[Bug] Parsing a valid saml assertion throws IDX13102

[blushingpenguin]

Which version of Microsoft.IdentityModel are you using?
7.5.0

Where is the issue?
M.IM.Tokens

Is this a new or an existing app?
New (reproduction)

Repro

using Microsoft.IdentityModel.Logging;
using Microsoft.IdentityModel.Tokens.Saml2;

internal class Program
{
    private static void Main()
    {
        string token = @"
<saml2:Assertion Version=""2.0"" ID=""test"" IssueInstant=""2024-03-27T17:49:12.345Z"" xmlns:saml2=""urn:oasis:names:tc:SAML:2.0:assertion"" xmlns:xs=""http://www.w3.org/2001/XMLSchema"">
  <saml2:Issuer xmlns:saml2=""urn:oasis:names:tc:SAML:2.0:assertion"" Format=""urn:oasis:names:tc:SAML:2.0:nameid-format:entity"">https://example.com/issuer</saml2:Issuer><Signature xmlns=""http://www.w3.org/2000/09/xmldsig#""><SignedInfo><CanonicalizationMethod Algorithm=""http://www.w3.org/2001/10/xml-exc-c14n#"" /><SignatureMethod Algorithm=""http://www.w3.org/2001/04/xmldsig-more#rsa-sha256"" /><Reference URI=""#_gzzLVaFPZNwsRusXqZl9b""><Transforms><Transform Algorithm=""http://www.w3.org/2000/09/xmldsig#enveloped-signature"" /><Transform Algorithm=""http://www.w3.org/2001/10/xml-exc-c14n#"" /></Transforms><DigestMethod Algorithm=""http://www.w3.org/2001/04/xmlenc#sha256"" /><DigestValue>digest</DigestValue></Reference></SignedInfo><SignatureValue>sign</SignatureValue><KeyInfo><X509Data><X509Certificate>cert</X509Certificate></X509Data></KeyInfo></Signature>
  <saml2:Subject xmlns:saml2=""urn:oasis:names:tc:SAML:2.0:assertion"">
    <saml2:NameID Format=""urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress"">test@example.com</saml2:NameID>
    <saml2:SubjectConfirmation Method=""urn:oasis:names:tc:SAML:2.0:cm:bearer"">
      <saml2:SubjectConfirmationData InResponseTo=""id1234"" Recipient=""https://example.com/acsurl"" NotOnOrAfter=""2024-03-27T18:49:12.345Z"" />
    </saml2:SubjectConfirmation>
  </saml2:Subject>
  <saml2:Conditions xmlns:saml2=""urn:oasis:names:tc:SAML:2.0:assertion"" NotBefore=""2024-03-27T17:49:12.345Z"" NotOnOrAfter=""2024-03-27T18:49:12.345Z"">
    <saml2:AudienceRestriction>
      <saml2:Audience>https://example.com</saml2:Audience>
    </saml2:AudienceRestriction>
  </saml2:Conditions>
  <saml2:AuthnStatement AuthnInstant=""2024-03-27T17:49:12.345Z"" SessionIndex=""id1234"" SessionNotOnOrAfter=""2024-03-27T18:49:12.345Z"" xmlns:saml2=""urn:oasis:names:tc:SAML:2.0:assertion"">
    <saml2:AuthnContext>
      <saml2:AuthnContextClassRef>urn:oasis:names:tc:SAML:2.0:ac:classes:PasswordProtectedTransport</saml2:AuthnContextClassRef>
    </saml2:AuthnContext>
  </saml2:AuthnStatement>
  <saml2:AttributeStatement xmlns:saml2=""urn:oasis:names:tc:SAML:2.0:assertion"">
    <saml2:Attribute Name=""groups"" NameFormat=""urn:oasis:names:tc:SAML:2.0:attrname-format:unspecified"">
      <saml2:AttributeValue>
        <name xmlns=""http://www.w3.org/2001/XMLSchema"" xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xsi:type=""xs:string"">Test Group</name>
        <id xmlns=""http://www.w3.org/2001/XMLSchema"" xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xsi:type=""xs:string"">1234</id>
        <email xmlns=""http://www.w3.org/2001/XMLSchema"" xmlns:xsi=""http://www.w3.org/2001/XMLSchema-instance"" xsi:type=""xs:string"">group@example.com</email>
      </saml2:AttributeValue>
    </saml2:Attribute>
  </saml2:AttributeStatement>
</saml2:Assertion>";
        try
        {
            IdentityModelEventSource.ShowPII = true;
            var th = new Saml2SecurityTokenHandler();
            th.ReadSaml2Token(token);
        }
        catch(Exception e)
        {
            Console.WriteLine("caught {0}", e);
        }
    }
}

Expected behavior
Assertion parsed without error

Actual behavior

caught Microsoft.IdentityModel.Tokens.Saml2.Saml2SecurityTokenReadException: IDX13102: Exception thrown while reading 'AttributeValue' for Saml2SecurityToken. Inner exception: 'System.Xml.XmlException: 'Element' is an invalid XmlNodeType. Line 23, position 131.
   at System.Xml.XmlReader.ReadEndElement()
   at Microsoft.IdentityModel.Tokens.Saml2.Saml2Serializer.ReadAttributeValue(XmlDictionaryReader reader, Saml2Attribute attribute)'.
 ---> System.Xml.XmlException: 'Element' is an invalid XmlNodeType. Line 23, position 131.
   at System.Xml.XmlReader.ReadEndElement()
   at Microsoft.IdentityModel.Tokens.Saml2.Saml2Serializer.ReadAttributeValue(XmlDictionaryReader reader, Saml2Attribute attribute)
   --- End of inner exception stack trace ---
   at Microsoft.IdentityModel.Tokens.Saml2.Saml2Serializer.ReadAttributeValue(XmlDictionaryReader reader, Saml2Attribute attribute)
   at Microsoft.IdentityModel.Tokens.Saml2.Saml2Serializer.ReadAttribute(XmlDictionaryReader reader)
   at Microsoft.IdentityModel.Tokens.Saml2.Saml2Serializer.ReadAttributeStatement(XmlDictionaryReader reader)
   at Microsoft.IdentityModel.Tokens.Saml2.Saml2Serializer.ReadAssertion(XmlReader reader)
   at Microsoft.IdentityModel.Tokens.Saml2.Saml2SecurityTokenHandler.ReadSaml2Token(XmlReader reader)
   at Microsoft.IdentityModel.Tokens.Saml2.Saml2SecurityTokenHandler.ReadSaml2Token(String token)

Possible solution

in Saml2Serializer.ReadAttributeValue, read more whitespace after the element start

            reader.ReadStartElement(Saml2Constants.Elements.AttributeValue, Saml2Constants.Namespace);
            // added block
            while (reader.NodeType == XmlNodeType.Whitespace)
            {
                whiteSpace += reader.Value;
                reader.Read();
            }

or just remove the "helpful" hack and parse the xml normally (not sure why it's in there though, presumably that will break something else).