Server security is a complicated subject. Servers are supposed to be used by a lot of people, which means that they have to be accessible. On the other hand, servers often have sensitive data and are important parts of the network so it is very important that they are secure.
A server can run a lot of different operating systems. Nowadays most servers are running some version of Windows or Linux. But many other operating systems are also used. It is a mixed blessing securing a server running an OS that is not very popular. Already in the 1990s, Novell pointed out that its NetWare networking operating system was quite safe, after all it was using IPX, not TCP/IP as most other computers on the Internet. Anyone who wanted to break into a Netware server had to design their own attacks. This of course required a fair bit of knowledge about both NetWare and the IPX protocol. Attacking TCP/IP computers on the other hand did not require much skills, you could find ready-made attack methods and programs on the Internet. But if an operating system is not much used, security flaws may also take longer time to detect and to patch.
Popular server platforms, such as Linux and Windows, may have a lot of known security flaws but they quickly detected and solved. But servers have to be patched regularly. Many successful attacks are taking advantage of security flaws that have been known, and solved, several months ago, sometimes even for years. One of the biggest problem is to keep up with all new patches. This is not a big problem if you only have one or two servers. But today, a lot of places have hundreds of servers. This creates a problem, it will generally takes quite some time to apply patches to all servers. Before all servers have been patched, new fixes have been released.
As mentioned, server security is dependent on the server platform. Someone who tries to break into a Linux server trying to exploit known Windows security flaws will not be very successful. The same goes for the good guys, how to secure a server depends on what kind of server you are securing. Here you can learn more about Windows server security. The success of Linux has made it a very lucrative market. Linux solutions used to be cheap but once Linux started to become popular with large companies, the money in the Linux world has become much bigger. This is true both for Linux software and jobs. Here is more information about Linux server security.
It should also be pointed out that although most threats come from the outside world. Most of the successful security breaches are done by insiders. Today’s firewalls are very good at keeping outsiders away from your servers but firewalls don’t help against attackers that already have access to the servers. So how can you protect yourself and your servers against inside attackers? Actually, there is no simple solution that will work for all servers. But most systems have features that let you divide users into different groups with different privileges. This combined with auditing features are the best way of protecting your servers. Here is more information about insider attacks
It is especially important to keep file servers secure. One single infected file on a file server can quickly infect a lot of client computers. Unfortunately, running a virus scanner on a file server may slow it down significantly. A busy file server can seldom scan files that are opened by clients, the CPU load required for this quickly makes the server non-responsive. Therefore, many sites disable virus scanning on file servers but make sure that all clients have virus scanning enabled. It is reasonable solution, not as secure as having the file server scanning the files but the server will be able to handle all requests within a reasonable amount of time.