CVE-2025-66568ruby-saml second Fragile Lock bypass
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: any user, including admins, on any SP still verifying 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 and SAML vs OIDC.
Sources
- NVD: CVE-2025-66568
- PortSwigger, "The Fragile Lock" (8 December 2025)
Related identity CVEs
Related on Start with Identity
- CVEruby-saml auth bypass, incomplete fix of CVE-2025-25292
The March 2025 ruby-saml patch did not close the parser differential. CVE-2025-54572 is the incomplete-fix follow-on: still a critical SSO impersonation if you
- CVEruby-saml 2024 auth bypass (ahacker1), still in the blast radius
The original 2024 ruby-saml authentication bypass (CVSS 10.0). It is outside the last twelve months, but every 2025 ruby-saml CVE is an incomplete-fix descendan
- CVEruby-saml companion signature-wrapping bypass
The pair to CVE-2025-25291. A second signature-wrapping path in ruby-saml lets an attacker forge assertions and impersonate any user. The March 2025 patch was l
- BlogFortinet's January SSO bypass hit boxes already patched for December's SAML bug
CVE-2026-24858 is the follow-on FortiCloud SSO SAML bypass. Devices patched for CVE-2025-59718 and 59719 were still exploitable. Actively exploited. CISA guidan
- GlossaryIdentity Federation
A trust relationship between identity providers and service providers that lets users authenticate once at their home IdP and access applications at the other p
- GlossaryService Provider (SP)
The application that consumes identity assertions from an IdP to grant the user access. In SAML it's the SP; in OIDC the equivalent is the Relying Party. The se
Technique
This CVE is an instance of Federation trust abuse and SAML forgery. A service provider that accepts a SAML assertion it should have rejected treats a forged identity as authenticated, because the failure sits in signature validation code, not in cryptography.