Introduction
A threat and a vulnerability are not one and the same. A threat is a person or event that has the potential for impacting a valuable resource in a negative manner. A vulnerability is that quality of a resource or its environment that allows the threat to be realized. An armed bank robber is an example of a threat. A bank teller is an example of a valuable resource that may be vulnerable during a bank robbery. Bullet-proof glass between the robber and the teller denies the robber the opportunity to shoot the teller. The threat remains present, but one of its harmful effects (a gun shot) has been mitigated by a protection mechanism (the glass).
In system and network security, the threats remain present but are mitigated through the proper use of security features and procedures. Mitigation is any effort to prevent the threat from having a negative impact, or to limit the damage where total prevention is not possible, or to improve the speed or effectiveness of the recovery effort.
Hardware and software systems and the data they process can be vulnerable to a wide variety of threats. The selection of security features and procedures must be based not only on general security objectives but also on the specific vulnerabilities of the system in question in light of the threats to which the system is exposed. It is possible to over-protect, which only wastes resources and inconveniences users.
As you can see, there is a relationship between threats and vulnerabilities. Sometimes it is easier to examine each potential threat and determine the extent to which you are vulnerable (e.g. fire, flood, earthquake). In other cases it is easier to look for potential vulnerabilities with no particular threat in mind (e.g. improper mounting of equipment, media failure, data entry error). In order to arrive at a complete risk assessment, both perspectives must be examined. Threats and vulnerabilities are intermixed in the following list and can be referred to collectively as potential "security concerns.