What is CVSS, and how does it help in vulnerability prioritization? Explain the different severity levels and what they indicate.

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CVSS stands for Common Vulnerability Scoring System. It is a standardized framework for assessing and communicating the severity of security vulnerabilities. CVSS provides a numerical score ranging from 0.0 to 10.0, where higher scores indicate more severe vulnerabilities. The current version, CVSS 3.1, is maintained by FIRST, the Forum of Incident Response and Security Teams.

CVSS helps in vulnerability prioritization by providing an objective, consistent measurement of severity. Without a standard scoring system, different analysts might assess the same vulnerability differently based on personal judgment. CVSS creates a common language that allows teams to compare vulnerabilities across different products and vendors. When facing thousands of vulnerabilities in a large environment, CVSS scores provide a starting point for deciding which issues to address first.

The severity levels based on CVSS scores are defined as follows. A score of 0.0 represents no vulnerability. Scores from 0.1 to 3.9 are classified as Low severity, indicating vulnerabilities with limited impact or difficult exploitation requirements. Scores from 4.0 to 6.9 fall into the Medium category, representing vulnerabilities that could cause moderate damage but may have mitigating factors. Scores from 7.0 to 8.9 are considered High severity, indicating serious vulnerabilities that should be addressed promptly. Scores from 9.0 to 10.0 are classified as Critical, representing the most dangerous vulnerabilities that require immediate attention.

CVSS scores are calculated based on multiple metrics grouped into three categories. Base metrics capture the intrinsic characteristics of a vulnerability, such as the attack vector, attack complexity, privileges required, and impact on confidentiality, integrity, and availability. Temporal metrics reflect characteristics that change over time, such as exploit availability and patch status. Environmental metrics allow organizations to customize scores based on their specific context, such as the importance of affected systems.

While CVSS provides valuable guidance, it should not be the only factor in prioritization. A Critical vulnerability on an isolated test server may be less urgent than a Medium vulnerability on a public-facing production system containing sensitive data. Effective vulnerability management combines CVSS scores with asset criticality, threat intelligence, and business context.