# ruby-saml second Fragile Lock bypass

Source: https://startwithidentity.com/cves/cve-2025-66568/
Last updated: 2026-08-29
License: content by Start with Identity. Cite the source URL.

---

## What broke

CVE-2025-66568 is the second of the two PortSwigger "Fragile Lock" ruby-saml bypasses disclosed on 8 December 2025. It affects ruby-saml below 1.18.0, including the earlier "patched" 1.12.4 line. Fixed in 1.18.1.

## Why it matters

Same blast radius as [CVE-2025-66567](https://startwithidentity.com/cves/cve-2025-66567/): any user, including admins, on any SP still verifying [SAML](https://startwithidentity.com/glossary/saml/) with that library. Two IDs exist because there were two distinct remaining paths, not because one is theoretical.

## What to do

- Upgrade once, to 1.18.1 or later, and close both tickets together.
- Rotate IdP signing material if the SP was internet-reachable on a vulnerable build.
- Stop treating each ruby-saml CVE as a separate risk acceptance. Track the library version as the control.

## After you patch

A SAML bypass means the service provider accepted an assertion it should have rejected, so anyone who exploited it authenticated as a real user and left a normal-looking log line.

- **Revoke every session** issued by the affected service provider, then rotate its session signing keys. Patching stops new forgeries and does nothing about sessions already minted.
- **Audit administrative accounts and group memberships** for changes during the exposure window. Signing in as an administrator is the point of this class, and adding a second account is the standard persistence step.
- **Rotate the identity provider signing certificate** if the flaw involved signature validation, and confirm the service provider pins the expected certificate rather than trusting anything in the assertion.
- **Check your own implementation for the same class**: exact-match comparison on verification results, rejection of unexpected signature algorithms, and audience and recency checks on every assertion. See [SAML 2.0](https://startwithidentity.com/standards/saml-2-0/) and [SAML vs OIDC](https://startwithidentity.com/guides/fundamentals/saml-vs-oidc/).

## Sources

- [NVD: CVE-2025-66568](https://nvd.nist.gov/vuln/detail/CVE-2025-66568)
- PortSwigger, "The Fragile Lock" (8 December 2025)