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I'm implementing a SAML2 Service provider that will be running on a public facing web site. The sign in functionality is publicly available, so anyone can get hold of a AuthnRequest from my site.

When I do receive a response from the Idp, do I have to validate the InResponseTo field to make sure that the response is to a AuthnRequest I've issued or can I ignore it?

I don't understand how it would add any extra security when I'm already validating the signature of the response and have a replay protection by only allowing each assertion id to be used once within the validity time of the assertion (as specified by the condition).

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It is explained in Security and Privacy Considerations for the OASIS Security Assertion Markup Language (SAML) V2.0 section 6.1.3 Message Insertion.

It is primarily a method for blocking fabricated requests inserted into the communication. As a complement to signing and/or SSL. It is not solving a separate security issue but is an in depth defense.

My interpretation is that checking InResponseTo is cheaper than validating the signature and a faster way to ditch invalid messages to avoid DOS attacks.

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Yes, according to the SAML spec this must be validated.

I'm not 100% sure of the reason or this. But just thinking out loud. With the contraint on time and sigature, you would still be able to do a replay of the message within the validity time.

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