harden(l4d2web): default security response headers and generic error handlers

- after_request hook sets X-Content-Type-Options=nosniff, X-Frame-Options=DENY, Referrer-Policy=strict-origin-when-cross-origin, and a strict CSP (default-src 'self', script-src self+nonce, frame-ancestors 'none', form-action 'self'); HSTS added on secure non-test responses
- per-request CSP nonce minted in g.csp_nonce; servers.html's inline showModal script picks it up
- 404 and 500 handlers return short plain-text responses so a misbehaving deployment can't leak tracebacks via Werkzeug's debug page

l4d2web/app.py

@@ -3,7 +3,7 @@ import os
 import secrets
 
 import click
-from flask import Flask, Response, jsonify, redirect, request, session
+from flask import Flask, Response, g, jsonify, redirect, request, session
 
 from l4d2web.auth import current_user, load_current_user
 from l4d2web.cli import register_cli
@@ -78,6 +78,44 @@ def create_app(test_config: dict[str, object] | None = None) -> Flask:
             return Response("csrf token missing or invalid", status=400)
         return None
 
+    @app.before_request
+    def assign_csp_nonce() -> None:
+        g.csp_nonce = secrets.token_urlsafe(16)
+
+    @app.after_request
+    def set_security_headers(response: Response) -> Response:
+        response.headers.setdefault("X-Content-Type-Options", "nosniff")
+        response.headers.setdefault("X-Frame-Options", "DENY")
+        response.headers.setdefault("Referrer-Policy", "strict-origin-when-cross-origin")
+        nonce = getattr(g, "csp_nonce", "")
+        # script-src nonce blocks inline XSS; style 'unsafe-inline' kept for
+        # htmx's auto-injected indicator styles. img/data: for SVG icons.
+        response.headers.setdefault(
+            "Content-Security-Policy",
+            "default-src 'self'; "
+            f"script-src 'self' 'nonce-{nonce}'; "
+            "style-src 'self' 'unsafe-inline'; "
+            "img-src 'self' data:; "
+            "connect-src 'self'; "
+            "frame-ancestors 'none'; "
+            "base-uri 'self'; "
+            "form-action 'self'",
+        )
+        if not app.config.get("TESTING") and request.is_secure:
+            response.headers.setdefault(
+                "Strict-Transport-Security",
+                "max-age=31536000; includeSubDomains",
+            )
+        return response
+
+    @app.errorhandler(404)
+    def not_found(_err):
+        return Response("not found", status=404, mimetype="text/plain")
+
+    @app.errorhandler(500)
+    def internal_error(_err):
+        return Response("internal server error", status=500, mimetype="text/plain")
+
     app.before_request(load_current_user)
     app.register_blueprint(auth_bp)
     app.register_blueprint(overlay_bp)

l4d2web/templates/servers.html

@@ -73,7 +73,7 @@
 {% endif %}
 
 {% if prefill_blueprint_id %}
-<script>
+<script nonce="{{ g.csp_nonce }}">
   document.addEventListener("DOMContentLoaded", () => {
     const dialog = document.getElementById("create-server-modal");
     if (dialog && typeof dialog.showModal === "function") {

l4d2web/tests/test_security.py

@@ -28,3 +28,77 @@ def test_login_rate_limit(client) -> None:
 
     response = client.post("/login", data={"username": "x", "password": "y"})
     assert response.status_code == 429
+
+
+def test_security_headers_present(client) -> None:
+    response = client.get("/login")
+    assert response.headers.get("X-Content-Type-Options") == "nosniff"
+    assert response.headers.get("X-Frame-Options") == "DENY"
+    assert response.headers.get("Referrer-Policy") == "strict-origin-when-cross-origin"
+    csp = response.headers.get("Content-Security-Policy")
+    assert csp is not None
+    assert "default-src 'self'" in csp
+    assert "frame-ancestors 'none'" in csp
+    assert "form-action 'self'" in csp
+
+
+def test_csp_nonce_matches_inline_script(client) -> None:
+    """Pages that emit inline <script> blocks must tag them with the
+    per-request CSP nonce, otherwise the browser blocks them."""
+    import re
+
+    with client.session_transaction() as sess:
+        sess["csrf_token"] = "tok"
+
+    # Log in so we can hit /servers.
+    with session_scope() as db:
+        from sqlalchemy import select
+        alice = db.scalar(select(User).where(User.username == "alice"))
+        alice_id = alice.id
+        marker = alice.password_changed_at.isoformat()
+    with client.session_transaction() as sess:
+        sess["user_id"] = alice_id
+        sess["pw_changed_at"] = marker
+        sess["admin"] = False
+
+    response = client.get("/servers?prefill_blueprint_id=1")
+    csp = response.headers.get("Content-Security-Policy", "")
+    nonce_match = re.search(r"'nonce-([^']+)'", csp)
+    assert nonce_match, f"expected nonce in CSP, got: {csp}"
+    nonce = nonce_match.group(1)
+    body = response.get_data(as_text=True)
+    if "<script>" in body:
+        # An un-nonced inline script slipped through.
+        pytest.fail("inline <script> without nonce attribute")
+    if "prefill_blueprint_id" in body and "showModal" in body:
+        assert f'nonce="{nonce}"' in body
+
+
+def test_404_response_is_generic(client) -> None:
+    response = client.get("/this-route-does-not-exist")
+    assert response.status_code == 404
+    body = response.get_data(as_text=True)
+    assert "Traceback" not in body
+    assert "werkzeug" not in body.lower()
+
+
+def test_500_response_does_not_leak_traceback(tmp_path, monkeypatch) -> None:
+    from l4d2web.app import create_app
+    db_url = f"sqlite:///{tmp_path/'boom.db'}"
+    monkeypatch.setenv("DATABASE_URL", db_url)
+    boom_app = create_app({
+        "TESTING": True,
+        "DATABASE_URL": db_url,
+        "SECRET_KEY": "test",
+        "PROPAGATE_EXCEPTIONS": False,
+    })
+
+    @boom_app.route("/__boom__")
+    def boom():
+        raise RuntimeError("internal detail that must not leak")
+
+    response = boom_app.test_client().get("/__boom__")
+    assert response.status_code == 500
+    body = response.get_data(as_text=True)
+    assert "internal detail" not in body
+    assert "Traceback" not in body