EPSS (Exploit Prediction Scoring System)

What is EPSS?

Exploit Prediction Scoring System (EPSS) is a data-driven risk assessment framework designed to estimate the likelihood of a Common Vulnerabilities and Exposures (CVE) being exploited in the wild within the near future. Developed and maintained by the Forum of Incident Response and Security Teams (FIRST), EPSS uses machine learning models trained on large datasets, including real-world exploitation events, to assign a probability score to vulnerabilities. This predictive approach enables security professionals to prioritize remediation efforts more effectively by focusing on vulnerabilities that pose the greatest threat to their organizations.

What is an EPSS Score?

An EPSS score is a numerical value ranging from 0 to 1, representing the estimated probability that a given CVE will be exploited within the next 30 days. Higher scores indicate a greater likelihood of exploitation, helping security teams prioritize which vulnerabilities to address first. Unlike traditional severity-based scoring systems, EPSS is dynamic and continuously updated based on new threat intelligence and exploitation trends.

Overview of EPSS Scoring

EPSS scoring is built upon statistical models that analyze various factors, such as:

• The presence of a publicly available exploit
• The complexity of exploitation
• The popularity of the affected software
• Real-world exploit data from honeypots, intrusion detection systems, and exploit databases
• Correlations between past vulnerabilities and their likelihood of exploitation

Because it is probability-based rather than impact-based, EPSS offers a different perspective than conventional vulnerability assessment methodologies, such as the Common Vulnerability Scoring System (CVSS).

EPSS Formula

The exact formula for EPSS is proprietary and maintained by FIRST, but it is publicly documented that the model incorporates features derived from multiple data sources, including exploit availability, vulnerability metadata, and machine learning inference based on historical attack data. The model undergoes periodic updates to refine its accuracy as new exploitation trends emerge.

What is a Normal EPSS Value?

EPSS scores typically follow a distribution where most vulnerabilities have low probabilities of exploitation. While there is no definitive "normal" EPSS score, values tend to cluster towards the lower end of the scale, with a small fraction of vulnerabilities exhibiting high EPSS scores. Security teams often set internal thresholds (e.g., prioritizing vulnerabilities with EPSS scores above 0.5 or 0.75) based on their risk appetite and resource constraints.

EPSS vs. CVSS

EPSS and CVSS serve complementary purposes in vulnerability management:

• CVSS (Common Vulnerability Scoring System): Measures the severity of a vulnerability based on its technical characteristics, such as impact on confidentiality, integrity, and availability. It provides a static score that does not account for real-world exploitation likelihood.
• EPSS: Focuses on the probability of exploitation, offering a dynamic, risk-based approach to vulnerability prioritization. Unlike CVSS, which remains static once assigned, EPSS scores are updated as new threat intelligence becomes available.

CVE vs. EPSS

• CVE (Common Vulnerabilities and Exposures): A unique identifier for publicly disclosed vulnerabilities. CVE entries provide structured descriptions but do not indicate the likelihood of exploitation.
• EPSS: Provides a predictive score for a given CVE, helping security teams understand the potential risk of exploitation rather than just the existence of a vulnerability.

EPSS and Vulnerability Management

Integrating EPSS into vulnerability management programs enables organizations to move beyond traditional severity-based patching strategies. By leveraging EPSS scores, security teams can:

• Prioritize patching efforts based on real-world exploitation risks rather than severity alone
• Reduce patching workloads by focusing on vulnerabilities most likely to be exploited
• Improve threat intelligence correlation with other security tools and risk assessment frameworks

As organizations struggle with an overwhelming number of vulnerabilities, EPSS serves as a valuable tool to enhance risk-based decision-making, optimize remediation strategies, and allocate security resources more effectively.

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