Can Mac and Linux boxes harbor malware that does not affect them, but could affect Windows PCs? Absolutely. It can and does happen. The Sophos white paper, “Protecting Mac and Linux computers: genuine need or nice to have?” presents a convincing case, describing just how Mac and UNIX/Linux machines threaten Windows PCs.
…it is very common for Windows networks to include a server running UNIX or Linux. Vulnerabilities, such as a weak SSH password, can allow hackers to convert a Linux server into a botnet controller, and install malware that will compromise desktop Windows computers.
Well, that’s one way, but consider this: Viruses, worms, and other types of malware are files, and can be stored on any digital media, regardless of the format or operating system that created them. A Mac/UNIX/Linux machine can store Windows files; a Windows machine can store Mac/UNIX/Linux files. That a Windows virus cannot damage a Mac/UNIX/Linux machine–and vice-versa–is irrelevant: Typhoid Mary harbored and transmitted typhoid fever yet never succumbed to it. She did, however, infect 47 others, three of whom died.
…computers harboring the malware can quietly transmit it to Windows computers. For example, UNIX computers can easily transmit the virus to Windows computers via the Samba fle-sharing system.
If you have a mixed network, it’s time to put some effort into protecting the non-Windows machines. Best practice now dictates that every server and desktop machine in your network be protected with some sort of anti-malware application.