Set SameSite=Lax and use the __Host- prefix for the session cookie

The own SessionInterface ignored SESSION_COOKIE_SAMESITE, so the cookie
was sent with every cross-site request in Firefox and Safari (CSRF,
clickjacking in iframes, keeping the idle timeout alive). The cookie
name "session" could also be set by other subdomains or via plain HTTP,
the precondition for session fixation.

- SameSite=Lax. No GET changes state, so links from mails keep working.
- In production the cookie is named __Host-session, which browsers only
  accept with Secure, Path=/ and without Domain. The debug server keeps
  "session", since it runs without Secure.
- set_cookie and delete_cookie use the same attributes, since browsers
  ignore the deletion of a __Host- cookie without Secure.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

max/app.py

@@ -47,8 +47,9 @@ def main():
         SECRET_KEY=(
             "123456" if app.debug else base64.b64encode(os.urandom(64)).decode("utf-8")
         ),
+        SESSION_COOKIE_NAME="session" if app.debug else "__Host-session",
         SESSION_COOKIE_SECURE=not app.debug,
-        SESSION_COOKIE_SAMESITE=False,
+        SESSION_COOKIE_SAMESITE="Lax",
     )
     add_middleware_stack(
         app,

max/sessions.py

@@ -139,19 +139,27 @@ class SessionInterface(FlaskSessionInterface):
             return self.make_session_object()
         return session
 
+    def get_cookie_attributes(self, app):
+        # Attributes for setting and deleting the cookie must match, because
+        # browsers ignore a deletion of a __Host- cookie without Secure and Path.
+        return dict(
+            domain=self.get_cookie_domain(app),
+            path=self.get_cookie_path(app),
+            secure=self.get_cookie_secure(app),
+            httponly=self.get_cookie_httponly(app),
+            samesite=self.get_cookie_samesite(app),
+        )
+
     def save_session(self, app, session, response):
-        domain = self.get_cookie_domain(app)
-        path = self.get_cookie_path(app)
         cookie_name = self.get_cookie_name(app)
+        cookie_attributes = self.get_cookie_attributes(app)
         if session.is_empty():
             if session.modified:
                 del session_storage[session.id]
             # Only send the deletion header if the client sent a cookie
             if cookie_name in request.cookies:
-                response.delete_cookie(cookie_name, domain=domain, path=path)
+                response.delete_cookie(cookie_name, **cookie_attributes)
         else:
-            httponly = self.get_cookie_httponly(app)
-            secure = self.get_cookie_secure(app)
             expires = self.get_expiration_time(app, session)
             session.last_seen = time.monotonic()
             if session.created_at is None:
@@ -170,11 +178,5 @@ class SessionInterface(FlaskSessionInterface):
                 self._get_signer(app).sign(want_bytes(session.id)).decode("utf-8")
             )
             response.set_cookie(
-                cookie_name,
-                session_id,
-                expires=expires,
-                httponly=httponly,
-                domain=domain,
-                path=path,
-                secure=secure,
+                cookie_name, session_id, expires=expires, **cookie_attributes
             )