OpenDJ Pre-Auth RCE via Java Deserialization in JMX RMI

OpenDJ is an LDAPv3 compliant directory service. Prior to 5.1.1, the JMX RMI connector in opendj-server-legacy/src/main/java/org/opends/server/protocols/jmx/RmiConnector.java processes attacker-controlled credential objects before authentication without a restrictive jmx.remote.rmi.server.credentials.filter.pattern, and RmiAuthenticator.authenticate in opendj-server-legacy/src/main/java/org/opends/server/protocols/jmx/RmiAuthenticator.java accepts an unconstrained Object array rather than a two-element String[]. When the JMX Connection Handler is enabled and its TCP listener is reachable, an unauthenticated remote attacker can submit a crafted serialized Java object and achieve code execution in the OpenDJ server process. The handler is disabled by default, and successful exploitation depends on the runtime classpath and Java version; remote code execution was demonstrated against OpenDJ 4.4.15 on JDK 11 with Jackson 2.12.6.1. This issue is fixed in 5.1.1.