Merge commit from fork

davidism · web-flow · commit 73d6504063bf · 2025-05-13T07:46:54.000-07:00
Sessions: fix signing key selection when key rotation is enabled
diff --git a/CHANGES.rst b/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`
diff --git a/docs/config.rst b/docs/config.rst
@@ -127,13 +127,16 @@ The following configuration values are used internally by Flask:
 
 .. py:data:: SECRET_KEY_FALLBACKS
 
-    A list of old secret keys that can still be used for unsigning, most recent
-    first. This allows a project to implement key rotation without invalidating
-    active sessions or other recently-signed secrets.
+    A list of old secret keys that can still be used for unsigning. This allows
+    a project to implement key rotation without invalidating active sessions or
+    other recently-signed secrets.
 
     Keys should be removed after an appropriate period of time, as checking each
     additional key adds some overhead.
 
+    Order should not matter, but the default implementation will test the last
+    key in the list first, so it might make sense to order oldest to newest.
+
     Flask's built-in secure cookie session supports this. Extensions that use
     :data:`SECRET_KEY` may not support this yet.
 
diff --git a/src/flask/sessions.py b/src/flask/sessions.py
@@ -318,11 +318,12 @@ def get_signing_serializer(self, app: Flask) -> URLSafeTimedSerializer | None:
         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,
diff --git a/tests/test_basic.py b/tests/test_basic.py
@@ -381,14 +381,21 @@ def set_session() -> str:
     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}
 
 
