url)
+    # href contains the full URL
+    assert long_url in out.replace("&amp;", "&")
+    # Display text is truncated to 57 chars + "..."
+    assert "..." in out
+
+
+def test_double_quote_in_url_escaped():
+    """A `"` in the source must not break out of the href attribute."""
+    hostile = 'https://attacker.test/"><img src=x>'
+    out = build_source_html(hostile)
+    # Raw <img> must NOT appear — html.escape converts < to &lt;
+    assert "<img" not in out
+    # The display text must also have the raw markup escaped.
+    assert "&gt;" in out or "&quot;" in out
+
+
+def test_html_tags_in_url_escaped():
+    hostile = 'https://attacker.test/<script>alert(1)</script>'
+    out = build_source_html(hostile)
+    assert "<script>" not in out
+    assert "&lt;script&gt;" in out
+
+
+def test_non_url_source_rendered_as_escaped_plain_text():
+    """A source string that isn't an http(s) URL is rendered as plain
+    text — no <a> tag, but still HTML-escaped."""
+    out = build_source_html("not a url <b>at all</b>")
+    assert "<a" not in out
+    assert "<b>" not in out
+    assert "&lt;b&gt;" in out
+
+
+def test_javascript_url_does_not_become_anchor():
+    """Sources that don't start with http(s) — including `javascript:` —
+    must NOT be wrapped in an <a> tag where they'd become a clickable
+    link target."""
+    out = build_source_html("javascript:alert(1)")
+    assert "<a " not in out
+    assert "alert(1)" in out  # text content preserved (escaped)
+
+
+def test_data_url_does_not_become_anchor():
+    out = build_source_html("data:text/html,<script>x</script>")
+    assert "<a " not in out
+    assert "<script>" not in out
+
+
+def test_ampersand_in_url_escaped():
+    out = build_source_html("https://example.test/?a=1&b=2")
+    # `&` must be `&amp;` inside the href attribute
+    assert "&amp;" in out
+    # Raw `&b=` is NOT acceptable as an attribute value
+    assert 'href="https://example.test/?a=1&amp;b=2"' in out
+
+
+def test_pixiv_real_world_source_unchanged_visually():
+    """Realistic input — a normal pixiv link — should pass through with
+    no surprising changes."""
+    out = build_source_html("https://www.pixiv.net/artworks/12345")
+    assert 'href="https://www.pixiv.net/artworks/12345"' in out
+    assert "https://www.pixiv.net/artworks/12345</a>" in out