Skip to content
Welcome, thoughtbot's Giant Robots listeners!Start here
Common Vulnerability Scoring System
AI-generated illustration
Technology

Common Vulnerability Scoring System

Industry-standard 0-10 vulnerability severity scale; a high score alone does not mean active exploitation.

Two Check Point firewall flaws rated 9.8 out of 10 were listed as exploited by US authorities on 22 September 2026, with three days to patch: the latest top score to arrive with attacks already under way.

Last refreshed: 12 October 2026 · Appears in 1 active topic

Key Question

Is a CVSS 7.5 in active exploitation more dangerous than a 9.8 that is not?

Timeline for Common Vulnerability Scoring System

#14 22 Sept
#13 6 Aug
#6 3 Jun

Mentioned in: Magento RCE forces 9-day patch race

Cybersecurity: Threats and Defences
#4 14 May

Mentioned in: UAT-8616 keeps Cisco SD-WAN under fire

Cybersecurity: Threats and Defences
#3 6 May
View full timeline →

Background

The Common Vulnerability Scoring System (CVSS) gives software flaws a severity score from 0.0 to 10.0. CISA, the UK's NCSC, the US National Vulnerability Database and software vendors all publish it. The current version, CVSS v4.0, was published in 2023 by FIRST, the Forum of Incident Response and Security Teams, and combines base, threat and environmental metrics; most vendor advisories publish only the base score.

Scores are set at disclosure, but the risk can change afterwards. F5 rated CVE-2025-53521 a moderate outage risk, then on 28 March 2026 reclassified it as a Remote Code Execution flaw scoring 9.8, the same day CISA listed it as exploited. Citrix scored CitrixBleed 3 (CVE-2026-3055) at 9.3 on disclosure on 23 March.

A high base score does not mean attackers are using a flaw, and a lower score does not mean it is SAFE. Defenders therefore weigh CVSS alongside exploitation data and CISA's known-exploited list. The scoring work itself is exposed to budget pressure: the US FY27 budget proposed on 7 April 2026 put NIST vulnerability-scoring programmes inside its cyber cuts.

Key Issues
Patch deadlines

Exploitation now sets the patch clock

Through 2026 the highest CVSS scores often arrived with attacks already running. On 14 May US authorities gave federal agencies three days to fix a maximum-severity flaw in Cisco's network-management software that a China-linked group was already exploiting. On 22 September two Check Point flaws rated 9.8 were listed as exploited, with a 25 September deadline.

The score itself did not set the timetable. When CISA listed a critical Palo Alto Networks firewall flaw on 6 May, its 9 May Deadline fell four days before the vendor's fix existed, so agencies could only apply a workaround. In each case confirmed exploitation, not the severity number, decided how fast defenders had to move.

Common Questions
What is the difference between CVSS v3.1 and CVSS v4.0?
CVSS v4.0, published by FIRST in 2023, replaced v3.1 with an expanded metric set covering base, threat and environmental scores. NVD now uses v4.0 as its primary scoring base. Vendor advisories often still publish only the base score, omitting the threat metrics that reflect real-world exploitation conditions.Source: FIRST
What is the CVSS score system and how does it work?
CVSS (Common Vulnerability Scoring System) is a standard framework for rating software vulnerability severity on a scale of 0 to 10. CVSS v4.0 is the current version, used by NIST's National Vulnerability Database and CISA to classify vulnerabilities for patch prioritisation.Source: FIRST / NIST
Can CVSS scores change after a vulnerability is disclosed?
Yes. Vendors can reclassify vulnerabilities after initial disclosure if exploitation evidence changes the assessed risk. F5's CVE-2025-53521 was reclassified from medium DOS to CVSS 9.8 RCE in March 2026 after active exploitation was confirmed.Source: F5 / CISA
Why do security teams not just patch the highest CVSS scores first?
CVSS scores are assigned at disclosure from theoretical analysis and can understate risk. A vendor may not know the true exploitation class until attackers demonstrate it, as happened with F5 CVE-2025-53521 (reclassified to 9.8 RCE months after a medium DOS initial rating). CISA KEV membership and active threat-feed data are required alongside CVSS to produce accurate prioritisation.Source: CISA / F5
Does a CVSS score of 10 mean a vulnerability is being actively exploited?
No. A CVSS score of 10.0 reflects worst-case theoretical severity, not confirmed attack activity. In June 2026, a 9.8-scored Magento flaw drew heavy attention while a lower-scored 7.5 Oracle WebLogic bug was already delivering ransomware payloads in the wild.Source: CISA / Sansec