The first time somebody asked me this it kinda caught me off guard. Then it hit me that most modern computer users will assume that if the computer isn’t connected to the Internet or another network then it’s not at risk. Anybody out there from the old school of computers will remember the days before the Internet (yes the Internet is a recent invention) and just how common viruses were back in the 90s. Of course back then a home computer was only at risk from viruses that could spread via floppy disks. Then again floppy disks were in use massively around the world and you had to be careful – some factories wound up producing batches of floppy disks with viruses already on them.
You couldn’t be more wrong.So we come back to the present and look at your typical home computer – laptop or desktop. We’ll assume that the computer is in no way connected to the Internet or any other network. It’s just sat there in your living room where you do your accounts, play a few games and maybe do a bit of word-processing. Your computer is totally offline so you’re assuming it’s safe.
Even an offline computer is extremely vulnerable to any number of different threats. The main threat is computer viruses. Any communication port on your computer (USB, serial or parallel) can be used to transfer data to and from your computer. This includes viruses. I had the misfortune of watching a guy I used to work with plugging a USB memory stick into his work laptop and massively infect it with viruses. The sad part is that this guy was a desktop technician who should know way better. He had to reimage the laptop to clean it up and he was only lucky that he wasn’t connected to our corporate network at the time. He could have caused massive problems!
Any device that can be plugged into your PC to communicate with it (transfer files for example) is capable of infecting your computer with a virus – it’s that simple.
You need to have some type of anti-virus software installed to prevent this or to try and clear up the damage afterwards. If you’re not using anti-virus protection you’re a sitting duck – plain and simple.
The next threat then is identity theft. True most identity theft does happen online but that’s not to say that somebody visiting your house can’t just dump some personal info onto a USB stick and leave with it. The basics of having restricted user accounts and an administrator password will help stop most of this type of problem. Or you can go a step further and install some software that prevents files being downloaded from your computer onto USB keys. Something like USB Port Blocker could help you secure your computers USB ports.
So to cut it short the answer is “No” an offline computer is not secure. Not at all.