<machineKey decryptionKey="AutoGenerate"... being ignored by IIS. Won't invalidate previous session's cookies

Question

(See question below for more context):

Are there any situations in which

<machineKey
      validationKey="AutoGenerate,IsolateApps"
      decryptionKey="AutoGenerate,IsolateApps"/>

in web.config would fail to AutoGenerate a new machineKey on App Pool recycle? This is the behavior I'm seeing...

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I'm using standard ASP.NET FormsAuthentication in an MVC app. If I log a user in using FormsAuthentication.GetAuthCookie and don't use a persistent cookie (relying on the browser's session to remember my authorized state), I would expect recycling the IIS App Pool to invalidate the session's knowledge of this cookie...and thus logout all users who don't have persistent cookies.

This DOES happen on one of my IIS installs (XP), but on a different IIS configuration (Server 2K3) the FormsAuthentication cookie (under the standard name ".ASPXAUTH") remains valid and continues to authorize the user.

Does anyone know why this is happening or what configuration controls this behavior?

Obviously recycling the app pool has no control over whether or not the browser still sends the .ASPXAUTH cookie (as long as I haven't closed my browser and the cookie hasn't expired).

In the case of the IIS install that properly denies authentication after a recycle, I can see the incoming cookie in Request.Cookies during the Application_BeginRequest event...but once control moves to the next event available in Global.asax.cs (Application_AuthenticateRequest), the cookie has been removed from the Request.Cookies collection.

Why does this not happen for both IIS/ASP.NET configurations?

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In case this isn't clear, a simpler way of forming the question is:

Why does HttpContext.Current.Request.Cookies[".ASPXAUTH"] change from {System.Web.HttpCookie} to null when I step, in a single request, from Application_BeginRequest to Application_AuthenticateRequest?

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More debugging information:

If I attach the following code to Global.asax.cs's FormsAuthentication_OnAuthenticate event...

var cookie = Request.Cookies[FormsAuthentication.FormsCookieName];
if (cookie != null)
{
    var val = cookie.Value;
    try
    {
        FormsAuthenticationTicket ticket = FormsAuthentication.Decrypt(val);
    }
    catch (Exception)
    {
    }
}

...then during a request before I recycle the IIS App Pool, no exception will be caught. After recycling the IIS App Pool, when the exact same .ASPXAUTH cookie is sent from the browser, a Cryptographic exception is caught ("Padding is invalid and cannot be removed.")

Why is this?

Answer 1

Our application is stateless (no session required), yet we had a situation where an app pool recycle caused invalidation of all machinekey-encrypted cookies on a server environment (above described issue). This was caused because the machinekey changes with every recycle, which should not be the case.

The AutoGenerate modifier specifies that ASP.NET generates a random key and stores it in the Local Security Authority (LSA)

https://msdn.microsoft.com/en-us/library/w8h3skw9%28v=vs.85%29.aspx?f=255&MSPPError=-2147217396

"Local Security Authority (LSA)" means the user assigned to app pool, see below for more details as this turned out to be the problem.

The issue lied in the fact that we are using a dedicated user account for running the application pool, and simply creating the user and then assigning it to the app pool did not seem to trigger the creation of the registry section where the machine key is then stored. You can verify this yourself by checking registry

1. HKLM/SOFTWARE/Microsoft/Windows NT/CurrentVersion/ProfileList (for getting the SID of the user you just created; if the user is not there, then this is already a bad sign)
2. HKU/[UserSIDFromBefore]/Software/Microsoft/ASP.NET/... (a machine key should be stored there)

The solution was to logon as that user once on the computer (normal Windows logon screen) so that the relevant registry sections are created. There might be quicker or more subtle ways to establish the registry sections though.

Answer 2

Internet Information Services (IIS) 7.0 (Windows Vista, Windows Server 2008) introduced application pool identity, a new isolation mechanism that helps provide increased security for servers that run ASP.NET applications. However, sites that are running under the application pool identity do not have access to the HKCU registry. This is where the ASP.NET runtime stores its auto-generated keys. The result is that ASP.NET cannot persist the auto-generated key when the application pool is reset. Therefore, every time w3wp.exe is reset, a new temporary key is generated. Note This is not an issue in IIS 7.5 (Windows 7, Windows Server 2008 R2) and later versions. On these versions of IIS, ASP.NET can persist its auto-generated keys in a different location that survives application pool resets.

https://support.microsoft.com/en-us/help/2915218/resolving-view-state-message-authentication-code-mac-errors