SimpleSAMLphp SP accepts a response from an unexpected IdP when unsigned `Response/InResponseTo` is combined with a signed assertion lacking `SubjectConfirmationData/InResponseTo`

• ECOSYSTEM: introduced 2.5.0; fixed 2.5.2

## 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 authentic

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• ECOSYSTEM: introduced 0; fixed 2.4.7

## 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 authentic

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