EPSS Score (Exploit Prediction Scoring System)

TL;DR: EPSS Score

The Exploit Prediction Scoring System (EPSS) is a data-driven standard that estimates the likelihood that a specific software vulnerability will be exploited in the wild.

This process will help you:

• Prioritize what to fix first based on real-world threat data.
• Reduce alert fatigue by ignoring high-severity vulnerabilities that attackers are not actually targeting.
• Optimize security resources by focusing on the 5% of vulnerabilities that pose a true risk.

The goal of EPSS is to tell you how likely a vulnerability is to be attacked, not just how damaging the attack would be.

What Is the EPSS Score

The EPSS Score is a metric between 0 and 1 (or 0% to 100%) that represents the probability that a specific vulnerability (CVE) will be exploited within the next 30 days.

It is managed by the Forum of Incident Response and Security Teams (FIRST), the same organization that manages CVSS. While CVSS measures the severity of a vulnerability (how bad it is), EPSS measures the threat (how likely it is to happen).

In simple terms :

CVSS tells you, “This window is broken, and it is a big window.”EPSS tells you, “There is a burglar standing right outside that specific window.”

Why EPSS Matters

Security teams are drowning in “Critical” alerts. A typical enterprise scan might show thousands of vulnerabilities with a CVSS score of 9.0 or higher. It is impossible to fix them all immediately.

So why is EPSS important :

**CVSS is not enough.**Research shows that less than 5% of all published CVEs are ever exploited in the wild. If you fix vulnerabilities based only on CVSS severity, you are wasting time fixing bugs that no one is attacking.

**Real-world prioritization.**EPSS uses current threat intelligence. A vulnerability might look dangerous on paper (High CVSS), but if no exploit code exists and no attackers are using it, the EPSS score will be low.

**Efficiency.**By filtering for High EPSS scores, teams can reduce their remediation backlog by up to 85% while still addressing the most dangerous threats.

How EPSS Works

EPSS is not a static number. It is a machine learning model that updates daily. It analyzes massive amounts of data to generate a probability score.

1. Data Collection

The model ingests data from multiple sources:

• CVE Lists: MITRE and NVD data.
• Exploit Code: Availability of exploit scripts in tools like Metasploit or ExploitDB.
• Wild Activity: Logs from firewalls, IDSs, and honeypots showing active attacks.
• Dark Web Chatter: Discussions on hacker forums.

2. Probability Calculation

The model calculates a score from 0.00 (0%) to 1.00 (100%).

• 0.95 means there is a 95% chance this vulnerability is being exploited right now or will be soon.
• 0.01 means it is highly unlikely to be exploited.

3. Application

Security tools ingest this score to sort vulnerability lists. Instead of sorting by “Severity,” you sort by “Probability of Attack.”

Example in Practice

Imagine your scanner finds two vulnerabilities.

Vulnerability A:

• CVSS: 9.8 (Critical)
• EPSS: 0.02 (2%)
• Context: It is a theoretical overflow in a library you use, but no one has figured out how to weaponize it yet.

Vulnerability B:

• CVSS: 7.5 (High)
• EPSS: 0.96 (96%)
• Context: This is the Log4j vulnerability or a known VPN bypass that ransomware gangs are actively using today.

**Without EPSS:**You might fix Vulnerability A first because 9.8 > 7.5.

With EPSS (using Plexicus):

1. You navigate to the Plexicus Dashboard.
2. You filter findings by EPSS > 0.5.
3. Plexicus highlights Vulnerability B immediately.
4. You patch Vulnerability B first because it is an immediate threat. Vulnerability A goes into the backlog.

Result: You stopped an active attack vector instead of patching a theoretical bug.

Who Uses EPSS

• Vulnerability Managers - to decide which patches to push to production this week.
• Threat Intelligence Analysts - to understand the current threat landscape.
• CISOs - to justify budget and resource allocation based on risk rather than fear.
• DevSecOps Teams - to automate breaking builds only for vulnerabilities that matter.

When to Apply EPSS

EPSS should be used during the Triage and Remediation phase of vulnerability management.

• During Triage - When you have 500 critical bugs and only time to fix 50.
• In Policy - Set rules like “Patch anything with EPSS > 50% within 24 hours.”
• In Reporting - Show leadership that you are reducing “Exploitable Risk,” not just closing tickets.

Key Capabilities of EPSS Tools

Tools that integrate EPSS typically provide:

• Dual Scoring: Displaying CVSS and EPSS side-by-side.
• Dynamic Prioritization: Re-ranking vulnerabilities daily as EPSS scores change.
• Risk Acceptance: Safely marking low-EPSS vulnerabilities as “Accept Risk” for a set period.
• Rich Context: Linking the score to the specific exploit families (e.g., “Used by Ransomware Group X”).

Example tools: Vulnerability management platforms and Plexicus ASPM,** which uses EPSS to filter out noise from code scans.

Best Practices for EPSS

• Combine CVSS and EPSS: Do not ignore CVSS. The “Holy Grail” of prioritization is High CVSS + High EPSS.
• Set Thresholds: Define what “High” means for your org. Many teams start prioritizing at EPSS > 0.1 (10%) because the average score is very low.
• Automate: Use APIs to pull EPSS scores into your ticketing system (Jira).
• Review Daily: EPSS scores change. A vulnerability with a 0.01 score today could jump to 0.80 tomorrow if a Proof of Concept (PoC) is published on Twitter.

Related Terms

• CVSS (Common Vulnerability Scoring System)
• Vulnerability Management
• CVE (Common Vulnerabilities and Exposures)
• Zero-Day Exploit