add SESSION_COOKIE_PARTITIONED config

CHANGES.rst

@@ -15,6 +15,8 @@ Unreleased
     through the ``MAX_CONTENT_LENGTH`` config. Added
     ``MAX_FORM_MEMORY_SIZE`` and ``MAX_FORM_PARTS`` config. Added documentation
     about resource limits to the security page. :issue:`5625`
+-   Add support for the ``Partitioned`` cookie attribute (CHIPS), with the
+    ``SESSION_COOKIE_PARTITIONED`` config. :issue`5472`
 
 
 Version 3.0.3

docs/config.rst

@@ -173,6 +173,23 @@ The following configuration values are used internally by Flask:
 
     Default: ``False``
 
+.. py:data:: SESSION_COOKIE_PARTITIONED
+
+    Browsers will send cookies based on the top-level document's domain, rather
+    than only the domain of the document setting the cookie. This prevents third
+    party cookies set in iframes from "leaking" between separate sites.
+
+    Browsers are beginning to disallow non-partitioned third party cookies, so
+    you need to mark your cookies partitioned if you expect them to work in such
+    embedded situations.
+
+    Enabling this implicitly enables :data:`SESSION_COOKIE_SECURE` as well, as
+    it is only valid when served over HTTPS.
+
+    Default: ``False``
+
+    .. versionadded:: 3.1
+
 .. py:data:: SESSION_COOKIE_SAMESITE
 
     Restrict how cookies are sent with requests from external sites. Can

src/flask/app.py

@@ -189,6 +189,7 @@ class Flask(App):
             "SESSION_COOKIE_PATH": None,
             "SESSION_COOKIE_HTTPONLY": True,
             "SESSION_COOKIE_SECURE": False,
+            "SESSION_COOKIE_PARTITIONED": False,
             "SESSION_COOKIE_SAMESITE": None,
             "SESSION_REFRESH_EACH_REQUEST": True,
             "MAX_CONTENT_LENGTH": None,

src/flask/sessions.py

@@ -224,6 +224,14 @@ def get_cookie_samesite(self, app: Flask) -> str | None:
         """
         return app.config["SESSION_COOKIE_SAMESITE"]  # type: ignore[no-any-return]
 
+    def get_cookie_partitioned(self, app: Flask) -> bool:
+        """Returns True if the cookie should be partitioned. By default, uses
+        the value of :data:`SESSION_COOKIE_PARTITIONED`.
+
+        .. versionadded:: 3.1
+        """
+        return app.config["SESSION_COOKIE_PARTITIONED"]  # type: ignore[no-any-return]
+
     def get_expiration_time(self, app: Flask, session: SessionMixin) -> datetime | None:
         """A helper method that returns an expiration date for the session
         or ``None`` if the session is linked to the browser session.  The
@@ -338,6 +346,7 @@ def save_session(
         domain = self.get_cookie_domain(app)
         path = self.get_cookie_path(app)
         secure = self.get_cookie_secure(app)
+        partitioned = self.get_cookie_partitioned(app)
         samesite = self.get_cookie_samesite(app)
         httponly = self.get_cookie_httponly(app)
 
@@ -354,6 +363,7 @@ def save_session(
                     domain=domain,
                     path=path,
                     secure=secure,
+                    partitioned=partitioned,
                     samesite=samesite,
                     httponly=httponly,
                 )
@@ -374,6 +384,7 @@ def save_session(
             domain=domain,
             path=path,
             secure=secure,
+            partitioned=partitioned,
             samesite=samesite,
         )
         response.vary.add("Cookie")

tests/test_basic.py

@@ -293,6 +293,7 @@ def test_session_using_session_settings(app, client):
         SESSION_COOKIE_DOMAIN=".example.com",
         SESSION_COOKIE_HTTPONLY=False,
         SESSION_COOKIE_SECURE=True,
+        SESSION_COOKIE_PARTITIONED=True,
         SESSION_COOKIE_SAMESITE="Lax",
         SESSION_COOKIE_PATH="/",
     )
@@ -315,6 +316,7 @@ def clear():
     assert "secure" in cookie
     assert "httponly" not in cookie
     assert "samesite" in cookie
+    assert "partitioned" in cookie
 
     rv = client.get("/clear", "http://www.example.com:8080/test/")
     cookie = rv.headers["set-cookie"].lower()
@@ -324,6 +326,7 @@ def clear():
     assert "path=/" in cookie
     assert "secure" in cookie
     assert "samesite" in cookie
+    assert "partitioned" in cookie
 
 
 def test_session_using_samesite_attribute(app, client):