title: ADFS 2.0 Error ID4175: The issuer of the security token was not recognized by the IssuerNameRegistry
body: <p>The error:</p>
  
  <blockquote>
    <p>ID4175: The issuer of the security
    token was not recognized by the
    IssuerNameRegistry. To accept security
    tokens from this issuer, configure the
    IssuerNameRegistry to return a valid
    name for this issuer.</p>
  </blockquote>
  
  <p>The situation: I have 3 or 4 asp.net apps running on a single IIS server (my QA environment), that this morning began returning this error. I start out on an anonymous site, click a link to a secure section, get redirected to my federation services proxy, authenticate, and am redirected back to my secure page, but this error appears. </p>
  
  <p><a href="http://samirvaidya.blogspot.com/2011/02/fixing-id4175-error-message.html" rel="noreferrer">This link</a> and a bunch of others indicate that the thumbprint in my web.config is wrong, but I can prove (via history in TFS) that the thumbprint in my web.config file has not changed. </p>
  
  <p>I've tried re-running the fedutil, but still get the same message (though that comes up with a different thumbprint). Any ideas?</p>