CVSS Frequently Asked Questions

What is CVSS?

A: CVSS stands for The Common Vulnerability Scoring System and is a vendor agnostic, industry open standard designed to convey vulnerability severity and help determine urgency and priority of response. It solves the problem of multiple, incompatible scoring systems and is usable and understandable by anyone.

Who developed CVSS?

A: CVSS was commissioned by the National Infrastructure Advisory Council (NIAC) tasked in support of the global Vulnerability Disclosure Framework. It is currently maintained by FIRST (Forum of Incident Response and Security Teams) http://www.first.org/. CVSS was a joint effort involving many groups including:

• CERT/CC
• Cisco
• DHS/MITRE
• eBay
• IBM Internet Security Systems
• Microsoft
• Qualys
• Symantec

Since the original release of CVSS, additional groups have joined the CVSS effort and assisted in developing version 2 of CVSS. The current list of major participants is available at www.first.org/cvss/team.

What does CVSS not do?

A: CVSS is not a threat scoring system (DHS color warning system), a vulnerability database or a real-time attack scoring system.

What is involved in CVSS?

A: The CVSS model is designed to provide the end user with an overall composite score representing the severity and risk of a vulnerability. It is derived from metrics and formulas. The metrics are in three distinct categories that can be quantitatively or qualitatively measured. Base Metrics contain qualities that are intrinsic to any given vulnerability that do not change over time or in different environments. Temporal Metrics contain characteristics of a vulnerability which evolve over the lifetime of vulnerability. Environmental Metrics contain those characteristics of a vulnerability which are tied to an implementation in a specific user’s environment.

What is the current version of CVSS?

A: The current version of CVSS is version 2. It was finalized and released to the public in June 2007. This FAQ addresses CVSS version 2 only, although there are many similarities between versions 1 and 2. Information on CVSS version 1 is available from the NIAC Paper on CVSS at http://www.first.org/cvss/cvss-dhs-12-02-04.pdf.

What are the details of the Base Metrics?

A:There are six Base Metrics which represent the most fundamental, immutable qualities of a vulnerability.

a) Access Vector measures how remote an attacker can be to attack a target.

• Local: Exploiting the vulnerability requires either physical access to the target or a local (shell) account on the target.
• Adjacent Network: Exploiting the vulnerability requires access to the local network of the target.
• Network: The vulnerability is exploitable from remote networks.

b) Access Complexity measures the complexity of attack required to exploit the vulnerability once an attacker has gained access to the target system.

• High: Specialized access conditions exist, such as a specific window of time (a race condition with a very narrow window), specific circumstance (a configuration rarely seen in practice), or social engineering methods that would be easily detected by knowledgeable people.
• Medium: Somewhat specialized access conditions exist, such as a non-default configuration that is not commonly used or social engineering methods that might occasionally fool cautious users.
• Low: Specialized access conditions or extenuating circumstances do not exist. In other words, it is usually or always exploitable. This is the most common case.

c) Authentication measures the number of times an attacker must authenticate to the target system in order to exploit the vulnerability.

• Multiple: Two or more instances of authentication are required to exploit the vulnerability, even if the same credentials are used each time.
• Single: One instance of authentication is required to exploit the vulnerability.
• None: Authentication is not required to exploit the vulnerability.

d) Confidentiality Impact measures the impact on confidentiality of a successful exploit of the vulnerability on the target system.

• None: No impact on confidentiality.
• Partial: Considerable informational disclosure.
• Complete: Total information disclosure.

e) Integrity Impact measures the impact on integrity of a successful exploit of the vulnerability on the target system.

• None: No impact on integrity.
• Partial: Considerable breach in integrity.
• Complete: Total compromise of system integrity.

f) Availability Impact measures the impact on availability of a successful exploit of the vulnerability on the target system.

• None: No impact on availability.
• Partial: Reduced performance or interruptions in resource availability.
• Complete: Total shutdown of the affected resource.

What are the details of the Temporal Metrics?

A: There are three Temporal Metrics which represent the time dependent qualities of a vulnerability.

a) Exploitability measures how complex the process is to exploit the vulnerability in the target system.

• Unproven: No exploit code is yet available
• Proof of Concept: Proof of concept exploit code is available
• Functional: Functional exploit code is available
• High: Exploitable by functional mobile autonomous code or no exploit required (manual trigger)

b) Remediation Level measures the level of an available solution.

• Official Fix: Complete vendor solution available
• Temporary Fix: There is an official temporary fix available
• Workaround: There is an unofficial non-vendor solution available
• Unavailable: There is either no solution available or it is impossible to apply

c) Report Confidence measures the degree of confidence in the existence of the vulnerability and the credibility of its report.

• Unconfirmed: A single unconfirmed source or possibly multiple conflicting reports
• Uncorroborated: Multiple non-official sources; possibly including independent security companies or research organizations
• Confirmed: Vendor has reported/confirmed a problem with its own product, or an external event such as widespread exploitation confirms the existence of the problem

What are the details of the Environment Metrics?

