CVE-2026-49283: SimpleSAMLphp HTTP-Artifact TLS validator confusion allows cross-IdP authentication bypass

SimpleSAMLphp’s HTTP-Artifact receive path can treat an unsigned embedded SAML Response as cryptographically valid for the wrong IdP.

In the HTTPArtifact::receive() flow, the SOAP ArtifactResponse receives a TLS-based validator from SOAPClient::addSSLValidator(). The embedded SAML Response then receives a validator that delegates signature validation to that outer ArtifactResponse. Later, the SP validates the embedded Response against metadata selected from the embedded response issuer, not necessarily the artifact issuer.

The critical issue is that SOAPClient::validateSSL() returns normally when the TLS public key does not match the key currently being validated. SAML2\Message::validate() treats any validator call that does not throw an exception as successful. As a result, an ArtifactResponse obtained from one IdP can validate an unsigned embedded SAML Response that claims to be issued by a different IdP.

In a multi-IdP/federation deployment where a malicious or lower-trust IdP can issue an HTTP-Artifact response to an SP, this can allow the attacker to authenticate to the SP as arbitrary users from a higher-trust victim IdP.