In the realm of cybersecurity, accurately assessing and quantifying the severity of vulnerabilities is crucial for organizations to effectively prioritize their remediation efforts. One widely adopted framework for quantifying the risk a vulnerability poses is the Common Vulnerability Scoring System (CVSS). Recently, the CVSS 4.0 specification and calculator were released, bringing significant enhancements to the process of vulnerability assessment and risk management. In this blog post, we will explore what CVSS is and highlight the key changes in version 4.0.

What is CVSS?

CVSS, or the Common Vulnerability Scoring System, is a framework designed to measure and communicate the severity of software vulnerabilities. It consists of four metric groups: Base, Temporal, Environmental, and Supplemental. The Base group represents the intrinsic qualities of a vulnerability that remain constant over time and different user environments. The Temporal group captures the characteristics of a vulnerability that may change over time. The Environmental group focuses on the unique characteristics of a vulnerability in a specific user environment. Finally, the Supplemental group allows for additional information and context to be included in the scoring process.

The Base metrics form the foundation of CVSS scoring and produce a score ranging from 0 to 10, indicating the severity of the vulnerability. This score can be further modified by incorporating Temporal and Environmental metrics, providing a more tailored assessment based on specific circumstances. To represent the CVSS score, a vector string is used, which is a compressed textual representation of the values used to derive the score. Read more about CVSS in the official specification document.

Changes in CVSS Version 4.0

CVSS version 4.0 brings several important changes that aim to enhance the standard and provide clearer guidance for vulnerability analysts. Let's explore some of the key changes:

CVSS Nomenclature: The introduction of a new nomenclature system allows for better communication of the metrics used in generating numerical CVSS scores. The nomenclature includes abbreviations such as CVSS-B (Base metrics), CVSS-BE (Base and Environmental metrics), CVSS-BT (Base and Threat metrics), and CVSS-BTE (Base, Threat, and Environmental metrics). This standardized nomenclature ensures that the meaning of a numerical CVSS score is understood in relation to the metrics employed.

CVSS Base Score (CVSS-B) Measures Severity, not Risk: The updated CVSS specification emphasizes that the CVSS Base Score should be used to measure the severity of a vulnerability and not as the sole indicator of risk. The CVSS Base Score focuses solely on the intrinsic characteristics of the vulnerability and is independent of external factors such as the threat landscape or the specific computing environment. To assess risk accurately, the Base Score should be supplemented with an analysis of the environment, leveraging CVSS Threat and Environmental Metrics. This comprehensive approach, using the resulting CVSS-BTE score, provides a more accurate representation of the risk associated with a vulnerability.

Changes to Scoring Guidance: The CVSS Specification Document and User Guide have been updated to provide additional guidance for scoring vulnerabilities, addressing previous ambiguities. Some of the notable changes include:

1. Scope Removed: The concept of Scope has been replaced with the concepts of a vulnerable system (VC, VI, VA) and a subsequent system (SC, SI, SA), where C, I, and A stand for confidentiality, integrity, and availability). This change captures the impacts from both systems where relevant.
2. Scoring Vulnerabilities in Software Libraries (and Similar): New guidance has been introduced to help analysts score the impact of vulnerabilities in libraries or similar components. This provides a more comprehensive approach to vulnerability assessment.
3. Multiple CVSS Base Scores: The updated guidance explicitly allows for multiple CVSS Base Scores to be generated for vulnerabilities that affect multiple product versions, platforms, or operating systems. This flexibility enables a more accurate representation of the impact across various environments.
4. Environmental Security Requirements Metrics: The Environmental Metric Group now includes three Security Requirement metrics: Confidentiality Requirement (CR), Integrity Requirement (IR), and Availability Requirement (AR) of the vulnerable system. The updated guidance offers examples and explanations on how to use these metrics effectively.
5. The CVSS Extensions Framework: CVSS 4.0 introduces a framework for extending CVSS with additional metrics and metric groups. This allows for industry-specific scoring factors to be included while retaining the official Base, Threat, and Environmental Metrics. It promotes flexibility and customization in vulnerability scoring.

Read more about the changes to scoring guidance in the official user guide.

A New Calculator: You can access the new calculator on the Forum of Incident Response and Security Teams (FIRST) website.

Conclusion:

CVSS version 4.0 represents an advancement in vulnerability severity assessment by enabling more accurate risk assessment and prioritization. The changes in CVSS 4.0 ensure that vulnerability analysts can better evaluate and communicate the impact of software vulnerabilities, leading to more effective risk mitigation and incident response strategies.

Remember, CVSS is a valuable tool, but it should be used in conjunction with other risk assessment techniques and the expertise of cybersecurity professionals. Regularly reviewing and updating your vulnerability management processes based on the latest industry standards and best practices is essential to maintaining a robust security posture.