The risk level of a specific vulnerability might change over time due to various factors:
Change in environment: If the system or environment where the vulnerability exists changes, the risk posed by the vulnerability might also change. For instance, a system previously not connected to the internet becoming internet-facing can increase the risk level.
Discovery of exploit: When a vulnerability is first discovered, there may not be any known exploits. However, if an exploit later becomes available or is actively being used in the wild, the risk level can significantly increase.
Application of patches or fixes: When patches or fixes are applied, the risk level of a vulnerability can decrease. However, it's also possible that a patch doesn't fully address the vulnerability or introduce new vulnerabilities, in which case the risk might not decrease as much as expected, or it might increase.
Change in asset importance: The importance of the asset where the vulnerability is located can also change over time. For instance, a server might start hosting more critical data or services, increasing the potential impact of a vulnerability and, therefore, its risk level.
Introduction of compensating controls: If new security controls are implemented that help protect against the vulnerability or limit its potential impact, the risk level can decrease. For example, a new firewall rule might limit network access to a vulnerable system, or an intrusion detection system might be set up to detect attempts to exploit the vulnerability.
False positives/negatives: Upon further investigation, what was initially identified as a vulnerability might turn out to be a false positive, or the severity of the vulnerability might have been over or underestimated. The initial risk assessment could also have missed some vulnerabilities (false negatives), and these could be identified in later assessments.