Securing cluster communication
Keycloak embeds Infinispan, which is leveraged when you create a cluster of Keycloak nodes. More sensitive data such as signing keys or user information is not sent across the cluster, as this information is only kept in a local cache in each node with the only communication across the cluster being invalidation messages. It does store information about user sessions in the cluster, which are distributed across the cluster. Sessions themselves contain some information such as the session ID, the expiration date, and associated client sessions. Even if an attacker gains access to this information, they are limited in terms of what they can do with it, since accessing any session through Keycloak requires a token or cookie to be signed by Keycloak.
It would still be a good idea to secure cluster communication, at the very least with a firewall. For additional protection, you can enable authentication and/or encryption for cluster communication.
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