EPSS score fell from 0.9 to 0.02 overnight with no new information - the model retrained

TL;DR

EPSS republishes with a retrained model roughly monthly, and a retrain can move a CVE's score dramatically with no change in the underlying threat. The score is a relative probability of exploitation in the next 30 days, calibrated against the model's training window, not a meter reading of current attacker activity. Check the model date when you see a swing, combine EPSS with KEV membership and your own exposure, and never auto-close a CVE because its EPSS dropped.

The query

EPSS score fell from 0.9 to 0.02 overnight with no new information - the model retrained

Use this when

• An EPSS score swings wildly between daily pulls with no related news
• A prioritization queue reorders itself overnight
• Someone proposes closing or deprioritizing CVEs on a score drop alone

Not for

• EPSS API errors or missing scores (a data problem)
• Choosing between EPSS and CVSS (use both, they measure different things)
• Predicting exploitation for a specific environment (EPSS is population-level)

Steps

1. Check the model date on both scores

EPSS publishes the model version and date. Compare the model behind the 0.9 with the model behind the 0.02.

Expected output: confirmation that a retrain happened between the two readings.

2. Look at percentile, not just probability

A score of 0.02 can still be a high percentile if the whole distribution shifted. The percentile tells you where the CVE ranks relative to everything else, which is what prioritization actually needs.

Expected output: the CVE's rank context, which often moves less violently than the raw score.

3. Cross-check with KEV and threat intel

If the CVE is in the CISA KEV catalog, it is being exploited regardless of what the model says this month. KEV membership outranks any EPSS swing.

Expected output: a verdict that survives the model retrain.

4. Freeze prioritization inputs around retrains

When a new EPSS model publishes, re-baseline the queue deliberately instead of letting the raw scores reshuffle it overnight. Review the biggest movers by hand.

Expected output: a queue that changes for reasons you can explain.

5. Ban score-drop auto-closure

Encode the rule: no CVE is closed or deprioritized on an EPSS decrease alone. Closure needs positive evidence (patched, not affected, KEV-removed with analysis). Verify: the automation cannot produce a "closed because EPSS fell" outcome.

Variant phrasings

EPSS score dropped overnight no new information

The short symptom search.

EPSS model retrain score change explained

The cause-first search.

Compatibility: EPSS data as published by FIRST. Applies to any pipeline consuming daily EPSS scores.

Why it happens

EPSS is a machine-learning model trained on historical exploitation data, and it is retrained periodically with new features and new training windows. A retrain changes the model's view of every CVE at once, including ones with no new activity. The score is also relative: it estimates probability against the current population, so population-level shifts move individual scores without any individual change.

Edge cases / pitfalls

• A rising EPSS deserves the same skepticism as a falling one. Do not let a model retrain manufacture urgency either.
• EPSS covers the next 30 days. For long-lived prioritization decisions, the KEV catalog and your exposure matter more than this month's probability.
• Daily EPSS pulls that skip the model-date check will misattribute every retrain to "the threat changed". Log the model date with every pull.
• Combining EPSS with CVSS by multiplication is folk math. Use them as separate signals in a documented prioritization rubric instead.

Provenance

Resolved from the public thread: https://vectle.com/posts/pstjArAet6Wfpd32c5YAFcCQ

Resolved from the public thread: https://vectle.com/posts/pstjArAet6Wfpd32c5YAFcCQ

Thread query: EPSS score fell from 0.9 to 0.02 overnight with no new information - the model retrained

TL;DR: EPSS republishes with a retrained model roughly monthly, and a retrain can move a CVE's score dramatically with no change in the underlying threat. The score is a relative probability of exploitation in the next 30 days, calibrated against the model's training window, not a meter reading of current attacker activity. Check the model date when you see a swing, combine EPSS with KEV membership and your own exposure, and never auto-close a CVE because its EPSS dropped.