Why are my forms authentication tickets expiring so fast?

Question

I'm using forms authentication in an ASP.NET application. I configure the FormsAuthenticationTicket to expire in 1 year but it actually expires after 1 hour or so. I can't figure out why.

Here is all the code involved in the login process:

public static bool Login(int id)
{
    try
    {
        string securityToken = UserHelper.AuthenticateUser(id);

        DateTime expiryDate = DateTime.Now.AddYears(1);
        FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(
             1, id.ToString(), DateTime.Now, expiryDate, true,
             securityToken, FormsAuthentication.FormsCookiePath);

        string encryptedTicket = FormsAuthentication.Encrypt(ticket);
        HttpCookie cookie = new HttpCookie(FormsAuthentication.FormsCookieName, encryptedTicket);
        cookie.Expires = expiryDate;

        HttpContext.Current.Response.Cookies.Add(cookie);

        return true;
    }
    catch
    {
        return false;
    }
}

Web.config:

<system.web>
    <machineKey validationKey="AutoGenerate"
    decryptionKey="AutoGenerate" validation="SHA1" />
    <compilation debug="true">
    <authentication mode="Forms">
        <forms loginUrl="~/Login.aspx" timeout="2880"/>
    </authentication>
...

Is something wrong with my approach? Why is it expiring so fast?

EDIT

Global.asax code:

protected void Application_AuthenticateRequest(object sender, EventArgs e)
{
    if (Request.PhysicalPath.EndsWith(".aspx") || Request.PhysicalPath.EndsWith(".axd") || Request.PhysicalPath.EndsWith(".ashx"))
        SecurityManager.SetPrincipal();
}

SetPrincipal Code:

public static void SetPrincipal()
{
    ILivrePrincipal principal = null;
    FormsIdentity identity;
    UrlParameters urlParameters = UrlParametersHelper.GetUrlParameters(HttpContext.Current.Request);

    if (HttpContext.Current.Request.IsAuthenticated)
    {
        identity = (FormsIdentity)HttpContext.Current.User.Identity;

        User userProfile;
        urlParameters.SecurityToken = (((FormsIdentity)identity).Ticket).UserData;
        try
        {
            userProfile = UserHelper.GetUser(urlParameters.SecurityToken);
            UserHelper.UpdateLastActiveOn(userProfile);
            principal = new AuthenticatedPrincipal(identity, userProfile);
        }
        catch
        {
            //TODO: Log an exception
            FormsAuthentication.SignOut();
            principal = new AnonymousPrincipal(new GuestIdentity(), UserHelper.GetUser(null));
        }
    }
    else
    {
        principal = new AnonymousPrincipal(new GuestIdentity(), UserHelper.GetUser(null));
    }

    HttpContext.Current.User = principal;
}

Answer 1

This is your problem.

<machineKey validationKey="AutoGenerate" 
            decryptionKey="AutoGenerate" 
            validation="SHA1"/>

ASP will generate a new machine key every time the app pool recycles. Which could reasonably happen every hour.

The machine key is used to encrypt and decrypt your FormsAuthentication cookie. If it changes, the cookie on your browser is no longer any good. So the system will treat you as if you have never logged on.

Try generating a static key and adding it to the configuration file. Should look something like this:

<machineKey  
    validationKey="21F090935F6E49C2C797F69(snip)F1B72A7F0A281B"          
    decryptionKey="ABAA84D7EC4BB56D75D(snip)B8BF91CFCD64568A145BE59719F"
    validation="SHA1"
    decryption="AES"
/>

Generate yourself a key here.