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FIRST EPSS documentation page.
Screenshot: FIRST, EPSS page defining EPSS as the probability that a published CVE will be exploited in the wild in the next 30 days (fair use).

Detection

Did You Know: EPSS Estimates Exploitation Probability, Not Severity

FIRST's Exploit Prediction Scoring System estimates the probability that a published CVE will be exploited in the wild in the next 30 days, while the CVSS v4.0 User Guide says CVSS Base scores measure severity and should not be used alone to assess risk. The two answer different questions, and multiplying one by the other produces no meaningful risk score.

Ava Okello, DetectionLondon3 min read

LONDON - Exploit Prediction Scoring System scores and Common Vulnerability Scoring System scores answer different questions. On the public FIRST EPSS site, FIRST defines EPSS as a data-driven machine-learning model that estimates the probability that a published CVE will be exploited in the wild in the next 30 days. Scores are published daily as a calibrated probability from 0 to 1, with ranking percentiles. On FIRST's Using EPSS page, the project states the limit in plain language: the EPSS score is a calibrated probability of observed exploitation activity across EPSS data partners. It is not a severity score and not a complete risk score.

FIRST Using EPSS page.
Screenshot: FIRST, Using EPSS page stating the score is a calibrated probability, not a severity score and not a complete risk score (fair use).

CVSS is FIRST's open framework for communicating the characteristics and severity of software vulnerabilities. The CVSS v4.0 User Guide states that CVSS Base (CVSS-B) scores measure the intrinsic severity of a vulnerability and should not be used alone to assess risk. The Base score is independent of threat factors and of the computing environment where the vulnerable system sits. FIRST's EPSS FAQ makes the same split from the other side: CVSS and EPSS measure different things, and multiplying an EPSS probability by a CVSS ordinal does not produce a meaningful combined risk score. A Critical CVSS rating with a low EPSS score can both be correct at once. The flaw may be severe if exploited, while current threat signals still make near-term observed exploitation unlikely.

FIRST CVSS v4.0 User Guide page.
Screenshot: FIRST, CVSS v4.0 User Guide, including the note that CVSS Base scores measure severity and should not be used alone to assess risk (fair use).
SOCtember diagram contrasting EPSS exploitation probability with CVSS severity.
Illustration: SOCtember, EPSS exploitation probability versus CVSS severity, paraphrased from FIRST EPSS and CVSS v4.0 guidance.

For Detection and vulnerability triage desks, the operational takeaway is practical. Use CVSS to understand how bad a vulnerability is on its own terms. Use EPSS as a forward-looking likelihood input for the next 30 days. Then layer local context FIRST recommends when converting a population-level EPSS score into an environment decision: presence in your estate, reachability, and consequence. When a CVE also appears on a confirmed-exploitation list such as CISA KEV, FIRST's Using EPSS guidance says treat that confirmation as precedence over the EPSS number. Do not treat EPSS as a severity label, and do not treat CVSS-B alone as complete risk.

SOCtember diagram of prioritizing with CVSS-B, EPSS, and local context.
Illustration: SOCtember, prioritizing with both signals plus local context, paraphrased from FIRST Using EPSS and the CVSS v4.0 User Guide.

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Ava Okello covers detection engineering and SOC telemetry for SOCtember from London.

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