A flaw was found in the keycloak-services component of Keycloak. The vulnerability exists in the OIDC backchannel logout endpoint located at /realms/realm/protocol/openid-connect/logout/backchannel-logout. When an OIDC identity provider is configured with validateSignature set to false, the endpoint fails to enforce cryptographic signature requirements for logout tokens, accepting tokens with the alg=none header.
An unauthenticated remote attacker who knows the provider issuer, the broker client ID, and a brokered user's upstream subject or session ID can craft and send a forged, unsigned logout token. Successful exploitation allows the attacker to force-logout targeted brokered users or revoke their offline sessions. This behavior violates the OIDC Back-Channel Logout specification which mandates signatures and prohibits the use of the none algorithm.