Published at LXer:

The latest MessageLabs spam index reveals that relative to its market share, any given Linux machine is five times more likely to be sending spam than any given Windows machine. But what are the facts behind those headline grabbing numbers and can Windows really get off the hook that easily?

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This article doesn't really say why there are some Linux spam bots. The only reason there would be a bot that is under Linux is because of a rootkit, or a more probable cause, a very careless and lax administrator. There are Linux distros that either have certain services turned on or off by default, and that is a factor, but if the person administering the Linux server doesn't take the time to make sure what needs to be running, and what doesn't, and doesn't keep up with security updates when needed, then Linux can be just as vulnerable as Windows is already.

Linux, like any other OS is only as secure or vulnerable you make it. Windows just makes vulnerability easier.