These are not vulnerabilities in Rack (necessarily), but in the way the cookie spec has been drafted and the way Chrome decides to implement it. There's nothing that can be "fixed" in Rack: the only definitive fix is a domain migration.

It looks like the fixes they have are messy (sending a header for each element in the path * each cookie key) and even then don't work in all browsers. Better to just not use subdomains for arbitrary hosting.