diff --git a/src/rights_filter/server/http_app.py b/src/rights_filter/server/http_app.py
index 26993b1..9044339 100644
--- a/src/rights_filter/server/http_app.py
+++ b/src/rights_filter/server/http_app.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import base64
+import hashlib
 import hmac
 import json
 import mimetypes
@@ -35,6 +36,17 @@ class _PayloadTooLarge(Exception):
     """Raised when a request body exceeds _MAX_BODY_BYTES."""
 
 
+# Content types treated as the trusted operator app shell, which receives the
+# strict script-src CSP + framing/nosniff headers. Both javascript spellings are
+# listed because mimetypes maps .js to text/javascript on Python 3.12+.
+_APP_SHELL_CONTENT_TYPES = {
+    "text/html",
+    "application/javascript",
+    "text/javascript",
+    "text/css",
+}
+
+
 # Media routes loaded via <img>/<link>, which cannot send an Authorization
 # header — these rely on the session cookie instead.
 _MEDIA_PREFIXES = (
@@ -51,6 +63,19 @@ _PROTECTED_PREFIXES = ("/api/", *_MEDIA_PREFIXES)
 # load media without ever carrying the token in a URL.
 _SESSION_COOKIE = "copyrighter_session"
 
+# Methods that change state — cookie-authenticated ones get an Origin/Referer
+# CSRF check (a header-authenticated request cannot be forged cross-site because
+# browsers never auto-attach an Authorization header).
+_STATE_CHANGING_METHODS = {"POST", "PATCH", "PUT", "DELETE"}
+
+
+def _session_tag(token: str) -> str:
+    # Deterministic HMAC of the token so the session cookie carries a derived
+    # tag, not the raw COPYRIGHTER_AUTH_TOKEN. A leaked cookie can still ride the
+    # session but no longer reveals the token itself (which is also the .env
+    # secret / Authorization bearer). No server-side session store needed.
+    return hmac.new(token.encode("utf-8"), b"copyrighter-session-v1", hashlib.sha256).hexdigest()
+
 
 def build_server(
     host: str,
@@ -64,6 +89,9 @@ def build_server(
 
     class CopyrighterHandler(BaseHTTPRequestHandler):
         server_version = "CopyrighterHTTP/0.1"
+        # Bound how long a slow/idle client can hold a worker thread's socket, so
+        # a handful of slowloris-style connections cannot exhaust the thread pool.
+        timeout = 30
 
         def do_GET(self) -> None:
             path = _path(self.path)
@@ -186,6 +214,9 @@ def build_server(
                 elif path.startswith("/api/collections/candidates/") and path.endswith("/promote"):
                     candidate_id = unquote(path.split("/")[4])
                     self._json(store.promote_collection_candidate(candidate_id, body))
+                elif path.startswith("/api/collections/candidates/") and path.endswith("/dismiss"):
+                    candidate_id = unquote(path.split("/")[4])
+                    self._json(store.dismiss_collection_candidate(candidate_id))
                 elif path.startswith("/api/knowledge/") and path.endswith("/promote-watchlist"):
                     entry_id = unquote(path.split("/")[3])
                     self._json(store.promote_watchlist_entry(entry_id))
@@ -268,13 +299,40 @@ def build_server(
                 jar = SimpleCookie()
                 jar.load(cookie_header)
                 morsel = jar.get(_SESSION_COOKIE)
-                if morsel is not None and hmac.compare_digest(morsel.value, auth_token):
+                if morsel is not None and hmac.compare_digest(morsel.value, _session_tag(auth_token)):
+                    # Cookie-authenticated state changes are CSRF-forgeable, so
+                    # require a same-origin Origin/Referer for them.
+                    if self.command in _STATE_CHANGING_METHODS and not self._origin_ok():
+                        self._json({"error": "invalid origin"}, HTTPStatus.FORBIDDEN)
+                        return False
                     return True
             self._json({"error": "unauthorized"}, HTTPStatus.UNAUTHORIZED)
             return False
 
+        def _origin_ok(self) -> bool:
+            # Accept only requests whose Origin (preferred) or Referer resolves to
+            # the same host we were reached on. If neither header is present we
+            # cannot verify, so allow (older/non-browser clients) rather than break
+            # the app — the classic cross-site POST always carries an Origin.
+            host = self.headers.get("Host", "")
+            origin = self.headers.get("Origin", "")
+            if origin:
+                return urlparse(origin).netloc == host
+            referer = self.headers.get("Referer", "")
+            if referer:
+                return urlparse(referer).netloc == host
+            return True
+
         def _body(self) -> dict[str, object]:
-            length = int(self.headers.get("Content-Length", "0") or "0")
+            # Parse defensively: a non-integer header must not leak Python's parse
+            # error, and a negative length must not slip past the size cap into
+            # rfile.read(-1) (an unbounded read-to-EOF that exhausts memory).
+            try:
+                length = int(self.headers.get("Content-Length", "0") or "0")
+            except ValueError:
+                raise ValueError("invalid Content-Length")
+            if length < 0:
+                raise ValueError("invalid Content-Length")
             if length > _MAX_BODY_BYTES:
                 raise _PayloadTooLarge()
             if not length:
@@ -286,9 +344,10 @@ def build_server(
 
