Traditional anti-virus tools can only protect us against hostile programs containing known patterns that the tools have been told to look for. Some folks will be unlucky enough to be the unfortunate discoverers of newly released hostile code. If the code replicates fast enough, a lot of people can be affected before anti-virus products can be updated. If the code stays quiet, it may not be discovered for some time and in the meantime may silently collect our passwords, passwords of neighboring computers seen on the network, or allow others to control our computer for their own purposes. As long as no noticeable action is taken, the person controlling our computer is free to use its resources and any connected to it as he or she sees fit.
Defense primarily consists of refusing to run unknown code .
Others Servers run software that constantly listens to the network for requests from web browsers, email clients, FTP clients, Napster clients, and the like. The most common method of breaking into server computers has probably been to force them to run hostile code by sending them these specially formatted network requests that exploit mistakes in the server's software. For decades, thousands of computers have been compromised through faulty web servers, faulty ftp servers, faulty e mail servers, faulty DNS servers, faulty RPC servers, etc. As shown on the SANS Top Ten list and CERT's list of current activity , quite often break-ins continue for months after the faulty server code is fixed by the vendor simply because computer owners don't apply the updates to their systems. Obviously, a very effective defense against such attacks is to update our computers regularly .