A computer-software vulnerability that is unknown to those who would be interested in mitigating the vulnerability is a zero-day vulnerability. This is because a zero-day vulnerability is a type of vulnerability that has not been reported or disclosed to the public or to the software vendor yet, and may be exploited by attackers before it is patched or fixed. A zero-day vulnerability poses a high risk to systems and applications that are affected by it, as there may be no known defense or solution against it. The other options are not computer-software vulnerabilities that are unknown to those who would be interested in mitigating the vulnerability, but rather types of vulnerabilities that are known and reported to the public or to the software vendor, such as cross-site scripting vulnerability (A), SQL injection vulnerability (B), or memory leakage vulnerability C.