Keycloak allows unrestricted admin use of system and environment variables

A security vulnerability has been identified that allows admin users to access sensitive server environment variables and system properties through user-configurable URLs. Specifically, when configuring backchannel logout URLs or admin URLs, admin users can include placeholders like ${env.VARNAME} or ${PROPNAME}. The server replaces these placeholders with the actual values of environment variables or system properties during URL processing.