Common Vulnerability Scoring System

The Common Vulnerability Scoring System (CVSS) is a free and open industry standard for assessing the severity of computer system security vulnerabilities. CVSS attempts to assign severity scores to vulnerabilities, allowing responders to prioritize responses and resources according to threat. Scores are calculated based on a formula that depends on several metrics that approximate ease of exploit and the impact of exploit. Scores range from 0 to 10, with 10 being the most severe. While many utilize only the CVSS Base score for determining severity, temporal and environmental scores also exist, to factor in availability of mitigations and how widespread vulnerable systems are within an organization, respectively.

The current version of CVSS (CVSSv3.1) was released in June 2019.^[1]

History

Research by the National Infrastructure Advisory Council (NIAC) in 2003/2004 led to the launch of CVSS version 1 (CVSSv1) in February 2005, with the goal of being "designed to provide open and universally standard severity ratings of software vulnerabilities". This initial draft had not been subject to peer review or review by other organizations. In April 2005, NIAC selected the Forum of Incident Response and Security Teams (FIRST) to become the custodian of CVSS for future development.^[2]

Feedback from vendors utilizing CVSSv1 in production suggested there were "significant issues with the initial draft of CVSS". Work on CVSS version 2 (CVSSv2) began in April 2005 with the final specification being launched in June 2007.^[3]

Further feedback resulted in work beginning on CVSS version 3^[4] in 2012, ending with CVSSv3.0 being released in June 2015.^[5]

Terminology

The CVSS assessment measures three areas of concern:

Base Metrics for qualities intrinsic to a vulnerability
Temporal Metrics for characteristics that evolve over the lifetime of vulnerability
Environmental Metrics for vulnerabilities that depend on a particular implementation or environment

A numerical score is generated for each of these metric groups. A vector string (or simply "vector" in CVSSv2), represents the values of all the metrics as a block of text.

Version 2

Complete documentation for CVSSv2 is available from FIRST.^[6] A summary is provided below.

Base metrics

Access Vector

The access vector (AV) shows how a vulnerability may be exploited.

Value	Description	Score
Local (L)	The attacker must either have physical access to the vulnerable system (e.g. firewire attacks) or a local account (e.g. a privilege escalation attack).	0.395
Adjacent Network (A)	The attacker must have access to the broadcast or collision domain of the vulnerable system (e.g. ARP spoofing, Bluetooth attacks).	0.646
Network (N)	The vulnerable interface is working at layer 3 or above of the OSI Network stack. These types of vulnerabilities are often described as remotely exploitable (e.g. a remote buffer overflow in a network service)	1.0

Access Complexity

The access complexity (AC) metric describes how easy or difficult it is to exploit the discovered vulnerability.

Value	Description	Score
High (H)	Specialised conditions exist, such as a race condition with a narrow window, or a requirement for social engineering methods that would be readily noticed by knowledgeable people.	0.35
Medium (M)	There are some additional requirements for the attack, such as a limit on the origin of the attack, or a requirement for the vulnerable system to be running with an uncommon, non-default configuration.	0.61
Low (L)	There are no special conditions for exploiting the vulnerability, such as when the system is available to large numbers of users, or the vulnerable configuration is ubiquitous.	0.71

Authentication

The authentication (Au) metric describes the number of times that an attacker must authenticate to a target to exploit it. It does not include (for example) authentication to a network in order to gain access. For locally exploitable vulnerabilities, this value should only be set to Single or Multiple if further authentication is required after initial access.

Value	Description	Score
Multiple (M)	Exploitation of the vulnerability requires that the attacker authenticate two or more times, even if the same credentials are used each time.	0.45
Single (S)	The attacker must authenticate once in order to exploit the vulnerability.	0.56
None (N)	There is no requirement for the attacker to authenticate.	0.704

Impact metrics

Confidentiality

The confidentiality (C) metric describes the impact on the confidentiality of data processed by the system.

