diff --git a/src/rights_filter/server/http_app.py b/src/rights_filter/server/http_app.py
index e61e757..26993b1 100644
--- a/src/rights_filter/server/http_app.py
+++ b/src/rights_filter/server/http_app.py
@@ -1,14 +1,16 @@
 from __future__ import annotations
 
 import base64
+import hmac
 import json
 import mimetypes
 import re
 import sqlite3
 from http import HTTPStatus
+from http.cookies import SimpleCookie
 from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
 from pathlib import Path
-from urllib.parse import parse_qs, unquote, urlparse
+from urllib.parse import unquote, urlparse
 
 from rights_filter.server.image_store import LocalSubmissionImageStore, SUPPORTED_IMAGE_SUFFIXES
 from rights_filter.server.sqlite_store import CopyrighterStore
@@ -33,15 +35,21 @@ class _PayloadTooLarge(Exception):
     """Raised when a request body exceeds _MAX_BODY_BYTES."""
 
 
-# Data routes requiring the shared token when COPYRIGHTER_AUTH_TOKEN is set.
-# The static GUI shell and /health stay open so the operator can bootstrap.
-_PROTECTED_PREFIXES = (
-    "/api/",
+# Media routes loaded via <img>/<link>, which cannot send an Authorization
+# header — these rely on the session cookie instead.
+_MEDIA_PREFIXES = (
     "/media/",
     "/knowledge-media/",
     "/collected-media/",
     "/face-crop-media/",
 )
+# Data routes requiring the shared token when COPYRIGHTER_AUTH_TOKEN is set.
+# The static GUI shell and /health stay open so the operator can bootstrap.
+_PROTECTED_PREFIXES = ("/api/", *_MEDIA_PREFIXES)
+
+# HttpOnly cookie minted after a header-authenticated request so the browser can
+# load media without ever carrying the token in a URL.
+_SESSION_COOKIE = "copyrighter_session"
 
 
 def build_server(
@@ -249,12 +257,19 @@ def build_server(
             if not any(path.startswith(prefix) for prefix in _PROTECTED_PREFIXES):
                 return True
             header = self.headers.get("Authorization", "")
-            if header == f"Bearer {auth_token}":
-                return True
-            # <img>/media cannot send headers, so accept ?token= for those.
-            query_token = parse_qs(urlparse(self.path).query).get("token", [""])[0]
-            if query_token == auth_token:
+            if hmac.compare_digest(header, f"Bearer {auth_token}"):
+                # First header-authenticated request mints an HttpOnly session
+                # cookie so the browser can load <img>/media (which cannot send an
+                # Authorization header) without ever putting the token in a URL.
+                self._issue_session_cookie = True
                 return True
+            cookie_header = self.headers.get("Cookie", "")
+            if cookie_header:
+                jar = SimpleCookie()
+                jar.load(cookie_header)
+                morsel = jar.get(_SESSION_COOKIE)
+                if morsel is not None and hmac.compare_digest(morsel.value, auth_token):
+                    return True
             self._json({"error": "unauthorized"}, HTTPStatus.UNAUTHORIZED)
             return False
 
@@ -269,11 +284,19 @@ def build_server(
             except json.JSONDecodeError as exc:
                 raise ValueError("invalid JSON body") from exc
 
+        def _maybe_set_session_cookie(self) -> None:
+            if getattr(self, "_issue_session_cookie", False):
+                self.send_header(
+                    "Set-Cookie",
+                    f"{_SESSION_COOKIE}={auth_token}; HttpOnly; SameSite=Strict; Path=/",
+                )
+
         def _json(self, payload: object, status: HTTPStatus = HTTPStatus.OK) -> None:
             data = json.dumps(payload, ensure_ascii=False).encode("utf-8")
             self.send_response(status)
             self.send_header("Content-Type", "application/json; charset=utf-8")
             self.send_header("Content-Length", str(len(data)))
+            self._maybe_set_session_cookie()
             self.end_headers()
             self.wfile.write(data)
 
@@ -313,6 +336,11 @@ def build_server(
                     "Content-Security-Policy",
                     "default-src 'none'; style-src 'unsafe-inline'; img-src 'self' data:; sandbox",
                 )
+                # Biometric/personal media: keep it out of intermediary caches and
+                # don't leak the URL via Referer.
+                self.send_header("Cache-Control", "no-store")
+                self.send_header("Referrer-Policy", "no-referrer")
+            self._maybe_set_session_cookie()
             self.end_headers()
             self.wfile.write(data)
 
diff --git a/tests/rights_filter/server/test_http_app.py b/tests/rights_filter/server/test_http_app.py
index 7d4d896..c0b7a3a 100644
--- a/tests/rights_filter/server/test_http_app.py
+++ b/tests/rights_filter/server/test_http_app.py
@@ -1325,8 +1325,23 @@ def test_protected_routes_require_token_when_configured(tmp_path: Path):
             urlopen(base + "/api/bootstrap")
         assert exc.value.code == 401
 
+        # Header auth succeeds and mints an HttpOnly session cookie.
         authed = Request(base + "/api/bootstrap", headers={"Authorization": "Bearer secret"})
-        assert urlopen(authed).status == 200
+        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
+
+        # The cookie alone authenticates (mimics a browser <img> media load).
+        cookie_req = Request(base + "/api/bootstrap", headers={"Cookie": "copyrighter_session=secret"})
+        assert urlopen(cookie_req).status == 200
+
+        # A wrong cookie is rejected; the token never travels in the URL.
+        bad = Request(base + "/api/bootstrap", headers={"Cookie": "copyrighter_session=nope"})
+        with pytest.raises(HTTPError) as exc_bad:
+            urlopen(bad)
+        assert exc_bad.value.code == 401
 
         # The GUI shell and liveness probe stay open so the operator can bootstrap.
         assert urlopen(base + "/health").status == 200
@@ -1379,6 +1394,33 @@ def test_malformed_json_body_returns_400(tmp_path: Path):
         server.shutdown()
 
 
+def test_upload_and_import_validation_returns_400(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)
+    _start(server)
+    base = f"http://127.0.0.1:{server.server_port}"
+
+    def post_expect(path: str, body: dict, expected: int) -> None:
+        data = json.dumps(body).encode("utf-8")
+        request = Request(base + path, data=data, method="POST", headers={"Content-Type": "application/json"})
+        with pytest.raises(HTTPError) as exc:
+            urlopen(request, timeout=10)
+        assert exc.value.code == expected
+
+    try:
+        good_b64 = base64.b64encode(b"x").decode("ascii")
+        # unsupported image type
+        post_expect("/api/submissions/upload-image", {"image": {"filename": "x.txt", "data": good_b64}}, 400)
+        # data is not valid base64
+        post_expect("/api/submissions/upload-image", {"image": {"filename": "x.png", "data": "!!notbase64!!"}}, 400)
+        # empty image data
+        post_expect("/api/submissions/upload-image", {"image": {"filename": "x.png", "data": ""}}, 400)
+        # import folder that does not exist
+        post_expect("/api/submissions/import-folder", {"path": str(tmp_path / "does-not-exist")}, 400)
+    finally:
+        server.shutdown()
+
+
 def test_operational_error_surfaces_as_503(tmp_path: Path, monkeypatch):
     static_dir, image_store, store = _fixtures(tmp_path)