Currently, any number could be used as a pid in
com.sun.tools.attach.VirtualMachine.attach(String vmid);
on Linux/Mac/Solaris

However, attach mechanism use signal (SIGQIT) as a part of attaching. Sending a signal to PID with a negative number is sending a signal to the process and all its childs.

So a user could just kill a bunch of processes using "-1" for example.