Unresolved Default SameSite Policy in Flask Sessions
0 reputation · 10 Apr 2022, 11:30 UTC
When upgrading from Flask 2.x to 3.x, the framework changes the default SAME_SITE attribute for session cookies from None to Lax. This adjustment improves CSRF protection but can unintentionally block legitimate cross‑site authentication flows that previously relied on SameSite=None. Because Flask ships with client‑side sessions by default, the entire session payload is serialized into the cookie, exposing sensitive data and limiting size. When a server‑side backend such as Flask‑Session is enabled, the session data is stored on the server and the cookie only holds a session identifier, yet the SameSite handling remains governed by the same configuration keys.
The documentation does not clearly state how SESSION_COOKIE_SAMESITE interacts with SESSION_TYPE when both client‑side and server‑side sessions are configured, leaving developers uncertain whether a single setting can simultaneously satisfy security and cross‑site requirements.
What is the precise impact of the default SAME_SITE=Lax on cross‑site authentication when using client‑side sessions? Which configuration combination restores SameSite=None while maintaining secure cookie handling? Does enabling Flask‑Session alter the SameSite attribute behavior, and if so, how should it be configured to preserve legacy cross‑site flows?
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