It’s a very unlikely scenario that could allow a turned-off computer to be hacked. I’ll describe it and show you how to prevent it.
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Fifty percent say no?
Welcome to the world of technology, where there are very rarely black-and-white answers.
One of my most common phrases (that I get kidded about by my assistants) is: “It depends.”
There are rarely yes-or-no answers to technological questions. The real answer is usually nuanced and depends on many factors.
This happens to be one of them.
Hacking a turned-off computer
The good news is that in general, the answer is “No.”
Your PC is off; it’s not doing anything. Unless you’ve taken extra steps described below, the PC will not be able to be restarted and hacked from outside if it’s turned off, even if you leave it connected to the internet and to power.
The exception to the rule is a feature that, when enabled, allows a PC to be remotely turned on and booted. In short, the network adapter for the computer is not turned off completely, but instead monitors for specific instructions that tell it to wake up. When those instructions are received, power is restored to the computer and it boots.
In such a scenario, if the PC has been configured to respond to the remote requests, then the PC could be turned on and booted remotely.
At that point, if the PC did not have appropriate security software installed (say it was not behind a firewall and it did not have anti-malware tools), then it is conceivable the PC could be hacked remotely, even though it was powered off.
One caveat: standby
It’s not uncommon for a laptop to be placed into standby mode only to wake up on its own a while later. The exact reasons this happens vary — I can’t give you a simple explanation.
There is a simple work around, though: if you find your laptop waking up on its own, and that concerns you, don’t use standby.
If it works for you, that’s great, but I generally avoid standby for a variety of reasons, including this one.
Allowing remote access
Hacking a turned-off machine is extremely unlikely.
A lot of things have to line up for it to happen. You have to have this remote power-on ability (Wake on LAN) turned on in the network settings, and most computers do not. In most cases, it’s a setting in the UEFI or BIOS you have to turn on yourself. If you haven’t turned it on, then it’s probably not on.
The PC also has to be fundamentally insecure. In other words, it has to be vulnerable to be being hacked when it’s running. That’s something you can control by putting appropriate security measures into place.
So, as I said, it’s possible.
It’s just extremely unlikely.