Value	Description	Score
None (N)	There is no impact on the confidentiality of the system.	0.0
Partial (P)	There is considerable disclosure of information, but the scope of the loss is constrained such that not all of the data is available.	0.275
Complete (C)	There is total information disclosure, providing access to any / all data on the system. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.	0.660

Integrity

The Integrity (I) metric describes the impact on the integrity of the exploited system.

Value	Description	Score
None (N)	There is no impact on the integrity of the system.	0.0
Partial (P)	Modification of some data or system files is possible, but the scope of the modification is limited.	0.275
Complete (C)	There is total loss of integrity; the attacker can modify any files or information on the target system.	0.660

Availability

The availability (A) metric describes the impact on the availability of the target system. Attacks that consume network bandwidth, processor cycles, memory or any other resources affect the availability of a system.

Value	Description	Score
None (N)	There is no impact on the availability of the system.	0.0
Partial (P)	There is reduced performance or loss of some functionality.	0.275
Complete (C)	There is total loss of availability of the attacked resource.	0.660

Calculations

These six metrics are used to calculate the exploitability and impact sub-scores of the vulnerability. These sub-scores are used to calculate the overall base score.

${\textsf {Exploitability}}=20\times {\textsf {AccessVector}}\times {\textsf {AttackComplexity}}\times {\textsf {Authentication}}$

${\textsf {Impact}}=10.41\times (1-(1-{\textsf {ConfImpact}})\times (1-{\textsf {IntegImpact}})\times (1-{\textsf {AvailImpact}}))$

$f({\textsf {Impact}})={\begin{cases}0,&{\text{if }}{\textsf {Impact}}{\text{ = 0}}\\1.176,&{\text{otherwise }}\end{cases}}$

${\textsf {BaseScore}}={\textsf {roundTo1Decimal}}(((0.6\times {\textsf {Impact}})+(0.4\times {\textsf {Exploitability}})-1.5)\times f({\textsf {Impact}}))$

The metrics are concatenated to produce the CVSS Vector for the vulnerability.

Example

A buffer overflow vulnerability affects web server software that allows a remote user to gain partial control of the system, including the ability to cause it to shut down:

Metric	Value	Description
Access Vector	Network	The vulnerability may be accessed from any network that can access the target system - typically the whole of the internet
Access Complexity	Low	There are no special requirements for access
Authentication	None	There is no requirement for authentication in order to exploit the vulnerability
Confidentiality	Partial	The attacker can read some files and data on the system
Integrity	Partial	The attacker can alter some files and data on the system
Availability	Complete	The attacker can cause the system and web service to become unavailable / unresponsive by shutting the system down

This would give an exploitability sub-score of 10, and an impact sub-score of 8.5, giving an overall base score of 9.0. The vector for the base score in this case would be AV:N/AC:L/Au:N/C:P/I:P/A:C. The score and vector are normally presented together to allow the recipient to fully understand the nature of the vulnerability and to calculate their own environmental score if necessary.

Temporal metrics

The value of temporal metrics change over the lifetime of the vulnerability, as exploits are developed, disclosed and automated and as mitigations and fixes are made available.

Exploitability

The exploitability (E) metric describes the current state of exploitation techniques or automated exploitation code.

Value	Description	Score
Unproven (U)	No exploit code is available, or the exploit is theoretical	0.85
Proof-of-concept (P)	Proof-of-concept exploit code or demonstration attacks are available, but not practical for widespread use. Not functional against all instances of the vulnerability.	0.9
Functional (F)	Functional exploit code is available, and works in most situations where the vulnerability is present.	0.95
High (H)	The vulnerability can be exploited by automated code, including mobile code (such as a worm or virus).	1.0
Not Defined (ND)	This is a signal to ignore this score.	1.0

Remediation Level

The remediation level (RL) of a vulnerability allows the temporal score of a vulnerability to decrease as mitigations and official fixes are made available.

