If someone can access my Ubuntu VirtualBox files, can they hack into it?
Is it hard?
"If someone can access my ubuntu virtualbox files, can they hack into it?"
If someone is able to access your files, why should he hack it?
Yes, they can. It has been discovered with VMWare images, if your image is not encrypted, then they can get full access to your virtual machine through the image. Obviously they need access to your image, once they have access they can mount it and boot it up without you even knowing. That's as much as I can remember from off the top of my head.
Hope that helps.
If someone is able to access the files associated with the virtual hard drive, then yes, it is possible to modify the content of the files from the virtual hard drive. I believe it is not hard to do it: as I remember, it is possible to mount the virtual hard drive and to access (read/write/delete/whatever) files from it.
If they have access to the hard disk image files, then they could mount those images in their own VirtualBox machines. This isn't hard, and would probably let them see the files on the disks (unless you have disk encryption turned on, or something similar). In that sense, yes they could "hack into it".
Getting access to the content within those files is a similar but separate matter. Are the files password-protected, encrypted, etc.? Are the passwords stored in the disk image?