SimpleSAMLphp HTTP-Artifact TLS validator confusion allows cross-IdP authentication bypass

Summary

The SimpleSAMLphp SAML2 library is a PHP library for SAML2 related functionality. Prior to versions 4.19.3, 4.20.2, 5.0.6, and 6.2.1, the HTTPArtifact::receive() flow can treat an unsigned embedded SAML Response as cryptographically valid for the wrong identity provider. SOAPClient::addSSLValidator() attaches a TLS-based validator to the outer SOAP ArtifactResponse, while the embedded Response receives a validator that delegates to the outer message and is later checked against metadata selected from the embedded response issuer rather than necessarily the artifact issuer. SOAPClient::validateSSL() returns normally when the TLS public key does not match the key being validated, and SAML2\Message::validate() treats a validator call that does not throw as successful. In a multi-IdP federation, a malicious or lower-trust IdP can therefore provide an ArtifactResponse containing an unsigned Response that claims a higher-trust victim IdP as issuer and authenticate as arbitrary users with attacker-chosen assertion attributes, NameID, and session data. This issue is fixed in versions 4.19.3, 4.20.2, 5.0.6, and 6.2.1.

Timeline

1. Aug 19, 2026, 2:47 PM UTCCVE published
2. Aug 19, 2026, 6:16 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: total

3. Aug 19, 2026, 6:19 PM UTCRecord last updated

Affected versions

• simplesamlphp saml2

  fixed in 4.19.3; From 4.20.0 · fixed in 4.20.2; From 5.0.0 · fixed in 5.0.6; From 6.0.0 · fixed in 6.2.1

• simplesamlphp/saml2Packagist

  From 6.0.0 · fixed in 6.2.1

• simplesamlphp/saml2Packagist

  From 5.0.0 · fixed in 5.0.6

• simplesamlphp/saml2Packagist

  From 4.20.0 · fixed in 4.20.2

• simplesamlphp/saml2-legacyPackagist

  From 4.20.0 · fixed in 4.20.2

• simplesamlphp/saml2Packagist

  fixed in 4.19.3

• simplesamlphp/saml2-legacyPackagist

  fixed in 4.19.3

References 5