diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 4f173bc..4f1535d 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -38,3 +38,7 @@
 **Vulnerability:** Missing input validation on `setLanguage()` could allow invalid strings (like Prototype Pollution payloads or arbitrary text) to be applied to the DOM (`lang` attribute) and stored in `localStorage`.
 **Learning:** The global `setLanguage` function assumed inputs would only come from predefined button clicks, skipping runtime validation.
 **Prevention:** Always sanitize and validate function arguments at the application boundary, even if the primary caller is trusted, to enforce defense in depth.
+## 2026-07-20 - Enforce strict default-src 'none' CSP on index.html
+**Vulnerability:** Weak CSP utilizing `default-src 'self'` could inadvertently allow loading unexpected resource types if they share the same origin, potentially widening the attack surface.
+**Learning:** It is more secure to use an explicit allowlist approach (`default-src 'none'`) and explicitly grant `self` only to required resource types (script, style, img, font, connect).
+**Prevention:** Always default to `default-src 'none'` and explicitly add directives for what is strictly needed, rather than relying on an implicitly broad `default-src 'self'`.
diff --git a/index.html b/index.html
index c40fea3..fd888db 100644
--- a/index.html
+++ b/index.html
@@ -3,7 +3,7 @@
   <head>
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
-    <meta http-equiv="Content-Security-Policy" content="default-src 'self'; img-src 'self'; object-src 'none'; base-uri 'self'; form-action 'none'; upgrade-insecure-requests; require-trusted-types-for 'script';">
+    <meta http-equiv="Content-Security-Policy" content="default-src 'none'; script-src 'self'; style-src 'self'; img-src 'self'; font-src 'self'; connect-src 'self'; object-src 'none'; base-uri 'none'; form-action 'none'; frame-src 'none'; upgrade-insecure-requests; require-trusted-types-for 'script';">
     <meta name="referrer" content="strict-origin-when-cross-origin">
     <title>맥락지혜 연구실 | Contextual Wisdom Lab</title>
     <meta
diff --git a/tests/test_index_security.py b/tests/test_index_security.py
new file mode 100644
index 0000000..d13d9c5
--- /dev/null
+++ b/tests/test_index_security.py
@@ -0,0 +1,47 @@
+"""Security tests for the main index.html."""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+INDEX = ROOT / "index.html"
+
+
+def _index_html() -> str:
+    """Return the index HTML source."""
+    return INDEX.read_text(encoding="utf-8")
+
+
+def _csp_content(html: str) -> str:
+    """Extract the CSP meta policy from the index HTML."""
+    match = re.search(
+        r'<meta\s+http-equiv="Content-Security-Policy"\s+content="([^"]+)"',
+        html,
+    )
+    assert match is not None, "index must declare a CSP meta policy"
+    return match.group(1)
+
+
+def test_index_declares_strict_csp() -> None:
+    """The main site limits active content to strict whitelist."""
+    policy = _csp_content(_index_html())
+
+    for directive in (
+        "default-src 'none'",
+        "script-src 'self'",
+        "style-src 'self'",
+        "img-src 'self'",
+        "font-src 'self'",
+        "connect-src 'self'",
+        "object-src 'none'",
+        "base-uri 'none'",
+        "form-action 'none'",
+        "frame-src 'none'",
+        "upgrade-insecure-requests",
+        "require-trusted-types-for 'script'",
+    ):
+        assert directive in policy, f"Missing {directive} in CSP"
+    assert "'unsafe-inline'" not in policy
+    assert "'unsafe-eval'" not in policy
