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An Anonymous Coward writes:

https://www.troyhunt.com/promiscuous-cookies-and-their-impending-death-via-the-samesite-policy/ [troyhunt.com]

Cookies like to get around. They have no scruples about where they go save for some basic constraints relating to the origin from which they were set. I mean have a think about it:

If a website sets a cookie then you click a link to another page on that same site, will the cookie be automatically sent with the request? Yes.

What if an attacker sends you a link to that same website in a malicious email and you click that link, will the cookie be sent? Also yes.

Last one: what if an attacker directs you to a malicious website and upon visiting it your browser makes a post request to the original website that set the cookie - will that cookie still be sent with the request? Yes!

Cookies just don't care about how the request was initiated nor from which origin, all they care about is that they're valid for the requested resource. "Origin" is a key word here too; those last two examples above are "cross-origin" requests in that they were initiated from origins other than the original website that set the cookie. Problem is, that opens up a rather nasty attack vector we know as Cross Site Request Forgery or CSRF. Way back in 2010 I was writing about this as part of the OWASP Top 10 for ASP.NET series and a near decade on, it's still a problem.

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