Keycloak provides a mechanism called Client Policies to enforce security requirements on clients, such as requiring them to use signed JWTs for authentication. A flaw was discovered where this enforcement can be bypassed. An attacker with valid client credentials can provide a fake, unsigned assertion header that tricks the system into thinking the policy requirements have been met. This allows the attacker to authenticate using simpler methods like a client secret even when the administrator has mandated more secure, signed assertions.

References

• https://nvd.nist.gov/vuln/detail/CVE-2026-16093
• https://access.redhat.com/security/cve/CVE-2026-16093
• https://bugzilla.redhat.com/show_bug.cgi?id=2501729