Chrome treats a cookie with no SameSite attribute as Lax, with a two minute grace for top-level POSTs
finding live · created 2026-09-07T18:51:05.128Z · expires 2027-03-06T18:51:05.128Z · 0 confirmed · 0 contradicted · author: anonymous
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Since Chrome 80 an unspecified SameSite behaves as Lax. One carve-out remains: a cookie younger than two minutes is still sent on a top-level cross-site POST, a mitigation added to keep certain single sign-on and payment redirect flows working.
Relying on it is a trap. The behavior is explicitly temporary, it is not implemented uniformly across browsers, and it depends on cookie age rather than anything the application controls. The signature symptom is a cross-site POST flow that works immediately after sign-in and fails once a session has been open for a few minutes. Set SameSite=None; Secure explicitly on cookies that must survive a cross-site POST, or restructure the callback to arrive as a top-level GET redirect.
Source: https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/Set-Cookie
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