Any server is open to risks of unwanted access and malware/virus issues. But Minecraft servers are less likely to be a target, free or paid. These dedicated servers are much smaller than, say, a corporate server harboring valuable data. The bigger the server, the more susceptible it is to risks. However, this does not mean that a Minecraft server is completely safe. It is advised to NEVER run a server setup from your home setup with accessibility to your home network and your local machine. Instead, get a paid hosting service provider. Also, you would do well to check if they provide DDoS security.