secret key rotation: fix key list ordering

The `itsdangerous` serializer interface[1] expects keys to be
provided with the oldest key at index zero and the active signing key
at the end of the list.

We document[2] that `SECRET_KEY_FALLBACKS` should be configured with
the most recent first (at index zero), so to achieve the expected
behaviour, those should be inserted in reverse-order at the head of
the list.

[1] - https://itsdangerous.palletsprojects.com/en/stable/serializer/#itsdangerous.serializer.Serializer

[2] - https://flask.palletsprojects.com/en/stable/config/#SECRET_KEY_FALLBACKS

CHANGES.rst

@@ -3,6 +3,8 @@ Version 3.1.1
 
 Unreleased
 
+-   Fix signing key selection order when key rotation is enabled via
+    ``SECRET_KEY_FALLBACKS``. :ghsa:`4grg-w6v8-c28g`
 -   Fix type hint for `cli_runner.invoke`. :issue:`5645`
 -   ``flask --help`` loads the app and plugins first to make sure all commands
     are shown. :issue:5673`

src/flask/sessions.py

@@ -318,11 +318,12 @@ class SecureCookieSessionInterface(SessionInterface):
         if not app.secret_key:
             return None
 
-        keys: list[str | bytes] = [app.secret_key]
+        keys: list[str | bytes] = []
 
         if fallbacks := app.config["SECRET_KEY_FALLBACKS"]:
             keys.extend(fallbacks)
 
+        keys.append(app.secret_key)  # itsdangerous expects current key at top
         return URLSafeTimedSerializer(
             keys,  # type: ignore[arg-type]
             salt=self.salt,

tests/test_basic.py

@@ -381,14 +381,21 @@ def test_session_secret_key_fallbacks(app, client) -> None:
     def get_session() -> dict[str, t.Any]:
         return dict(flask.session)
 
-    # Set session with initial secret key
+    # Set session with initial secret key, and two valid expiring keys
+    app.secret_key, app.config["SECRET_KEY_FALLBACKS"] = (
+        "0 key",
+        ["-1 key", "-2 key"],
+    )
     client.post()
     assert client.get().json == {"a": 1}
     # Change secret key, session can't be loaded and appears empty
-    app.secret_key = "new test key"
+    app.secret_key = "? key"
     assert client.get().json == {}
-    # Add initial secret key as fallback, session can be loaded
-    app.config["SECRET_KEY_FALLBACKS"] = ["test key"]
+    # Rotate the valid keys, session can be loaded
+    app.secret_key, app.config["SECRET_KEY_FALLBACKS"] = (
+        "+1 key",
+        ["0 key", "-1 key"],
+    )
     assert client.get().json == {"a": 1}