Implementing EPSS Score for Vulnerability Prioritization

Overview

The Exploit Prediction Scoring System (EPSS) is a data-driven model developed by FIRST (Forum of Incident Response and Security Teams) that estimates the probability of a CVE being exploited in the wild within the next 30 days. EPSS produces scores from 0.0 to 1.0 (0% to 100%) using machine learning trained on real-world exploitation data. Unlike CVSS which measures severity, EPSS measures likelihood of exploitation, making it essential for risk-based vulnerability prioritization.

When to Use

• When deploying or configuring implementing epss score for vulnerability prioritization capabilities in your environment
• When establishing security controls aligned to compliance requirements
• When building or improving security architecture for this domain
• When conducting security assessments that require this implementation

Prerequisites

• Python 3.9+ with requests, pandas, matplotlib
• Access to FIRST EPSS API (https://api.first.org/data/v1/epss)
• Vulnerability scan results with CVE identifiers
• Optional: NVD API key for CVSS enrichment

EPSS API Usage

Query Single CVE

# Get EPSS score for a specific CVE
curl -s "https://api.first.org/data/v1/epss?cve=CVE-2024-3400" | python3 -m json.tool

# Response:
# {
#   "status": "OK",
#   "status-code": 200,
#   "version": "1.0",
#   "total": 1,
#   "data": [
#     {
#       "cve": "CVE-2024-3400",
#       "epss": "0.95732",
#       "percentile": "0.99721",
#       "date": "2024-04-15"
#     }
#   ]
# }

Query Multiple CVEs

# Batch query up to 100 CVEs
curl -s "https://api.first.org/data/v1/epss?cve=CVE-2024-3400,CVE-2024-21887,CVE-2023-44228" | \
  python3 -c "
import sys, json
data = json.load(sys.stdin)
for item in data['data']:
    pct = float(item['epss']) * 100
    print(f\"{item['cve']}: {pct:.2f}% exploitation probability (percentile: {item['percentile']})\")
"

Download Full EPSS Dataset

# Download complete daily EPSS scores (CSV format)
curl -s "https://epss.cyentia.com/epss_scores-current.csv.gz" | gunzip > epss_scores_current.csv

# Check size and preview
wc -l epss_scores_current.csv
head -5 epss_scores_current.csv

Query Historical EPSS Scores

# Get EPSS score for a specific date
curl -s "https://api.first.org/data/v1/epss?cve=CVE-2024-3400&date=2024-04-12"

# Get time series data
curl -s "https://api.first.org/data/v1/epss?cve=CVE-2024-3400&scope=time-series"

Prioritization Strategy

EPSS + CVSS Combined Approach

EPSS Percentile Thresholds

• Top 1% (percentile >= 0.99): Extremely likely to be exploited; treat as Critical
• Top 5% (percentile >= 0.95): High exploitation probability; prioritize remediation
• Top 10% (percentile >= 0.90): Elevated risk; schedule for near-term remediation
• Bottom 50%: Low exploitation probability; handle in normal patch cycle

Implementation

import requests
import pandas as pd
from datetime import datetime

def fetch_epss_scores(cve_list):
    """Fetch EPSS scores for a list of CVEs from FIRST API."""
    scores = {}
    batch_size = 100
    for i in range(0, len(cve_list), batch_size):
        batch = cve_list[i:i + batch_size]
        resp = requests.get(
            "https://api.first.org/data/v1/epss",
            params={"cve": ",".join(batch)},
            timeout=30
        )
        if resp.status_code == 200:
            for entry in resp.json().get("data", []):
                scores[entry["cve"]] = {
                    "epss": float(entry["epss"]),
                    "percentile": float(entry["percentile"]),
                    "date": entry.get("date", ""),
                }
    return scores

def prioritize_vulnerabilities(scan_results_csv, output_csv):
    """Enrich scan results with EPSS scores and assign priorities."""
    df = pd.read_csv(scan_results_csv)
    cve_list = df["cve_id"].dropna().unique().tolist()

    epss_data = fetch_epss_scores(cve_list)


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