Viruses date back to long before the Internet was in every home. Originally, even before the first Windows, viruses were doing their thing, corrupting and formatting entire hard drives on systems that generally used MS-DOS. However, as the Internet has reached all users, it has become the main source of malware distribution. However, today is it still possible to infect a computer with a virus without being connected?
The short answer is yes. Just as before, viruses were generally distributed on floppy disks (pirated), today the second most used form of malware distribution, behind the Internet, is USB memory sticks . If our PC does not have Internet, it cannot download threats, even if it is not updated with the latest patches. But another computer, which is infected, could put you in danger with relative ease.
How an offline PC gets infected with a virus
The most normal thing is that the virus has previously been downloaded to another computer. If a PC connected to the Internet has a worm, and we connect a USB or hard drive to it, it will replicate on it. And, when we connect it to the computer without connection, it will be copied, leaving the PC infected. We can also get infected if we have downloaded emails, files or programs from unreliable sources. And, above all, if we have downloaded any pirated content.
It is true that many viruses cannot do anything without an Internet connection . Modern malware is designed to receive instructions from a remote server, so without the Internet, they are practically useless. A Trojan, for example, could not send the information to the server or receive orders, so it would be useless. A ransomware would not be able to send the encryption key to the server and will (generally) not encrypt our data.
But there are viruses that can work without the Internet, and they are almost the most dangerous, because they are designed to do harm. They can erase our hard drive, even damage it. They can also make the computer go bad, deceive us that we have a virus and we have to connect to the Internet to erase it or even remain on the PC, residents, without making noise, waiting for a connection.
There are also ways to infect computers without the need for the Internet or another USB, although they are already used to carry out very specific attacks. For example, through the Meltdown and Specter vulnerabilities of the CPUs they can spy on us, or by EternalBlue execute malware on Windows through a local SMB.
Do I need an antivirus if I don’t connect to the Internet?
Taking into account all the above, is it necessary to install an antivirus if we do not connect to the Internet? Most of the time, not only would it not be necessary, it would not even be recommended . We must bear in mind that, if we do not have Internet, the database will not be updated and, therefore, the truth is that it will not be of much use. There are ways to update virus databases without an Internet connection, but in practice it is not worth it.
What we must do is be careful whenever we are going to copy any file to the computer and thus we will not run risks. As we are sure that we have a PC with an Internet connection, and its corresponding antivirus, we can analyze the USB memories or hard drives that we are going to connect to the computer without Internet with said computer to make sure they are clean.