enable secret key rotation

CHANGES.rst

@@ -20,6 +20,9 @@ Unreleased
 -   ``-e path`` takes precedence over default ``.env`` and ``.flaskenv`` files.
     ``load_dotenv`` loads default files in addition to a path unless
     ``load_defaults=False`` is passed. :issue:`5628`
+-   Support key rotation with the ``SECRET_KEY_FALLBACKS`` config, a list of old
+    secret keys that can still be used for unsigning. Extensions will need to
+    add support. :issue:`5621`
 
 
 Version 3.0.3

docs/config.rst

@@ -125,6 +125,22 @@ The following configuration values are used internally by Flask:
 
     Default: ``None``
 
+.. 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.
+
+    Keys should be removed after an appropriate period of time, as checking each
+    additional key adds some overhead.
+
+    Flask's built-in secure cookie session supports this. Extensions that use
+    :data:`SECRET_KEY` may not support this yet.
+
+    Default: ``None``
+
+    .. versionadded:: 3.1
+
 .. py:data:: SESSION_COOKIE_NAME
 
     The name of the session cookie. Can be changed in case you already have a

pyproject.toml

@@ -79,7 +79,7 @@ source = ["src", "*/site-packages"]
 
 [tool.mypy]
 python_version = "3.9"
-files = ["src/flask", "tests/typing"]
+files = ["src/flask", "tests/type_check"]
 show_error_codes = true
 pretty = true
 strict = true
@@ -95,7 +95,7 @@ ignore_missing_imports = true
 
 [tool.pyright]
 pythonVersion = "3.9"
-include = ["src/flask", "tests/typing"]
+include = ["src/flask", "tests/type_check"]
 typeCheckingMode = "basic"
 
 [tool.ruff]

src/flask/app.py

@@ -180,6 +180,7 @@ class Flask(App):
             "TESTING": False,
             "PROPAGATE_EXCEPTIONS": None,
             "SECRET_KEY": None,
+            "SECRET_KEY_FALLBACKS": None,
             "PERMANENT_SESSION_LIFETIME": timedelta(days=31),
             "USE_X_SENDFILE": False,
             "SERVER_NAME": None,

src/flask/sessions.py

@@ -315,14 +315,20 @@ class SecureCookieSessionInterface(SessionInterface):
     def get_signing_serializer(self, app: Flask) -> URLSafeTimedSerializer | None:
         if not app.secret_key:
             return None
-        signer_kwargs = dict(
-            key_derivation=self.key_derivation, digest_method=self.digest_method
-        )
+
+        keys: list[str | bytes] = [app.secret_key]
+
+        if fallbacks := app.config["SECRET_KEY_FALLBACKS"]:
+            keys.extend(fallbacks)
+
         return URLSafeTimedSerializer(
-            app.secret_key,
+            keys,  # type: ignore[arg-type]
             salt=self.salt,
             serializer=self.serializer,
-            signer_kwargs=signer_kwargs,
+            signer_kwargs={
+                "key_derivation": self.key_derivation,
+                "digest_method": self.digest_method,
+            },
         )
 
     def open_session(self, app: Flask, request: Request) -> SecureCookieSession | None:

tests/test_basic.py

@@ -1,5 +1,6 @@
 import gc
 import re
+import typing as t
 import uuid
 import warnings
 import weakref
@@ -369,6 +370,27 @@ def expect_exception(f, *args, **kwargs):
         expect_exception(flask.session.pop, "foo")
 
 
+def test_session_secret_key_fallbacks(app, client) -> None:
+    @app.post("/")
+    def set_session() -> str:
+        flask.session["a"] = 1
+        return ""
+
+    @app.get("/")
+    def get_session() -> dict[str, t.Any]:
+        return dict(flask.session)
+
+    # Set session with initial secret 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"
+    assert client.get().json == {}
+    # Add initial secret key as fallback, session can be loaded
+    app.config["SECRET_KEY_FALLBACKS"] = ["test key"]
+    assert client.get().json == {"a": 1}
+
+
 def test_session_expiration(app, client):
     permanent = True