While most malware these days has to be “invited” in – by downloading and running an infected file, or opening an infected attachment – another, perhaps even older class of malware is still present: the network-based threat.
Network-based malware simply uses your machine’s network connection to probe for other machines on the network, and then attempts to exploit any network-based vulnerabilities on that machine. If successful, the malware can spread from machine to machine simply by using the network connection.
For the most part, these types of malware aren’t as prevalent as they once were. But if you connect directly to the internet without any kind of protection, you can actually see that there are infected machines continually reaching out and probing for other vulnerable machines. These machines – often older, unattended machines that have never been updated or properly secured – are responsible for generating what some refer to as “internet background noise” as they keep plugging away, looking for other vulnerable machines.