EPSS vs CVSS: How to Prioritize Vulnerabilities in Practice

Security teams face a constant stream of vulnerability alerts, each with its own risk implications. Two scoring approaches stand out for helping teams translate those alerts into actionable remediation plans: the Exploit Prediction Scoring System (EPSS) and the Common Vulnerability Scoring System (CVSS). While CVSS has long been the industry standard for measuring how severe a vulnerability could be, EPSS adds a probabilistic view of likelihood—that a vulnerability will be exploited in the wild within a given timeframe. Used together, these tools provide a more complete picture of risk and enable smarter prioritization decisions.

What CVSS Measures

CVSS, short for Common Vulnerability Scoring System, is a framework designed to communicate the severity of software vulnerabilities. It is widely adopted across industries and products because it provides a consistent, reproducible score that helps teams compare vulnerabilities. CVSS scores come in three layers: Base, Temporal, and Environmental. The Base score captures the intrinsic characteristics of a vulnerability, while Temporal and Environmental scores adjust that assessment for changing circumstances and specific environments.

The Base score is built from a set of metrics that describe how the exploit works and what impact it could have. Key components include:

• Attack Vector (Network, Adjacent, Local, Physical)
• Attack Complexity
• Privileges Required
• User Interaction required
• Scope (whether a vulnerability changes the scope of access)
• Impact on Confidentiality, Integrity, and Availability

Scores range from 0.0 to 10.0. Higher scores indicate greater potential harm. In practice, CVSS is often used to categorize vulnerabilities into severity levels such as Low, Medium, High, and Critical. However, CVSS is not a probabilistic forecast; it reflects potential impact and ease of exploitation rather than the actual likelihood of exploitation in the real world. This distinction matters for mature risk management, as a vulnerability with a high base score might see little real-world exploitation, while another with a moderate score could be widely exploited due to widespread exposure or weaponization.

What EPSS Measures

EPSS stands for Exploit Prediction Scoring System. Unlike CVSS, EPSS is explicitly focused on the probability that a vulnerability will be exploited in the wild within a defined future window—typically the next 12 months. EPSS combines historical exploitation data, exploit trends, vendor advisories, exploit availability, and other telemetry to estimate likelihood. The result is a probability between 0 and 1 that a given vulnerability will be exploited within the target period.

The purpose of EPSS is to inform prioritization decisions by emphasizing exploitability probability rather than intrinsic impact alone. A vulnerability with a lower CVSS base score but a high EPSS probability might warrant urgent remediation because attackers are actively exploiting similar flaws in the current landscape. Conversely, a vulnerability with a high CVSS score but a low EPSS probability might be deprioritized temporarily, if the threat landscape does not indicate active exploitation.

Key Differences Between EPSS and CVSS

Understanding the core differences helps security teams use both systems effectively. Here are the main points of contrast:

• What they measure: CVSS measures potential severity and exploitability; EPSS measures real-world exploitation likelihood in the near term.
• Time horizon: CVSS is static or can be updated with Temporal and Environmental scores, but it does not inherently forecast exploitation by time horizon. EPSS is time-bound, focused on the next 12 months.
• Data inputs: CVSS relies on vulnerability characteristics and potential impact; EPSS relies on historical exploitation data, telemetry, and trend signals.
• Usage in prioritization: CVSS informs severity and risk appetite; EPSS informs urgency based on probability of exploitation. Together, they enable risk-based prioritization that aligns with both impact and likelihood.

Using EPSS and CVSS Together: A Practical Approach

For most organizations, the most effective vulnerability prioritization combines CVSS-based impact with EPSS-based exploitation likelihood. A practical workflow might look like this:

1. Collect CVSS base scores for all known vulnerabilities from trusted sources (for example, CVSS v3.1 data in national or vendor advisories).
2. Pull EPSS probabilities for the same vulnerabilities from a reputable EPSS feed or internal telemetry.
3. Normalize both inputs to a consistent scale and map them to your risk model. A common approach is to treat CVSS as a proxy for impact and EPSS as the likelihood of exploitation.
4. Compute a composite risk indicator. One simple method is to multiply a mapped CVSS-based impact score by the EPSS probability to produce an Expected Risk Score. Note that organizations should tailor the calculation to their risk tolerance and patching cadence.
5. Rank vulnerabilities by the composite score and align remediation SLAs with business criticality, asset importance, and exposure (e.g., internet-facing systems get accelerated remediation).
6. Review and adjust regularly. As CVSS and EPSS data evolve, re-prioritize the queue to reflect new exploitation trends and changing configurations.

Illustrative example: Vulnerability A has a CVSS base score of 9.2 (high impact) and an EPSS probability of 0.85, while Vulnerability B has a CVSS base score of 6.5 and an EPSS probability of 0.15. The composite risk score would typically favor Vulnerability A, assuming the environment justifies prioritizing high-impact, high-likelihood exploits.

Practical Steps for Teams

• Automate data feeds: Integrate CVSS data from standard catalogs and EPSS probabilities into your vulnerability management platform to ensure timely updates.
• Contextualize with environment: Use Environmental CVSS scores when possible to reflect asset-specific exposure, compensating controls, and deployment context.
• Define thresholds and SLAs: Establish clear thresholds for urgent remediation (for example, treat vulnerabilities with high CVSS and high EPSS as critical) and set realistic patching timelines based on asset criticality.
• Include risk-aligned remediation plans: Pair remediation with functional risk controls (e.g., compensating controls, network segmentation) when patching is not immediately feasible.
• Monitor over time: Track remediation progress against the evolving risk picture, not just the count of open vulnerabilities.

Common Pitfalls to Avoid

• Relying on CVSS alone: A high CVSS score does not necessarily imply immediate exploitation risk if EPSS is very low due to lack of active exploitation.
• Ignoring data freshness: Outdated CVSS or EPSS data can mislead prioritization; ensure feeds are current and synchronized with your asset inventory.
• Overcomplicating the model: An overly complex integration can hinder timely responses. Start with a simple, repeatable workflow and iterate.
• Neglecting context: Asset criticality, data sensitivity, and network exposure should influence prioritization alongside scores.

When to Emphasize One Score Over the Other

CVSS often remains essential for understanding the potential damage a vulnerability could cause within a given system, especially when evaluating the impact on critical assets. EPSS becomes especially valuable in fast-moving threat environments where exploitation activity is a key driver of urgency. In practice, neither score should be used in isolation. A balanced approach leverages CVSS to gauge impact and EPSS to gauge exploit probability, delivering a risk-informed path to remediation.

Conclusion

In modern vulnerability management, EPSS and CVSS are complementary tools. CVSS gives you a view of potential harm, while EPSS provides a forecast of real-world exploitation. By combining them, security teams can prioritize patches more effectively, allocate resources to the most urgent risks, and align remediation with the realities of attacker behavior. The result is a more resilient security posture that adapts to evolving threats without sacrificing clarity or agility.