Monday, September 17, 2018

Parsing SAML 1.1 (WS-Federation) tokens without the WSFam module

Ocassionally there's a scenario where a SAML token must be parsed without the WSFederationAuthentication module. Note that when the WSFam can be used, parsing is straightforward.

For us, it was one of our old applications that still can't be upgraded to .NET 4.5, because of reasons ;), and we wanted to drop the old WIF runtime (the one that targets older .NET versions). For someone else it can be another scenario, e.g. you have the SAML token as string and just want the IPrincipal out of the token.

The solution is to think of the token as it was the regular XMLDsig signed XML - the assertion node is signed and the signature's reference points back to it:

<?xml version="1.0"?>
<t:RequestSecurityTokenResponse xmlns:t="">
    <wsu:Created xmlns:wsu="">2018-09-18</wsu:Created>
    <wsu:Expires xmlns:wsu="">2018-09-18</wsu:Expires>
  <wsp:AppliesTo xmlns:wsp="">
    <wsa:EndpointReference xmlns:wsa="">
    <saml:Assertion xmlns:saml="urn:oasis:names:tc:SAML:1.0:assertion" MajorVersion="1" MinorVersion="1" AssertionID="_assertionID" 
        Issuer="http://issuer" IssueInstant="2018-09-18">
        <saml:Attribute AttributeName="windowsaccountname" AttributeNamespace="">
      <ds:Signature xmlns:ds="">
          <ds:CanonicalizationMethod Algorithm=""/>
          <ds:SignatureMethod Algorithm=""/>
          <ds:Reference URI="#_assertionID">
              <ds:Transform Algorithm=""/>
              <ds:Transform Algorithm=""/>
            <ds:DigestMethod Algorithm=""/>
        <KeyInfo xmlns="">
What you should do is to
  1. validate the signature
  2. accept or reject the signature's certificate
  3. parse the token to retrieve claims required to create the IPrincipal
The code is rather simple, what's interesting however is that the SignedXml class has to be inherited to have the signature validator that follows the AssertionID attribute (the default convention is that the signed node's id attribute is called just ID and the default validator just won't find the node that has the id attribute called differently):
    public class SamlSignedXml : SignedXml
        public SamlSignedXml(XmlElement e) : base(e) { }

        public override XmlElement GetIdElement(XmlDocument document, string idValue)
            XmlNamespaceManager mgr = new XmlNamespaceManager(document.NameTable);
            mgr.AddNamespace("trust", "");
            mgr.AddNamespace("wsu", "");
            mgr.AddNamespace("saml", "urn:oasis:names:tc:SAML:1.0:assertion");

            XmlElement assertionNode = 
                                                         "trust:RequestedSecurityToken/saml:Assertion", mgr);

            if (assertionNode.Attributes["AssertionID"] != null &&
                string.Equals(assertionNode.Attributes["AssertionID"].Value, idValue, StringComparison.InvariantCultureIgnoreCase)
                return assertionNode;

            return null;
Note that the XPath assumes the token has the RequestSecurityTokenResponseCollection in the root, make sure your tokens follow this convention (in case of a single token, the collection node can be missing and the token's root could be just RequestSecurityTokenResponse, update the code accordingly).

The validation code is then

// token is the string representation of the SAML1 token
// expectedCertThumb is the expected certificate's thumbprint
protected bool ValidateToken( string token, string expectedCertThumb, out string userName )
 userName = string.Empty;

 if (string.IsNullOrEmpty(token)) return false;

 var xd = new XmlDocument();
 xd.PreserveWhitespace = true;

 XmlNamespaceManager mgr = new XmlNamespaceManager(xd.NameTable);
 mgr.AddNamespace("trust", "");
 mgr.AddNamespace("wsu", "");
 mgr.AddNamespace("saml", "urn:oasis:names:tc:SAML:1.0:assertion");

 // assertion
 XmlElement assertionNode = (XmlElement)xd.SelectSingleNode("//trust:RequestSecurityTokenResponseCollection/trust:RequestSecurityTokenResponse/trust:RequestedSecurityToken/saml:Assertion", mgr);

 // signature
 XmlElement signatureNode = (XmlElement)xd.GetElementsByTagName("Signature")[0];

 var signedXml = new SamlSignedXml( assertionNode );

 X509Certificate2 certificate = null;
 foreach (KeyInfoClause clause in signedXml.KeyInfo)
  if (clause is KeyInfoX509Data)
   if (((KeyInfoX509Data)clause).Certificates.Count > 0)
    certificate =

 // cert node missing
 if (certificate == null) return false;

 // check the signature and return the result.
 var signatureValidationResult = signedXml.CheckSignature(certificate, true);

 if (signatureValidationResult == false) return false;

 // validate cert thumb
 if ( !string.IsNullOrEmpty( expectedCertThumb ) )
  if ( !string.Equals( expectedCertThumb, certificate.Thumbprint ) )
   return false;

 // retrieve username

 // expires = 
 var expNode = xd.SelectSingleNode("//trust:RequestSecurityTokenResponseCollection/trust:RequestSecurityTokenResponse/trust:Lifetime/wsu:Expires", mgr );

 DateTime expireDate;

 if (!DateTime.TryParse(expNode.InnerText, out expireDate)) return false; // wrong date

 if (DateTime.UtcNow > expireDate) return false; // token too old

 // claims
 var claimNodes =                 
                  "saml:Assertion/saml:AttributeStatement/saml:Attribute", mgr );
 foreach ( XmlNode claimNode in claimNodes )
  if ( claimNode.Attributes["AttributeName"] != null && 
              claimNode.Attributes["AttributeNamespace"] != null &&
       string.Equals( claimNode.Attributes["AttributeName"].Value, "name", StringComparison.InvariantCultureIgnoreCase ) &&   
                     string.Equals( claimNode.Attributes["AttributeNamespace"].Value, "", StringComparison.InvariantCultureIgnoreCase ) &&
         claimNode.ChildNodes.Count == 1 
   userName = claimNode.ChildNodes[0].InnerText;
   return true;

 return false;
A couple of comments here.

First, the XPath could possibly be shortened to reflect the possibility of a missing collection node.

Then, the code assumes there's the name claim that contains the username but it could be the windowsaccountname or maybe yet another claim type.

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