Harden SAML 2.0 service provider against XML signature wrapping and related attacks

Steps

1. Validate the XML schema of the SAML Response before any signature verification; reject documents that fail schema validation — do not pass malformed XML to the signature verifier
2. Verify that the Signature element's Reference URI points to the Assertion being validated; check that the ID attribute of the validated element matches the Reference URI exactly, and that only one Assertion is signed
3. Use a StaticKeySelector seeded with the IdP's certificate (fetched via metadata and pinned locally) rather than trusting KeyInfo in the received document — an attacker can inject a KeyInfo with their own key
4. After signature verification, extract data from the same element node that was verified — many vulnerabilities arise from verifying one node and reading data from a different node injected by the attacker
5. Enable ForceAuthn and validate AuthnInstant recency to prevent replay attacks; maintain a used-assertion cache indexed by the Assertion ID to detect duplicate submissions
6. Keep the SAML library dependency current; CVE-2025-47949 (samlify < 2.10.0) and Ruby-SAML CVEs (CVE-2025-66568/66567 in versions prior to 1.18.0) are active signature wrapping vulnerabilities from 2025