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.
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:
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)
Temporal Score Metrics
Reflect characteristics that change over time
Consider exploit code maturity, remediation level, and report confidence
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
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
Probability-Based Approach
Provides a probability score between 0 and 1
Based on real-world exploitation data
Updated daily to reflect current threat landscape
Machine Learning Foundation
Uses various data points and features to make predictions
Considers factors like vulnerability characteristics, social media mentions, and exploit availability
Dynamic Nature
Scores change based on new data and observations
Reflects real-world exploitation patterns
Comparison Table: CVSS vs 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
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