Some of these bugs will have security implications, often allowing unintended access to files and processes. These problems are often exposed, particularly on machines connected to networks, as most computers run, by default, with all options turned on, whether they are needed or not. Configure the remaining options to reduce the chance of misuse. Update the system promptly in response to warnings of newly discovered problems. New bugs are discovered all the time, so even brand new software may well be insecure.
Many vendors also have mailing lists where alerts are issued.
We strongly recommend that anyone responsible for a computer system should subscribe to the appropriate vendor lists, learn how patches are distributed and how to install them from trusted sources before this knowledge is needed to deal with a vital security problem. NB Patches should only be obtained from trusted sources. It is rare for genuine patches to be distributed by e-mail, much more common for malicious fake patches to be mailed, often with forged headers, by intruders in order to provide them with access to your machines. The following links are best read in the order in which they are given, as these range from introductory to advanced topics. CERT-CC have specific notes on other aspects of Windows and Unix systems in their tech tip collection.