So would the signed 'object' contain code? Or is it just data? And even if it is code, does deserializing mean execution? I guess it could mean execution at some other stage in the process.

What is the end-goal of this... would it be data exfiltration vs ransomware.

Java object serialization can be super dangerous as it just works on any class that implements serializable.

That means if the shape of your object is something like

    class Foo implements Serializable {
      SerializableFunction bar;
      
      void doBar() {
        bar.apply();
      }
    }

Here's an article detailing exactly how that works: https://www.baeldung.com/java-serialize-lambda

> What is the end-goal of this... would it be data exfiltration vs ransomware.

The end-goal is to gain complete access to the system - the outcome (data theft or ransomware) is customers choice

It often results in remote code/command execution, its data that de-serializes into java objects. But during the instantiation or sometimes deconstruction of objects, code can be executed. Popular tool for java: https://github.com/frohoff/ysoserial