It is a software , hardware or procedural weakness that may provide an attacker the open door he is looking for to enter a computer or network and have unauthorized access to resources within the environment. A vulnerability characterizes the absence or weakness of a safeguard that could be exploited. This vulnerability may be a service running on a server, unpatched applications or operating system software etc.
The following answers are incorrect because: Threat: A threat is defined as a potential danger to information or systems. The threat is someone or something will identify a specific vulnerability and use it against the company or individual. The entity that takes advantage of a vulnerability is referred to as a ‘Threat Agent’. A threat agent could be an intruder accessing the network through a port on the firewall , a process accessing data that violates the security policy. Risk:A risk is the likelihood of a threat agent taking advantage of a vulnerability and the corresponding business impact. If a firewall has several ports open , there is a higher likelihood that an intruder will use one to access the network in an unauthorized method.
Exposure: An exposure is an instance of being exposed to losses from a threat agent.
REFERENCES: SHON HARRIS , ALL IN ONE THIRD EDITION : Chapter 3 : Security Management Practices , Pages: 57-59