diff --git a/.env.example b/.env.example
index 8c6db39..b01fa5b 100644
--- a/.env.example
+++ b/.env.example
@@ -39,6 +39,11 @@ STATION_CODE=ST-DEFAULT
 # floor the station identity comes from the local install, and measuring against
 # another station's recipes would silently break traceability.
 STATION_SWITCH_ENABLED=0
+# Cookie di sessione "Secure": di default segue FLASK_DEBUG (sicuro salvo
+# debug). Impostare a false solo per l'installazione on-premise che serve
+# HTTP in LAN (#38.4) - un cookie Secure su un'origine non-HTTPS viene
+# scartato dal browser in silenzio e il login non regge mai.
+# SESSION_COOKIE_SECURE=false
 
 # --- Vision ---
 # Internal address of the vision worker. Never exposed outside tmflow-net.
diff --git a/src/frontend/flask_app/config.py b/src/frontend/flask_app/config.py
index 7a805c3..c1528a4 100644
--- a/src/frontend/flask_app/config.py
+++ b/src/frontend/flask_app/config.py
@@ -23,7 +23,18 @@ class Config:
     # Session
     SESSION_COOKIE_HTTPONLY = True
     SESSION_COOKIE_SAMESITE = "Lax"
-    SESSION_COOKIE_SECURE = not DEBUG  # Only secure cookies in production (HTTPS)
+    # Default: secure unless FLASK_DEBUG=1 (unchanged). Explicit override via
+    # SESSION_COOKIE_SECURE=false for the on-premise install (#38.4): nginx
+    # there listens on plain HTTP inside the LAN, and a cookie marked Secure
+    # is silently dropped by the browser on a non-HTTPS origin - login looks
+    # like it works and the session never sticks.
+    _session_cookie_secure_env = os.getenv("SESSION_COOKIE_SECURE")
+    if _session_cookie_secure_env is not None:
+        SESSION_COOKIE_SECURE = _session_cookie_secure_env.strip().lower() not in (
+            "false", "0", "no",
+        )
+    else:
+        SESSION_COOKIE_SECURE = not DEBUG
     PERMANENT_SESSION_LIFETIME = 28800  # 8 hours
     # None = token valid for the life of the session (#12): a measurement page
     # stays open for hours between saves, and a 1h-expired token turned every
diff --git a/src/frontend/flask_app/tests/test_config_session_cookie_secure.py b/src/frontend/flask_app/tests/test_config_session_cookie_secure.py
new file mode 100644
index 0000000..469d66a
--- /dev/null
+++ b/src/frontend/flask_app/tests/test_config_session_cookie_secure.py
@@ -0,0 +1,50 @@
+"""SESSION_COOKIE_SECURE reads from the environment (#38.4).
+
+Default unchanged (secure unless FLASK_DEBUG=1); SESSION_COOKIE_SECURE=false
+overrides it for the on-premise HTTP-in-LAN install.
+"""
+import importlib
+
+
+def _reload_config():
+    import config
+    importlib.reload(config)
+    return config
+
+
+def test_defaults_secure_when_not_debug(monkeypatch):
+    monkeypatch.delenv("SESSION_COOKIE_SECURE", raising=False)
+    monkeypatch.delenv("FLASK_DEBUG", raising=False)
+    config = _reload_config()
+    assert config.Config.SESSION_COOKIE_SECURE is True
+
+
+def test_defaults_insecure_when_debug(monkeypatch):
+    monkeypatch.delenv("SESSION_COOKIE_SECURE", raising=False)
+    monkeypatch.setenv("FLASK_DEBUG", "1")
+    config = _reload_config()
+    assert config.Config.SESSION_COOKIE_SECURE is False
+
+
+def test_explicit_false_overrides_default_in_production(monkeypatch):
+    """On-premise over plain HTTP: FLASK_DEBUG stays unset (real prod), but
+    the cookie must not be marked Secure or the browser silently drops it."""
+    monkeypatch.delenv("FLASK_DEBUG", raising=False)
+    monkeypatch.setenv("SESSION_COOKIE_SECURE", "false")
+    config = _reload_config()
+    assert config.Config.SESSION_COOKIE_SECURE is False
+
+
+def test_explicit_true_keeps_secure(monkeypatch):
+    monkeypatch.setenv("FLASK_DEBUG", "1")
+    monkeypatch.setenv("SESSION_COOKIE_SECURE", "true")
+    config = _reload_config()
+    assert config.Config.SESSION_COOKIE_SECURE is True
+
+
+def test_falsy_values_are_case_insensitive(monkeypatch):
+    monkeypatch.delenv("FLASK_DEBUG", raising=False)
+    for value in ("false", "FALSE", "0", "no", "  False  "):
+        monkeypatch.setenv("SESSION_COOKIE_SECURE", value)
+        config = _reload_config()
+        assert config.Config.SESSION_COOKIE_SECURE is False, value