I wonder which party should pay the bounty in this case. He seems to have reported it to Twitter, however wouldn't it be Google's responsibility to fix this?
Seeing as how it's a combination of an issue in the web server parsing cookies, and the client setting malicious cookies, I'm not sure which party should be responsible for the bounty.
They should both pay a bounty. But if you had to assign blame only one place it would be ECMAscript. Setting the value of an unintended cookie should not be so easy to do by accident.
I sent information about this vulnerability to:
1) Google (2 fix in Google Analytics)
2) Django / Python (https://hg.python.org/cpython/rev/270f61ec1157)
3) Twitter (changed CSRF protection)
4) Instagram [Facebook] (sent me to Django)
Seeing as how it's a combination of an issue in the web server parsing cookies, and the client setting malicious cookies, I'm not sure which party should be responsible for the bounty.