diff --git a/README.md b/README.md
index f5b0b48..17d5da8 100644
--- a/README.md
+++ b/README.md
@@ -211,7 +211,7 @@ If ConnectHub sits behind a reverse proxy or tunnel (Cloudflare Tunnel, Traefik,
 TRUSTED_PROXIES=172.18.0.0/16
 ```
 
-Without it, failed logins from anyone count against everyone: ten wrong passwords lock all visitors out for 15 minutes, and logs show the proxy's address. Only list addresses that are proxies: a listed address can claim to forward for any visitor.
+Without it, failed logins from anyone count against everyone: ten wrong passwords lock all visitors out for 15 minutes, and logs show the proxy's address. It also lets `COOKIE_SECURE=auto` see that the proxy serves HTTPS when it talks plain HTTP to ConnectHub, so the session cookie is marked `Secure`. Only list addresses that are proxies: a listed address can claim to forward for any visitor.
 
 ### Which pages may open the stream
 
diff --git a/root/app/backend/auth.py b/root/app/backend/auth.py
index c45c0a4..292a85a 100644
--- a/root/app/backend/auth.py
+++ b/root/app/backend/auth.py
@@ -345,6 +345,11 @@ def _cookie_secure(request: Optional[Request]) -> bool:
     if request is None:
         return False
     proto = request.headers.get("X-Forwarded-Proto", request.url.scheme)
+    # nginx sets X-Forwarded-Proto to its own scheme, which is http when a TLS-terminating
+    # proxy or tunnel talks plain HTTP to it. From a trusted proxy, use what that proxy received.
+    upstream = request.headers.get("X-Proxy-Forwarded-Proto")
+    if upstream and _in_networks(peer_ip(request), PROXY_NETS):
+        proto = upstream
     return proto.split(",")[0].strip().lower() == "https"
 
 def create_session_cookie(response: Response, user_data: dict, request: Optional[Request] = None):
diff --git a/root/defaults/default.conf b/root/defaults/default.conf
index 404b127..bb37818 100644
--- a/root/defaults/default.conf
+++ b/root/defaults/default.conf
@@ -65,6 +65,8 @@ server {
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $scheme;
+        # What a proxy in front of us received; the backend only believes it from TRUSTED_PROXIES
+        proxy_set_header X-Proxy-Forwarded-Proto $http_x_forwarded_proto;
         proxy_read_timeout 300s;
     }
 
@@ -75,6 +77,7 @@ server {
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_set_header X-Proxy-Forwarded-Proto $http_x_forwarded_proto;
     }
 
     location /stream/ {
diff --git a/tests/test_auth.py b/tests/test_auth.py
index 04514dd..874586c 100644
--- a/tests/test_auth.py
+++ b/tests/test_auth.py
@@ -299,7 +299,8 @@ def proxied(headers):
     """A request as nginx hands it on: X-Real-IP is the address that connected to nginx."""
     from starlette.requests import Request
     raw = [(k.lower().encode(), v.encode()) for k, v in headers.items()]
-    return Request({"type": "http", "headers": raw, "client": ("127.0.0.1", 0)})
+    return Request({"type": "http", "headers": raw, "client": ("127.0.0.1", 0), "scheme": "http",
+                    "server": ("127.0.0.1", 8000), "path": "/auth/login", "query_string": b""})
 
 
 @pytest.mark.parametrize("trusted, headers, expected", [
@@ -333,3 +334,28 @@ def test_forward_auth_trusts_the_peer_not_a_forwarded_address(auth, monkeypatch)
     with pytest.raises(auth.HTTPException) as exc:
         auth.get_current_user(request)
     assert exc.value.status_code == 401
+
+
+# ----------------- Secure cookie behind a TLS-terminating proxy -----------------
+
+@pytest.mark.parametrize("trusted, headers, secure", [
+    # direct over HTTPS (port 3001)
+    ("", {"X-Real-IP": "203.0.113.9", "X-Forwarded-Proto": "https"}, True),
+    # tunnel terminates TLS and talks HTTP to port 3000: nginx says http, the tunnel said https
+    ("172.21.0.0/16", {"X-Real-IP": "172.21.0.4", "X-Forwarded-Proto": "http",
+                       "X-Proxy-Forwarded-Proto": "https"}, True),
+    # the same without TRUSTED_PROXIES: the forwarded scheme isn't believed
+    ("", {"X-Real-IP": "172.21.0.4", "X-Forwarded-Proto": "http", "X-Proxy-Forwarded-Proto": "https"}, False),
+    # a direct visitor can't claim https
+    ("172.21.0.0/16", {"X-Real-IP": "203.0.113.9", "X-Forwarded-Proto": "http",
+                       "X-Proxy-Forwarded-Proto": "https"}, False),
+    # trusted proxy that received plain http
+    ("172.21.0.0/16", {"X-Real-IP": "172.21.0.4", "X-Forwarded-Proto": "http",
+                       "X-Proxy-Forwarded-Proto": "http"}, False),
+    # trusted proxy that sent no scheme: nginx's
+    ("172.21.0.0/16", {"X-Real-IP": "172.21.0.4", "X-Forwarded-Proto": "https"}, True),
+])
+def test_cookie_secure_auto_behind_a_proxy(auth, monkeypatch, trusted, headers, secure):
+    monkeypatch.setattr(auth, "COOKIE_SECURE", "auto")
+    monkeypatch.setattr(auth, "PROXY_NETS", auth._parse_networks(trusted))
+    assert auth._cookie_secure(proxied(headers)) is secure