Are attributes allowed in a SAML authentication request?

Question

Is it possible to send attributes in a SAML authentication request?

<samlp:AuthnRequest
   xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol"
   xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion"
   ID="aaf23196-1773-2113-474a-fe114412ab72"
   Version="2.0"
   IssueInstant="2004-12-05T09:21:59Z"
   AssertionConsumerServiceIndex="0"
   AttributeConsumingServiceIndex="0">
   <saml:Issuer>https://sp.example.com/SAML2&lt;/saml:Issuer&gt;
   <samlp:NameIDPolicy
     AllowCreate="true"
     Format="urn:oasis:names:tc:SAML:2.0:nameid-format:transient"/>
 </samlp:AuthnRequest>

Answer 1

Technically, yes, it is possible, since AuthnRequest can contain an Extensions element, which can contain anything - see the SAML 'core' spec: AuthnRequest (section 3.4.1) is derived from RequestAbstractType (section 3.2.1) which has an optional Extensions. The sender and recipient would have to agree on the syntax and semantics of data sent this way.

I can't see a more 'conventional' way to do this, since Attributes are usually in Assertions, rather than AuthnRequests.

Answer 2

Yes, it's possible to do so. If you are a .NET developer, you may want to see more info about SAML and C# on this blog