Understanding Vulnerability Scoring Systems: CVSS and EPSS

This guide provides a comprehensive comparison between two major vulnerability scoring systems: the Common Vulnerability Scoring System (CVSS) and the Exploit Prediction Scoring System (EPSS).

CVSS and EPSS

In today's rapidly evolving cybersecurity landscape, effectively prioritizing vulnerability remediation has become more critical than ever. Security teams face an overwhelming number of vulnerabilities across their systems, making it crucial to have reliable methods for assessing which ones pose the greatest risk and require immediate attention.

For many years, the Common Vulnerability Scoring System (CVSS) has served as the primary framework for evaluating vulnerability severity. However, the emergence of the Exploit Prediction Scoring System (EPSS) represents a significant shift in how we approach vulnerability prioritization. While CVSS focuses on the theoretical severity of vulnerabilities, EPSS takes a data-driven approach to predict the likelihood of actual exploitation in the wild.

This guide explores both scoring systems, their methodologies, and how they complement each other in modern vulnerability management practices. Whether you're a security practitioner, manager, or stakeholder, understanding these systems is essential for making informed decisions about vulnerability prioritization and resource allocation.

CVSS

Common Vulnerability Scoring System (CVSS)

CVSS is a framework for assessing the severity of computer system security vulnerabilities. It aims to provide a standardized method for rating vulnerabilities.

Key Components of CVSS

CVSS consists of three metric groups:

1. Base Score Metrics

   • Assess the intrinsic characteristics of a vulnerability

   • Include exploitability metrics (attack vector, complexity, privileges required, user interaction)

   • Consider impact metrics (confidentiality, integrity, availability)

2. Temporal Score Metrics

   • Reflect characteristics that change over time

   • Consider exploit code maturity, remediation level, and report confidence

3. Environmental Score Metrics

   • Allow for context-specific adjustments

   • Account for the security requirements of your implementation

CVSS Scoring Scale

• Scores range from 0.0 to 10.0

• Critical: 9.0-10.0

• High: 7.0-8.9

• Medium: 4.0-6.9

• Low: 0.1-3.9

• None: 0.0

EPSS

Exploit Prediction Scoring System (EPSS)

EPSS is a data-driven effort for estimating the probability that a software vulnerability will be exploited in the wild. Unlike CVSS, which measures severity, EPSS predicts the likelihood of exploitation.

Key Features of EPSS

1. Probability-Based Approach

   • Provides a probability score between 0 and 1

   • Based on real-world exploitation data

   • Updated daily to reflect current threat landscape

2. Machine Learning Foundation

   • Uses various data points and features to make predictions

   • Considers factors like vulnerability characteristics, social media mentions, and exploit availability

3. Dynamic Nature

   • Scores change based on new data and observations

   • Reflects real-world exploitation patterns

Comparison Table: CVSS vs EPSS

Aspect	CVSS	EPSS
Primary Purpose	Measures vulnerability severity	Predicts exploitation probability
Score Range	0.0 to 10.0	0 to 1 (probability)
Update Frequency	Static (unless manually updated)	Daily
Methodology	Expert-driven framework	Data-driven machine learning
Complexity	Complex (3 metric groups, multiple sub-scores)	Simple (single probability score)
Context Consideration	Through environmental metrics	Through real-world exploitation data
Industry Maturity	Well-established (20+ years)	Emerging standard
Primary Use Case	Vulnerability severity assessment	Exploitation risk prioritization
Data Source	Theoretical assessment	Real-world exploitation data
Adaptability	Manual updates needed	Automatically adapts to new threats