Value	Description	Score
Official Fix (O)	A complete vendor solution is available - either a patch or an upgrade.	0.87
Temporary Fix (T)	There is an official but temporary fix / mitigation available from the vendor.	0.90
Workaround (W)	There is an unofficial, non-vendor solution or mitigation available - perhaps developed or suggested by users of the affected product or another third party.	0.95
Unavailable (U)	There is no solution available, or it is impossible to apply a suggested solution. This is the usual initial state of the remediation level when a vulnerability is identified.	1.0
Not Defined (ND)	This is a signal to ignore this score.	1.0

Report Confidence

The report confidence (RC) of a vulnerability measures the level of confidence in the existence of the vulnerability and also the credibility of the technical details of the vulnerability.

Value	Description	Score
Unconfirmed (UC)	A single unconfirmed source, or multiple conflicting sources. Rumored vulnerability.	0.9
Uncorroborated (UR)	Multiple sources that broadly agree - there may be a level of remaining uncertainty about the vulnerability	0.95
Confirmed (C)	Acknowledged and confirmed by the vendor or manufacturer of the affected product.	1.0
Not Defined (ND)	This is a signal to ignore this score.	1.0

Calculations

These three metrics are used in conjunction with the base score that has already been calculated to produce the temporal score for the vulnerability with its associated vector.

The formula used to calculate the temporal score is:

${\textsf {TemporalScore}}={\textsf {roundTo1Decimal}}({\textsf {BaseScore}}\times {\textsf {Exploitability}}\times {\textsf {RemediationLevel}}\times {\textsf {ReportConfidence}})$

Example

To continue with the example above, if the vendor was first informed of the vulnerability by a posting of proof-of-concept code to a mailing list, the initial temporal score would be calculated using the values shown below:

Metric	Value	Description
Exploitability	Proof-of-concept	Proof-of concept, non-automated code is provided to show basic exploit functionality.
Remediation Level	Unavailable	The vendor has not yet had the opportunity to provide a mitigation or fix.
Report Confidence	Unconfirmed	There has been a single report of the vulnerability

This would give a temporal score of 7.3, with a temporal vector of E:P/RL:U/RC:UC (or a full vector of AV:N/AC:L/Au:N/C:P/I:P/A:C/E:P/RL:U/RC:UC).

If the vendor then confirms the vulnerability, the score rises to 8.1, with a temporal vector of E:P/RL:U/RC:C

A temporary fix from the vendor would reduce the score back to 7.3 (E:P/RL:T/RC:C), while an official fix would reduce it further to 7.0 (E:P/RL:O/RC:C). As it is not possible to be confident that every affected system has been fixed or patched, the temporal score cannot reduce below a certain level based on the vendor's actions, and may increase if an automated exploit for the vulnerability is developed.

Environmental metrics

The environmental metrics use the base and current temporal score to assess the severity of a vulnerability in the context of the way that the vulnerable product or software is deployed. This measure is calculated subjectively, typically by affected parties.

Collateral Damage Potential

The collateral damage potential (CDP) metric measures the potential loss or impact on either physical assets such as equipment (and lives), or the financial impact upon the affected organisation if the vulnerability is exploited.

Value	Description	Score
None (N)	No potential for loss of property, revenue or productivity	0
Low (L)	Slight damage to assets, or minor loss of revenue or productivity	0.1
Low-Medium (LM)	Moderate damage or loss	0.3
Medium-High (MH)	Significant damage or loss	0.4
High (H)	Catastrophic damage or loss	0.5
Not Defined (ND)	This is a signal to ignore this score.	0

Target Distribution

The target distribution (TD) metric measures the proportion of vulnerable systems in the environment.

Value	Description	Score
None (N)	No target systems exist, or they only exist in laboratory settings	0
Low (L)	1–25% of systems at risk	0.25
Medium (M)	26–75% of systems at risk	0.75
High (H)	76–100% of systems at risk	1.0
Not Defined (ND)	This is a signal to ignore this score.	1.0

Impact Subscore Modifier

Three further metrics assess the specific security requirements for confidentiality (CR), integrity (IR) and availability (AR), allowing the environmental score to be fine-tuned according to the users' environment.