         def _maybe_set_session_cookie(self) -> None:
             if getattr(self, "_issue_session_cookie", False):
+                # Store the derived tag, never the raw token.
                 self.send_header(
                     "Set-Cookie",
-                    f"{_SESSION_COOKIE}={auth_token}; HttpOnly; SameSite=Strict; Path=/",
+                    f"{_SESSION_COOKIE}={_session_tag(auth_token)}; HttpOnly; SameSite=Strict; Path=/",
                 )
 
         def _json(self, payload: object, status: HTTPStatus = HTTPStatus.OK) -> None:
@@ -340,6 +399,21 @@ def build_server(
                 # don't leak the URL via Referer.
                 self.send_header("Cache-Control", "no-store")
                 self.send_header("Referrer-Policy", "no-referrer")
+            elif content_type.split(";", 1)[0] in _APP_SHELL_CONTENT_TYPES:
+                # Trusted app shell (HTML/JS/CSS). The GUI loads only same-origin
+                # scripts (no inline <script>, no CDN), so a script-src 'self'
+                # policy is compatible and blocks injected scripts. img-src stays
+                # permissive because evidence thumbnails are external result URLs.
+                self.send_header("X-Content-Type-Options", "nosniff")
+                self.send_header("X-Frame-Options", "DENY")
+                self.send_header(
+                    "Content-Security-Policy",
+                    "default-src 'self'; script-src 'self'; "
+                    "style-src 'self' 'unsafe-inline'; "
+                    "img-src 'self' data: blob: https: http:; "
+                    "font-src 'self' data:; connect-src 'self'; "
+                    "base-uri 'self'; object-src 'none'; frame-ancestors 'none'",
+                )
             self._maybe_set_session_cookie()
             self.end_headers()
             self.wfile.write(data)
@@ -373,8 +447,7 @@ def _save_submission_upload(image_store: LocalSubmissionImageStore, raw_image: o
     image_dir = image_store.root / "images"
     image_dir.mkdir(parents=True, exist_ok=True)
     safe_name = _safe_upload_name(filename)
-    target = _unique_upload_path(image_dir, safe_name)
-    target.write_bytes(content)
+    target = _write_unique_upload(image_dir, safe_name, content)
     return target.stem
 
 
@@ -384,15 +457,20 @@ def _safe_upload_name(filename: str) -> str:
     return f"{stem}{path.suffix.lower()}"
 
 
-def _unique_upload_path(directory: Path, filename: str) -> Path:
-    candidate = directory / filename
-    if not candidate.exists():
-        return candidate
+def _write_unique_upload(directory: Path, filename: str, content: bytes) -> Path:
+    # Exclusive create ('xb') so two concurrent uploads of the same name can't
+    # both pass an exists() check and then overwrite each other — the loser gets
+    # FileExistsError and advances to the next -N suffix.
     path = Path(filename)
-    for index in range(2, 1000):
-        candidate = directory / f"{path.stem}-{index}{path.suffix}"
-        if not candidate.exists():
+    candidates = [directory / filename]
+    candidates.extend(directory / f"{path.stem}-{index}{path.suffix}" for index in range(2, 1000))
+    for candidate in candidates:
+        try:
+            with open(candidate, "xb") as handle:
+                handle.write(content)
             return candidate
+        except FileExistsError:
+            continue
     raise ValueError("too many files with the same submission image name")
 
 
diff --git a/tests/rights_filter/server/test_http_app.py b/tests/rights_filter/server/test_http_app.py
index fd4af85..9ec4d3d 100644
--- a/tests/rights_filter/server/test_http_app.py
+++ b/tests/rights_filter/server/test_http_app.py
@@ -266,6 +266,54 @@ def test_http_server_exposes_health_bootstrap_and_mutations(tmp_path: Path):
     assert next(provider for provider in providers if provider["id"] == "naver")["enabled"] is False
 
