Many commercial organisations now use a range of Internet servers to host their presence on the Internet. All of these servers can be subject to attacks. Some worries are
Denial of service attacks can essentially shutdown their presence for a period of time.
You need to ensure that third parties cannot alter or delete company data on their public servers.
Private Network Confidentiality
You want to ensure that the public servers cannot be used a means of entering private networks, which may well contain confidential information.
Activity - DOS Attacks
Have a look at the article "Peer-to-peer networks co-opted for DOS attacks" on the Register website to see how DOS attacks exploit vulnerabilities.
If the above link is unavailable go The Register home page and use the security tab to search for current DOS attacks.
Next: Cyber Crime