Single Sign On between sub domains : Forms Authentication

Today, I am going to discuss one of the feature, that I was working last few days and spent sleepless nights at office and Home as well. Here I am going to discuss Single Sign On (SSO) feature and every other developer implements SSO on some or other day.

So Actually I was having two applications. The requirement was like, user logins from one application and on clicking a link it is navigated to another application. So when user clicks on the link it redirected to application2, it checks, whether the user is authenticated or not. If authenticated it can access the application2 else get redirected to first application for Authentication, And once authenticated again directly reaches the second application.

Here my both application were Web application and form Authentication is used to authenticate the user. I had to devise some solution to implement SSO. I didn’t want to use the authentication code written on both applications so used first application only for Authentication. I thought of using the same authentication cookie that was created by First Application after authentication. And implemented this.

It worked very well. I was able to read the authentication cookie in another application and wrote the logic accordingly. I deployed both applications on my local machine and Test QA Server and it worked like a charm.

But… as soon as my application went actual test environment it started barfing. Nothing was working. No SSO.. Nothing.

Actually, what happened both the application were deployed on different web server in different domain. Like my first application has URL like and another one

As I was not aware earlier I didn’t keep it in mind. As we know by default cookie is limited to domain. I was not able to access auth cookie in application2 as it was in separate domain. After some research on Internet, I happened to make the changes in auth cookie and updated the domain property as I found solutions on Google. The code was like

System.Web.HttpCookie MyCookie =
MyCookie.Domain = “”;//the second level domain name

You can find it easily on internet. But the actual nightmare started after this only. Users were not able to logout. It kept looping as I checked in Fiddler and finally redirection error. Started searching on internet and found lots of people have problem but no thread ended with proper solution. I found at some places the auth cookie doesn’t get deleted if domain is set. Some workaround were there, like deleting the auth cookie manually when user logs out. But this also didn’t work. And ultimately did not find any solution on internet.

So I did some brainstorming. Actually there is a method provided by Form Authentication as


which is supposed to logout the user once called but it was not doing it. Actually as suggested over internet I set the domain for Auth cookie. Which was creating some problem? FormAutetication itself provides a way to set the domain for cookie. This is a static property of the FormAuthentication Class. But this is read only. One need to set it in web.config file and it will also be easy you can change the domain whenever you want. So one can set it as

<forms name=".ASPXAUTH" loginUrl="Login/" protection="Validation" timeout="120" path="/" domain=""/>

After setting my application again started working perfectly fine. So wanted to share this to you all. One does not need to update the cookie manually as I did in first code sample above and later deleting it while logging out. It may not work. Just one need to set the domain at config file, one will be away from this endless problem and surfing the Internet.

Hope this helps.

Thanks a lot


8 thoughts on “Single Sign On between sub domains : Forms Authentication

  1. Hi, I apologize for this newbie question – but does this method only work for sub-domains (like, and Is there a way, that you know of, to accomplish SSO in a similar way for multiple top-level domains (like, and ?
    Thanks for your advice and for sharing an excellent bit of knowledge,

    • You must have seen in the last section of my post, where I wrote that we’ll require the following to add in the file

      If you see here I added domain=”” specifically.

      Hope it answers you.

      Please let me know if you have any other question.


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