The main point is the file system with full, umlimited write access. Okay, you are not able to write outside of the computer directory, but there are other problems.
First: See this script as an example for disk exhaustion. Workaround: Add an option to limit space used by single computers/disks.
Second: Another (not obvious) hole is the possibilty to create files / folders. There are several options to destroy/damage the real computer's file system.
- Create many files: By creating really many files (could be possible for multiple computers) you can make your file system instable. I had a HDD with a folder containing some sub-folders (IDK how many, guess something greater 2,000,000,000) and was not able to delete the folder! I tried for hours. Once I opened (or tried to open) the folder with Windows Explorer it crashed. Finally I had to format the whole thing.
- Create deep structures. I managed to create pure Java code that generated a folder structure in just a few moments. This folder pointed me to a big problem: Once names get longer than 256 characters almost every OS and software will be unable to handle it! I needed hours to find a trick to remove the folder. And YES: This is possible in java so I guess it is using fs API, too.
Maybe I am wrong but please have a look at these facts.