Vulnerability databases, such as the National Vulnerability Database (NVD), are populated through a process of responsible disclosure by security researchers, vendors, and other ethical entities. A significant limitation is that they cannot account for vulnerabilities discovered and kept secret by malicious actors for exploitation. These are often referred to as "zero-day" vulnerabilities. Because attackers do not report their findings, these security flaws remain absent from public databases until they are independently discovered and disclosed by others, often after an attack has already occurred. This creates a critical gap between publicly documented vulnerabilities and the actual threats present in the wild.

B. Maintaining and updating large-scale vulnerability databases requires substantial, not minimal, human effort for analysis, scoring (e.g., CVSS), and validation.

C. The process of discovering, reporting, analyzing, and publishing a vulnerability introduces significant delays; therefore, these databases do not provide real-time information.

D. The scope of software and hardware is vast, and no database can achieve complete coverage of all vulnerabilities in every product, especially in custom or niche systems.

1. National Institute of Standards and Technology (NIST), Special Publication 800-161 Rev. 1, Cybersecurity Supply Chain Risk Management Practices for Systems and Organizations, May 2022.

Section 2.3.2, "Threat Events": This section discusses threat sources, including adversaries who exploit vulnerabilities. It implicitly supports the idea that adversaries discover and use vulnerabilities that may not be publicly known, stating, "Adversaries may also develop their own exploits or purchase them on the black market." This highlights the existence of a threat landscape beyond publicly cataloged vulnerabilities.

2. Cisco, Security Vulnerability Policy, Document ID: 13322.

Section: "Vulnerability Disclosure": The policy outlines a structured process for reporting vulnerabilities to Cisco. It states, "Cisco encourages individuals to report vulnerabilities to us privately." This formal, private disclosure model is what populates vendor advisories and, subsequently, public databases. It contrasts directly with the behavior of malicious actors who do not report, creating the limitation described in the correct answer.

3. Souppaya, M., & Scarfone, K., National Institute of Standards and Technology (NIST), Special Publication 800-40 Rev. 3, Guide to Enterprise Patch Management Technologies, July 2013.

Section 2.2, "Vulnerability and Patch Information Sources" (Page 8): The document notes that organizations rely on sources like "software vendors, and public vulnerability databases." It describes a reactive process based on available information, which inherently excludes vulnerabilities that have not been publicly disclosed by any of these sources.