It has nothing to do with Express. It matters the setting of the cookie itself. Set your .mydomain.com domain and everything will be fine.
EDIT : OP wanted more details, so here are some examples from the code.
connect.createServer( connect.cookieParser() , connect.session({ cookie: { domain : ".mydomain.com" }}) );
and
res.cookie('remember', 1, { domain : ".mydomain.com" });
must work.
Malvolio
source share