 
+def test_app_shell_sends_security_headers(tmp_path: Path):
+    # 2026-07-11 #6: the trusted operator shell must carry CSP / framing / nosniff
+    # headers so an injected script or a framing attempt is blocked by the browser.
+    import http.client
+
+    static_dir, image_store, store = _fixtures(tmp_path)
+    server = build_server(
+        host="127.0.0.1", port=0, store=store, image_store=image_store, static_dir=static_dir
+    )
+    _start(server)
+    try:
+        conn = http.client.HTTPConnection("127.0.0.1", server.server_port, timeout=5)
+        conn.request("GET", "/")
+        resp = conn.getresponse()
+        resp.read()
+        csp = resp.getheader("Content-Security-Policy")
+        assert csp is not None and "script-src 'self'" in csp
+        assert resp.getheader("X-Frame-Options") == "DENY"
+        assert resp.getheader("X-Content-Type-Options") == "nosniff"
+        conn.close()
+    finally:
+        server.shutdown()
+
+
+def test_negative_content_length_is_rejected(tmp_path: Path):
+    # 2026-07-11 M10: a negative Content-Length must be rejected (400), never
+    # reach rfile.read(-1) which would read to EOF and exhaust memory.
+    import http.client
+
+    static_dir, image_store, store = _fixtures(tmp_path)
+    server = build_server(
+        host="127.0.0.1", port=0, store=store, image_store=image_store, static_dir=static_dir
+    )
+    _start(server)
+    try:
+        conn = http.client.HTTPConnection("127.0.0.1", server.server_port, timeout=5)
+        conn.putrequest("POST", "/api/submissions/SUB-API1/decision")
+        conn.putheader("Content-Type", "application/json")
+        conn.putheader("Content-Length", "-1")
+        conn.endheaders()
+        resp = conn.getresponse()
+        resp.read()
+        assert resp.status == 400
+        conn.close()
+    finally:
+        server.shutdown()
+
+
 def test_http_server_rejects_enabling_unconfigured_google_custom_search(tmp_path: Path):
     static_dir, image_store, _ = _fixtures(tmp_path)
     store = CopyrighterStore(
@@ -1333,11 +1381,13 @@ def test_protected_routes_require_token_when_configured(tmp_path: Path):
         response = urlopen(authed)
         assert response.status == 200
         set_cookie = response.headers.get("Set-Cookie")
-        assert set_cookie and "copyrighter_session=secret" in set_cookie
-        assert "HttpOnly" in set_cookie
+        assert set_cookie and "HttpOnly" in set_cookie
+        # 2026-07-11 #7: the cookie carries a derived tag, never the raw token.
+        session_value = set_cookie.split("copyrighter_session=")[1].split(";")[0]
+        assert session_value != "secret"
 
         # The cookie alone authenticates (mimics a browser <img> media load).
-        cookie_req = Request(base + "/api/bootstrap", headers={"Cookie": "copyrighter_session=secret"})
+        cookie_req = Request(base + "/api/bootstrap", headers={"Cookie": f"copyrighter_session={session_value}"})
         assert urlopen(cookie_req).status == 200
 
         # A wrong cookie is rejected; the token never travels in the URL.
@@ -1353,6 +1403,63 @@ def test_protected_routes_require_token_when_configured(tmp_path: Path):
         server.shutdown()
 
 
+def test_cookie_authed_state_change_requires_same_origin(tmp_path: Path):
+    # 2026-07-11 #10: a cookie-authenticated POST from a foreign Origin (classic
+    # CSRF) is rejected; header-authed requests are not forgeable.
+    import http.client
+
+    static_dir, image_store, store = _fixtures(tmp_path)
+    server = build_server(
+        host="127.0.0.1", port=0, store=store, image_store=image_store,
+        static_dir=static_dir, auth_token="secret",
+    )
+    _start(server)
+    port = server.server_port
+    try:
+        conn = http.client.HTTPConnection("127.0.0.1", port, timeout=5)
+        conn.request("GET", "/api/bootstrap", headers={"Authorization": "Bearer secret"})
+        resp = conn.getresponse()
+        resp.read()
+        session = resp.getheader("Set-Cookie").split("copyrighter_session=")[1].split(";")[0]
+        conn.close()
+
+        body = json.dumps({"decision": "held", "memo": "x"})
+        conn2 = http.client.HTTPConnection("127.0.0.1", port, timeout=5)
+        conn2.request(
+            "POST",
+            "/api/submissions/SUB-API1/decision",
+            body=body,
+            headers={
+                "Cookie": f"copyrighter_session={session}",
+                "Origin": "http://evil.example",
+                "Content-Type": "application/json",
+            },
+        )
+        resp2 = conn2.getresponse()
+        resp2.read()
+        assert resp2.status == 403
+        conn2.close()
+
+        # Same-origin cookie POST succeeds.
+        conn3 = http.client.HTTPConnection("127.0.0.1", port, timeout=5)
+        conn3.request(
+            "POST",
+            "/api/submissions/SUB-API1/decision",
+            body=body,
+            headers={
+                "Cookie": f"copyrighter_session={session}",
+                "Origin": f"http://127.0.0.1:{port}",
+                "Content-Type": "application/json",
+            },
+        )
+        resp3 = conn3.getresponse()
+        resp3.read()
+        assert resp3.status == 200
+        conn3.close()
+    finally:
+        server.shutdown()
+
+
 def test_oversized_body_rejected_with_413(tmp_path: Path):
     static_dir, image_store, store = _fixtures(tmp_path)
     server = build_server(host="127.0.0.1", port=0, store=store, image_store=image_store, static_dir=static_dir)