better-auth/.changeset/saml-response-validation-hardening.md

@better-auth/sso

@better-auth/sso
minor

fix(sso)!: harden SAML response validation (InResponseTo, Audience, SessionIndex)

Breaking Changes

• allowIdpInitiated now defaults to false — IdP-initiated SSO (unsolicited SAML responses) is disabled by default. Set saml.allowIdpInitiated: true to restore the previous behavior. This aligns with the SAML2Int interoperability profile which recommends against IdP-initiated SSO due to its susceptibility to injection attacks.

Bug Fixes

• InResponseTo validation was completely non-functional — The code read extract.inResponseTo (always undefined) instead of samlify's actual path extract.response.inResponseTo. SP-initiated InResponseTo validation now works as intended in both ACS handlers.
• Audience Restriction was never validated — SAML assertions issued for a different service provider were accepted without checking the <AudienceRestriction> element. Audience is now validated against the configured samlConfig.audience value per SAML 2.0 Core §2.5.1.
• SessionIndex stored as object instead of string — samlify returns sessionIndex from login responses as { authnInstant, sessionNotOnOrAfter, sessionIndex }, but the code stored the whole object. SLO session-index comparisons always failed silently. The correct inner sessionIndex string is now extracted.

Improvements

• Extracted shared validateInResponseTo() and validateAudience() into packages/sso/src/saml/response-validation.ts, eliminating ~160 lines of duplicated validation logic between the two ACS handlers.
• Fixed SAMLAssertionExtract type to match samlify's actual extractor output shape.