Comment by peterwaller

It lets the hacker take over the machine because there are strings which are interpreted as (IIUC, I am not a java engineer) variables within a class, and you can express a remote URL to load a class from, apparently (through something like (jndi://... ldap... URL), resulting in fetching code from somewhere and running it, in the service of writing a log message. This is apparently being exploited in the minecraft ecosystem by simply writing chat messages containing the full exploit, which gets executed by both servers and clients.