Value	Description	Score
Low (L)	Loss of (confidentiality / integrity / availability) is likely to have only a limited effect on the organisation.	0.5
Medium (M)	Loss of (confidentiality / integrity / availability) is likely to have a serious effect on the organisation.	1.0
High (H)	Loss of (confidentiality / integrity / availability) is likely to have a catastrophic effect on the organisation.	1.51
Not Defined (ND)	This is a signal to ignore this score.	1.0

Calculations

The five environmental metrics are used in conjunction with the previously assessed base and temporal metrics to calculate the environmental score and to produce the associated environmental vector.

${\textsf {AdjustedImpact}}=\min(10,10.41\times (1-(1-{\textsf {ConfImpact}}\times {\textsf {ConfReq}})\times (1-{\textsf {IntegImpact}}\times {\textsf {IntegReq}})\times (1-{\textsf {AvailImpact}}\times {\textsf {AvailReq}})))$

${\textsf {AdjustedTemporal}}={\textsf {TemporalScore}}{\text{ recomputed with the }}{\textsf {BaseScore}}{\text{s }}{\textsf {Impact}}{\text{ sub-equation replaced with the }}{\textsf {AdjustedImpact}}{\text{ equation}}$

${\textsf {EnvironmentalScore}}={\textsf {roundTo1Decimal}}(({\textsf {AdjustedTemporal}}+(10-{\textsf {AdjustedTemporal}})\times {\textsf {CollateralDamagePotential}})\times {\textsf {TargetDistribution}})$

Example

If the aforementioned vulnerable web server were used by a bank to provide online banking services, and a temporary fix was available from the vendor, then the environmental score could be assessed as:

Metric	Value	Description
Collateral Damage Potential	Medium-High	This value would depend on what information the attacker is able to access if a vulnerable system is exploited. In this case I am assuming that some personal banking information is available, therefore there is a significant reputational impact on the bank.
Target Distribution	High	All of the bank's web servers run the vulnerable software.
Confidentiality Requirement	High	Customers expect their banking information to be confidential.
Integrity Requirement	High	Financial and personal information should not be changeable without authorization.
Availability Requirement	Low	Unavailability of online banking services is likely to be an inconvenience for customers, but not catastrophic.

This would give an environmental score of 8.2, and an environmental vector of CDP:MH/TD:H/CR:H/IR:H/AR:L. This score is within the range 7.0-10.0, and therefore constitutes a critical vulnerability in the context of the affected bank's business.

Criticism of Version 2

Several vendors and organizations expressed dissatisfaction with CVSSv2.

Risk Based Security, which manages the Open Sourced Vulnerability Database, and the Open Security Foundation jointly published a public letter to FIRST regarding the shortcomings and failures of CVSSv2.^[7] The authors cited a lack of granularity in several metrics which results in CVSS vectors and scores that do not properly distinguish vulnerabilities of different type and risk profiles. The CVSS scoring system was also noted as requiring too much knowledge of the exact impact of the vulnerability.

Oracle introduced the new metric value of "Partial+" for Confidentiality, Integrity, and Availability, to fill perceived gaps in the description between Partial and Complete in the official CVSS specifications.^[8]

Version 3

To address some of these criticisms, development of CVSS version 3 was started in 2012. The final specification was named CVSS v3.0 and released in June 2015. In addition to a Specification Document, a User Guide and Examples document were also released.^[9]

Several metrics were changed, added, and removed. The numerical formulas were updated to incorporate the new metrics while retaining the existing scoring range of 0-10. Textual severity ratings of None (0), Low (0.1-3.9), Medium (4.0-6.9), High (7.0-8.9), and Critical (9.0-10.0)^[10] were defined, similar to the categories NVD defined for CVSS v2 that were not part of that standard .^[11]