A: There are three Environmental Metrics which represent the implementation and environment specific qualities of a vulnerability.

a) Collateral Damage Potential measures the potential for a loss of life or physical assets through damage or theft of property or equipment.

• None: There is no potential for loss of life, physical assets, productivity or revenue.
• Low: A successful exploit of this vulnerability may result in slight physical or property damage, or slight loss of revenue or productivity.
• Low-Medium: A successful exploit of this vulnerability may result in moderate physical or property damage, or moderate loss of revenue or productivity.
• Medium-High: A successful exploit of this vulnerability may result in significant physical or property damage or loss, or significant loss of revenue or productivity.
• High: A successful exploit of this vulnerability may result in catastrophic physical or property damage and loss, or catastrophic loss of revenue or productivity.

b) Target Distribution measures the percentage of vulnerable systems.

• None: No target systems exist, or targets are so highly specialized that they only exist in a laboratory setting (effectively 0% of the environment is at risk).
• Low: Targets exist inside the environment, but on a small scale (between 1% - 25% of the total environment is at risk).
• Medium: Targets exist inside the environment, but on a medium scale (between 26% - 75% of the total environment is at risk).
• High: Targets exist inside the environment on a considerable scale (between 76% - 100% of the total environment is at risk).

c) Security Requirements allows a score to be customized depending on the criticality of the affected IT asset, such as giving greater weight to availability if an asset supports a business function for which availability is most important. The Security Requirements consist of three metrics: confidentiality, integrity, and availability. The possible values for each metric are:

• Low: Loss of [confidentiality|integrity|availability] is likely to have only a limited adverse effect on the organization or individuals associated with the organization (e.g., employees, customers).
• Medium: Loss of [confidentiality|integrity|availability] is likely to have a serious adverse effect on the organization or individuals associated with the organization.
• High: Loss of [confidentiality|integrity|availability] is likely to have a catastrophic adverse effect on the organization or individuals associated with the organization.

How is the scoring done?

A: Scoring is the process of combining all the metric values according to specific formulas. Base Scoring is computed by the vendor or originator with the intention of being published and once set, is not expected to change. It is computed from “the big three” confidentiality, integrity and availability. This is the “foundation” which is modified by the Temporal and Environmental metrics. The base score has the largest bearing on the final score and represents vulnerability severity.

Temporal Scoring is also computed by vendors and coordinators for publication, and modifies the Base score. It allows for the introduction of mitigating factors to reduce the score of a vulnerability and is designed to be re-evaluated at specific intervals as a vulnerability ages. The temporal score represents vulnerability urgency at specific points in time.

Environmental Scoring is optionally computed by end-user organizations and adjusts combined Base-Temporal score. This should be considered the FINAL score and represents a snapshot in time, tailored to a specific environment. User organizations should use this to prioritize responses within their own environments.

Is there an easier way to understand all this?

A: Yes. This flowchart shows each metric group and how they interrelate with each other.

Where can I get the hardcore details of the scoring formulas?

A: Full details on the CVSS version 2 formulas are available from A Complete Guide to the Common Vulnerability Scoring System Version 2.0, at http://www.first.org/cvss/cvss-guide.html.

Who is using CVSS?

A: NIAC was submitted to the President in January 2005. DHS (Department of Homeland Security) and CVSS developers are encouraging widespread, voluntary adoption. Many organizations have since adopted CVSS, including several NIAC member companies (Akamai, American Water, Symantec, Union Pacific) and other organizations (CERT/CC, Cisco, HP, IBM, NIST, Oracle, Qualys, US-CERT).

I am an end-user (CISO/CSO/operations security person), is there anything I need to do?

A: Typically, application and security product vendors will provide both the Base and Temporal scores. As the end user, you need only calculate your Environmental score.

I am an application or product security vendor, why should I use CVSS and publish CVSS temporal scores?

A: As more vendors begin publishing CVSS scores, more customers will understand and appreciate the advantages. They will grow to appreciate the ability to tailor scores to their environment and begin expect CVSS scores of all their suppliers. The more it is used, the better it works.

I am an end-user, and really like other vendors scoring methods, why should I change to CVSS?

A: Other systems are closed competing standards, do not offer a mutable scoring framework, and do not consider different environments.

What does CVSS really offer that other scoring methodologies do not?

A: An open framework that can be used, understood, and improved upon by anybody to score vulnerabilities.

Where can I get CVSS scores?

A: There are many sources of CVSS scores. Several major sources of CVSS scores are posted at http://www.first.org/cvss/scores.html.

Where can I get the CVSS code?

A: CVSS is a framework that you can use to develop an application suitable to your needs, your environment or your customers. There is no established code as of yet. However, there are several CVSS calculators available; a listing of some calculators is posted at http://www.first.org/cvss/scores.html.

How can I help establish CVSS throughout the industry?

A: Urge your vendors to support CVSS scoring.

Where can I get more information on CVSS?