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SameSite is a 2016 extension to HTTP cookies
intended to mitigate cross site request forgery (CSRF). The original design was an opt-in feature which could be used by adding a new SameSite property to cookies. It had two values, Lax and Strict.
Google is now updating the standard
However browsers which adhere to the original standard and are unaware of the new value have a different behavior to browsers which use the new standard as the SameSite standard states that if a browser sees a value for SameSite it does not understand it should treat that value as «Strict». This means your .NET website will now have to add user agent sniffing to decide whether you send the new None value, or not send the attribute at all.