n the Startup.Auth.cs file, add the CookieDomain parameter with your domain:

var cookieAuthenticationOptions = new CookieAuthenticationOptions

{

    AuthenticationType  = DefaultAuthenticationTypes.ApplicationCookie,

    LoginPath           = new PathString("/Account/Login"),

    CookieDomain        = ".mydomain.com"

};

Then for all websites you need to set a unique machine key. The easiest way to generate a new one is using IIS:

Find the "Machine Key" option on your site:

Click the "Generate Keys" button to get your keys.

In the Startup.Auth.cs file, add the CookieDomain parameter with your domain:

var cookieAuthenticationOptions = new CookieAuthenticationOptions

{

    AuthenticationType  = DefaultAuthenticationTypes.ApplicationCookie,

    LoginPath           = new PathString("/Account/Login"),

    CookieDomain        = ".mydomain.com"

};

Then for all websites you need to set a unique machine key. The easiest way to generate a new one is using IIS:

Find the "Machine Key" option on your site:

Click the "Generate Keys" button to get your keys.

Finally, the above process will add the following to your web.config and you need to ensure that this is copied into each of your sites.

<machineKey

  validationKey="DAD9E2B0F9..."

  decryptionKey="ADD1C39C02..."

  validation="SHA1"

  decryption="AES"

/>
原文地址：https://*.com/questions/19166599/asp-net-identity-cookie-across-subdomains
Asp.Net Identity介绍：http://www.360doc.com/content/17/0227/08/1355383_632319876.shtml