User impersonation in WCF message inspector

I'm facing a problem while using WCF message inspectors, the problem is I can't get the impersonated user in message inspector and when I try to get current user with WindowIdentity.GetCurrent() it returns AppPool\DefaultAppPool instead of impersonated user.

On the other hand if I try to get the user in OperationContract the impersonated user is available.

private void RequestValidationStart(ref Message request)
{
    // WindowIdentity.GetCurrent() returns IIS AppPool\DefaultAppPool
    // instead of impersonated user
    var user = System.Security.Principal.WindowIdentity.GetCurrent();
}

How to get the impersonated user in WCF Message Inspector?