How should I be implementing the HTTP POST Protocol Binding for SAML WebSSO Profile?
Asked Answered
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I've implemented my Service Provider and Identify Provider following the SAML Profile for Web SSO using HTTP POST Protocol Binding. However, I am a bit confused as to how the Identity Provider will provide an <AuthnStatement> if the HTTP POST coming from the Service Provider is not tied to a session on the Identity Provider.

Could someone enlighten me how one would be able to do this?

The other approach I could use is the HTTP Redirect Binding, but that requires User-Agent intervention (i.e., the browser), often using the User-Agent simply as a pass-thru intermediary to faciliate the Request-Response message exchange. I'd rather use HTTP POST for this reason, because the message exchange occurs server-side, so the user sees nothing happening on their screen.

However, using HTTP Redirect makes more sense to me with respect to how I'd be able to tie a session to a request. Since the HTTP Redirect is facilitated via a User-Agent, the request to the IdP will have a session (if previously authenticated). What I don't get though is how to send an <AuthnRequest> on a HTTP Redirect. Answered by JST

So I'm a bit confused and would love to hear what other people are doing. Here are my questions again:

  1. Using the HTTP POST Protocol Binding with the IsPassive option the <AuthnRequest>, how do I tie a request made by the Service Provider to a session on the Identity Provider? In other words, how does the Identity Provider know who is making the request if the POST is coming from the Service Provider which is technically an anonymous session?
  2. Using the HTTP Redirect Protocol Binding, how do I send an <AuthnRequest> to the Identity Provider if I am using a HTTP Redirect? Answered by JST

UPDATE

Sorry for the confusion if I was unclear in my explanation above. I am implementing both the IdP and SP (via a plugin). The IdP is an existing application for which I want the SP (a third-party system) to use for authentication (i.e., Web SSO). I am developing a simple PoC at the moment. The SP is actually a third-party Spring application for which I am developing a plugin to perform the SAML operations.

I should have mentioned that I am trying to do this using the IsPassive option, that meaning the User-Agent doesn't come into play during the message exchange. It is simply the catalyst that gets the SAML-party started. Right? With that in mind, given that the user is anonymous at Step 1, what does the SP send to the IdP to allow the IdP figure out whether the user is already authenticated? Because of IsPassive, the HTTP POST isn't sent via the User-Agent


UPDATE

Question 1 Revised: How does the IdP resolve the Principal when the AuthnRequset is sent with the IsPassive option on?

Straight from the SAML 2.0 Profiles document, page 15, lines 417 to 419:

In step 4, the principal is identified by the identity provide by some means outside the scope of this profile.

What I'm really after is an explanation how to implement some means.

Henriettahenriette answered 3/2, 2011 at 9:15 Comment(1)
Ian and JST, I got it working thanks to both of your helpful pointers. Problem now is: Who do I give the CHECKMARK to?! I'll review both of your answers again and hopefully I can come to a fair conclusion. In the meantime, I've upvoted every answer and comment you two have posted so far.Henriettahenriette
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The thing to keep in mind is that there's no connection between a session on the IdP and a session on the SP. They don't know about each other, and communicate only through the SAML messages. The general steps for SP-initiated SAML SSO are:

  1. Anonymous user visits resource (page) at SP.
  2. SP identifies that user needs to be authenticated at IdP.
  3. SP constructs AuthnRequest and sends to IdP.
  4. IdP does some sort of authentication, constructs SAML Response and sends to SP.
  5. SP validates Response and, if valid, does whatever is necessary to identify user at SP and get them to originally requested resource.

Yes, there does need to be some way to connect the SP's AuthnRequest to the IdP's Response. That's covered by the SAML spec: the SP's AuthnRequest includes an ID value, and the corresponding response from the IdP MUST include an InResponseTo attribute (on its SubjectConfirmationData element) with that ID value. The Authentication Request Protocol also allows the SP to pass a RelayState parameter to the IdP, which the IdP is then REQUIRED to pass along unchanged with the SAML Response. You (in the SP role) can use that RelayState value to capture state information allowing the user to be relayed to the originally requested resource.

That implies that when you implement an SP, you'll need some mechanism for recording ID and RelayState values, and your Response processing needs to validate InResponseTo and RelayState values it receives. How you choose to create and interpret RelayState values is up to you, but keep in mind that there is a length limit. (We use random GUID values corresponding to locally saved state data, which has the extra advantage of not giving any hint of meaning to the RelayState values.)

