# Understanding Vulnerability Scoring Systems: CVSS and 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.

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### 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
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#### 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
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### 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  |