Software vulnerabilities are weaknesses in source code that can be potentially exploited to cause loss or harm. While researchers have been devising a number of methods to deal with vulnerabilities, there is still a noticeable lack of knowledge on their software engineering life cycle, for example how vulnerabilities are introduced and removed by developers. This information can be exploited to design more effective methods for vulnerability prevention and detection, as well as to understand the granularity at which these methods should aim. To investigate the life cycle of known software vulnerabilities, we focus on how, when, and under which circumstances the contributions to theintroductionof vulnerabilities in software projects are made, as well as how long, and how they areremoved. We consider 3,663 vulnerabilities with public patches from the National Vulnerability Database—pertaining to 1,096 open-source software projects onGitHub—and define an eight-step process involving both automated parts (e.g., using a procedure based on the SZZ algorithm to find the vulnerability-contributing commits) and manual analyses (e.g., how vulnerabilities were fixed). The investigated vulnerabilities can be classified in 144 categories, take on average at least 4 contributing commits before being introduced, and half of them remain unfixed for at least more than one year. Most of thecontributionsare done by developers with high workload, often when doing maintenance activities, andremovedmostly with the addition of new source code aiming at implementing further checks on inputs. We conclude by distilling practical implications on how vulnerability detectors should work to assist developers in timely identifying these issues.