According to various documentation, the cookie domain has to contain at least 2 dots for a browser to accept this as a wildcard cookie (e.g. .google.com) and use this for all sub-domains. So .foo.bar.com should work - should. Opera doesn’t like it and only accepts it domain-wide if there are exactly 2 dots in the domain name. In this case .bar.com. If you use .foo.bar.com the string is ignored and the cookie only gets set for the current domain.