How does the IdP know who is making the request? The AuthnRequest must include an Issuer element that identifies the SP. It might also contain an AssertionConsumerServiceURL (the URL to which the Response is to be sent), or the IdP may have a local mapping of the Issuer to the proper URL.

How do you send an AuthnRequest using HTTP Redirect? The only difference between AuthnRequest sent using POST vs. Redirect, besides using GET rather than POST, is that the AuthnRequest XML has to get compressed (using the DEFLATE encoding).

Hope that answers most of your questions.

Sardanapalus answered 3/2, 2011 at 19:33 Comment(2)
I should have mentioned that I am trying to do this using the IsPassive option, that meaning the User-Agent doesn't come into play during the message exchange. It is simply the catalyst that gets the SAML-party started. Right? With that in mind, given that the user is anonymous at Step 1, what does the SP send to the IdP to allow the IdP figure out whether the user is already authenticated? Because of IsPassive, the HTTP POST isn't sent via the User-Agent.Henriettahenriette
Authentication (as you have noticed) is not part of the SAML specification. The IDP can generally use any method it wants (Username/Password, Digital Certs, two-factor auth, etc). However, if you are sending an "IsPassive" in the AuthnRequest the IDP must not prompt the end user for any authentication information. Typically, something like IWA/Kerberos performs this way. If the IDP cannot determine the identity of the user passively, then you must return the appropriate error response to the SP. BTW - using HTTP Post the AuthnRequest is sent via the User-Agent.Trevatrevah
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John,

I might suggest taking a step back and doing some more research before you decide to write your own SAML IDP/SP Implementation. You appear to be mixing Bindings with Profiles, Unsolicited vs Solicited Web SSO as well as the fact that SAML requires that the User Agent (aka Browser) is the bearer of almost all the messages between the IDP and SP. There is also a ton of info in the spec that will will have to implement to ensure your solution is actually secure.

I would suggest starting with our SAML Knowledge Base and then moving on to the OASIS SAML 2.0 Technical Overview for information on these flows.

Alternatively, if you decide to go best-of-breed you can check out our PingFederate product which can enable ALL the SAML IDP/SP use cases for you in < a day.

Hope this helps - Ian

Trevatrevah answered 3/2, 2011 at 15:7 Comment(3)
Thanks Ian. I definitely appreciate the additional learning resources. I've been reading the specification documents provided by OASIS but as I say, it has left me a bit confused. I'll take a look at the information you provided. Hopefully it'll give me some clarity. Thanks again!Henriettahenriette
Just watched the tutorial video here: pingfederate.com/knowledge-center/…. So when a user clicks on a link to access a SP resource, that request is actually processed through the IdP first before redirecting to the resource?Henriettahenriette
For Unsolicited Responses (aka IDP-Init SSO), yes. For SP-Init (what you are attempting) then the SP must generate an AuthnRequest for the IDP to act upon. I'll comment above on your IsPassive and Authentication question.Trevatrevah
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Unlike Ian, I am not associated with a company producing SAML-related products. However, I'd give somewhat similar advice: step back and identify why you are implementing SP or IdP. Are you really acting as both SP and IdP, or are you really just one or the other? If you're implementing/acting as IdP only, then it's fairly likely that a product like PingFederate or something similar offers all you need through configuration rather than requiring you to write custom code. If you're implementing SP, then such a product MAY be able to help you out, but it depends to a large extent on the characteristics of the system you're integrating it into. I am speaking as a developer who has done both IdP and SP implementations, and evaluated several tools before determining that because of our specific system, clients, and requirements, a custom implementation was our best option. It's been in place for over a year, with several clients using it (including some using varying commercial IdP tools).

If you can identify your use cases in terms of SAML profiles/bindings, then you'll be better equipped to make a buy-vs-build decision.

Sardanapalus answered 3/2, 2011 at 17:56 Comment(2)
Thanks for the help. Sorry for the confusion if I was unclear in my explanation above. I'll revise it for clarity. I am implementing both the IdP and SP (via a plugin). The IdP is an existing application for which I want the SP (a third-party system) to use for authentication (i.e., Web SSO). I am developing a simple PoC at the moment. The SP is actually a third-party Spring application for which I am developing a plugin to perform the SAML operations.Henriettahenriette
OK, that's helpful. (So is that you're using Java.) I'll put a separate answer to address your specific question.Sardanapalus

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