SimpleSAMLphp SP accepts a response from an unexpected IdP when unsigned Response/InResponseTo is combined with a signed assertion lacking SubjectConfirmationData/InResponseTo
Summary
SimpleSAMLphp's SAML SP ACS path does not enforce the IdP selected for an SP-initiated login. If a saved SP state contains ExpectedIssuer = IdP A, but the ACS receives a valid response from IdP B, the code logs a warning and continues processing instead of rejecting the response.
That behavior becomes security-relevant when combined with the response-processing rule that accepts an unsigned samlp:Response/@InResponseTo outside the signed assertion whenever the signed assertion's SubjectConfirmationData does not carry its own InResponseTo. A response issued by one trusted IdP can therefore be bound to SP state created for another IdP.
Impact
In a multi-IdP deployment, a lower-trust IdP can satisfy SP state created for a different expected IdP. This can bypass an SP flow that intentionally routes the user to a specific IdP, including deployments that set enable_unsolicited to false to prevent IdP-initiated logins.
The impact is highest when the SP trusts multiple IdPs with different assurance levels, tenant boundaries, or attribute namespaces, and application authorization depends on the selected/expected IdP. In those deployments this is an authentication/authorization bypass candidate. Impact strongly depends on whether an attacker can obtain a signed IdP-initiated assertion from a lower-trust trusted IdP and whether the downstream application maps identifiers globally.