Repository: pallets/flask

Issue:
Add support for partitioned session cookies
Related issue with historical context: https://github.com/pallets/werkzeug/issues/2797

Partitioned cookies are now supported in Werkzeug: https://github.com/pallets/werkzeug/pull/2855

It would be nice to have first-class support for the flask session cookie using the `Partitioned` flag to provide support for Chrome's CHIPS and allow sessions to continue to work within iframes.

Example usage might be:

```python
app.config['SESSION_COOKIE_SAMESITE'] = "None"
app.config['SESSION_COOKIE_SECURE'] = True
app.config['SESSION_COOKIE_PARTITIONED'] = True # add this?
```

Hints from issue comments:
@odanoburu: This is related to the warning I get on Firefox, right?

> Cookie “session” does not have a proper “SameSite” attribute value. Soon, cookies without the “SameSite” attribute or with an invalid value will be treated as “Lax”. This means that the cookie will no longer be sent in third-party contexts. If your application depends on this cookie being available in such contexts, please add the “SameSite=None“ attribute to it. To know more about the “SameSite“ attribute, read https://developer.mozilla.org/docs/Web/HTTP/Headers/Set-Cookie/SameSite

Although as I understand it, it won't break anything for me.
@davidism: No, that's related to same site, which is already possible to set in the app config.@Josecespedesant: Hi! I'd like to take a look into collaborate in this issue

I was going through the context of this issue and saw that implicitly if partitioned is set to true it should also set secure to true as mentioned here: https://github.com/pallets/werkzeug/pull/2855/files#diff-cd95d3a95564f3230d4875ac34fc9d039856b033c6b1430d5d3cc864f87cf89aL1300

Would that be the case for here as well?@davidism: Yes, but you shouldn't need to do anything extra for that, it's already how it's implemented, as you've linked to.

Solution patch:
diff --git a/CHANGES.rst b/CHANGES.rst
index 1cfbc5fe9f..118751e90f 100644
--- a/CHANGES.rst
+++ b/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
diff --git a/docs/config.rst b/docs/config.rst
index 4f7d902614..c17fc93277 100644
--- a/docs/config.rst
+++ b/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
diff --git a/src/flask/app.py b/src/flask/app.py
index 8074ed4dad..5133052fc4 100644
--- a/src/flask/app.py
+++ b/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,
diff --git a/src/flask/sessions.py b/src/flask/sessions.py
index 05b367a25f..beb2f3f3ac 100644
--- a/src/flask/sessions.py
+++ b/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")


Test patch:
diff --git a/tests/test_basic.py b/tests/test_basic.py
index 64337ab38c..1fc0d67aa4 100644
--- a/tests/test_basic.py
+++ b/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):