Why did ASP.NET generate the same cookie key for a domain and subdomain? - asp.net

Question

Bug:
I've got an ASP.NET web application that occasionally sets identical cookie keys for ".www.mydomain.com" and "www.mydomain.com". I'm trying to figure out what default cookie domain ASP.NET sets, and how I accidentally coded the site to sometimes prepend a "." to the cookie domain.
When 2 cookies have the same key and are sent up from the browser, the ASP.NET web application is unable to differentiate between the two because the domain value is not sent in the header. (See my previous question)
Evidence:
I've enabled W3C logging on the web server and verified that both cookies are sent from the client. Here's an example from the log file (paired down for brevity).
80 GET /default.aspx page= 200 0 0 - - - - - +MyCookie2=sessionID=559ddb9b-0f38-4878-bb07-834c2ca9caae;+MyCookie2=sessionID=e13d83cd-eac2-46fc-b39d-01826b91cb2c;
Possible Factor:
I am using subdomain enabled forms authentication.
Here's my web.config settings:
<authentication mode="Forms">
<forms domain="mydomain.com" enableCrossAppRedirects="true" loginUrl="/login" requireSSL="false" timeout="5259600" />
</authentication>
Here's and example of setting custom cookies:
HttpCookie cookie1 = new HttpCookie("MyCookie1") {HttpOnly = true, Expires = expiration};
logosCookie["email"] = user.Email;
logosCookie["keycode"] = user.PasswordHash;
logosCookie["version"] = currentCookieVersion;
context.Response.Cookies.Remove("cookie1");
context.Response.Cookies.Add(cookie1);
// set FormsAuth cookie manually so we can add the UserId to the ticket UserData
var userData = "UserId=" + user.UserID;
FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(2, user.Email, now, expiration, true, userData);
string str = FormsAuthentication.Encrypt(ticket);
HttpCookie cookie = new HttpCookie(FormsAuthentication.FormsCookieName, str)
{
HttpOnly = true,
Path = FormsAuthentication.FormsCookiePath,
Secure = FormsAuthentication.RequireSSL,
Expires = ticket.Expiration
};
if (FormsAuthentication.CookieDomain != null)
{
cookie.Domain = FormsAuthentication.CookieDomain;
}
context.Response.Cookies.Remove(FormsAuthentication.FormsCookieName);
context.Response.Cookies.Add(cookie1 );
Here's another example of setting a cookie.
var cookie2 = new HttpCookie("MyCookie2");
cookie2[CookieSessionIdKey] = Guid.NewGuid();
cookie2.Expires = DateTime.Now.AddYears(10);
HttpContext.Current.Response.Cookies.Set(cookie2);
Undesirable Resolution:
I can manually force the cookie domain to be a specific value, but I'd like to avoid explicitly declaring the domain. I'd prefer to use the default framework behavior and change my use of ASP.NET to avoid prepend the "." to the cookie domain for custom cookies.

Answer 1

When no domain is explicitly set by the server on the response, the browser is free to assign the cookie domain value. I haven't figured out exactly what conditions result in the browser setting "www.mydomain.com" vs ".mydomain.com" on a cookie domain when no domain is provided on the response, but it happened.
I have a feeling it's a result of explicitly setting the .ASPAUTH cookie domain value to ".mydomain.com" to enable cross subdomain authentication, while leaving other custom cookie domains set to the default (empty string, or "").
I'm going to go with the undesired solution, and explicitly set the cookie domain for all custom cookies to avoid browser quirks.