R1:38c83ef846c1

Defuse a "Host:" header attack

Summary:
Django released a security update recently dealing with malicious "Host" headers:

https://www.djangoproject.com/weblog/2012/oct/17/security/

We're vulnerable to the same attack. Plug the hole.

The risk here is that an attacker does something like this:

# Register "evil.com".
# Point it at secure.phabricator.com in DNS.
# Send a legitimate user a link to "secure.phabricator.com:ignored@evil.com".
# They login and get cookies. Normally Phabricator refuses…