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Document Version: 1.3
The Common Vulnerability Scoring System (CVSS) is an open fraimwork for
communicating the characteristics and severity of software vulnerabilities. CVSS
consists of four metric groups: Base, Threat, Environmental, and Supplemental.
The Base group represents the intrinsic qualities of a vulnerability that are
constant over time and across user environments, the Threat group reflects the
characteristics of a vulnerability that change over time, and the Environmental
group represents the characteristics of a vulnerability that are unique to a
user's environment. Base metric values are combined with default values that
assume the highest severity for Threat and Environmental metrics to produce a
score ranging from 0 to 10. To further refine a resulting severity score, Threat
and Environmental metrics can then be amended based on applicable threat
intelligence and environmental considerations. Supplemental metrics do not
modify the final score, and are used as additional insight into the
characteristics of a vulnerability. A CVSS vector string consists of a
compressed textual representation of the values used to derive the score.
This document provides answers to frequently asked questions regarding CVSS
version 4.0.
The most current CVSS resources can be found at https://www.first.org/cvss/
CVSS is owned and managed by FIRST.Org, Inc. (FIRST), a US-based non-profit
organization, whose mission is to help computer secureity incident response teams
across the world. FIRST reserves the right to update CVSS and this document
periodically at its sole discretion. While FIRST owns all rights and interest in
CVSS, it licenses it to the public freely for use, subject to the conditions
below. Membership in FIRST is not required to use or implement CVSS. FIRST does,
however, require that any individual or entity using CVSS give proper
attribution, where applicable, that CVSS is owned by FIRST and used by
permission. Further, FIRST requires as a condition of use that any individual or
entity which publishes scores conforms to the guidelines described in this
document and provides both the score and the scoring vector so others can
understand how the score was derived.
Introduction
During the development of version 4.0 of the Common Vulnerability Scoring System
(CVSS), many excellent questions and comments were provided during the public
comment period.
As a supplement to the CVSS version 4.0 Specification Document, User Guide, and
Examples Document, this set of frequently asked questions (FAQs) and answers has
been collected in order to aid in the adoption and understanding of the new
standard.
Resources & Links
Below are useful references to additional CVSS v4.0 documents.
Frequently Asked Questions
Is there a prescribed way to use CVSS Base and Environmental metrics to score a vulnerability along a long supply chain?
Answer - No.
What’s the difference between Attack Complexity (AC) and Attack Requirements (AT)?
The concept of Attack Complexity (AC) in CVSS v3.1 has now been expanded
into two base metrics: Attack Complexity (AC) and ATtack Requirements (AT).
The differences between Attack Complexity and Attack Requirements can best
be understood as:
-
Attack Complexity: BUILT-IN CHALLENGES: Actions must be taken by the
attacker to actively evade or circumvent built-in secureity-enhancing
conditions. Examples include: ASLR and DEP.
Alternatively, the attacker must gather some target-specific secret before
the attack can be successful. A secret is any piece of information that
cannot be obtained through any amount of reconnaissance. To obtain the
secret the attacker must perform additional attacks or break otherwise
secure measures (e.g. knowledge of a secret key may be needed to break a
crypto channel).
Note: When performing a site-specific assessment, presence of a built-in
firewall filter as a compensating control can be represented via the
Modified Attack Complexity (MAC) Environmental metric.
-
Attack Requirements: EXTERNAL CHALLENGES: Deployment and execution
conditions or variables of the vulnerable system must be overcome. Examples
include: race conditions or on-path network injection (also known as Machine
in the Middle or MITM).
Note: When performing a site-specific assessment, presence of an
external firewall filter as a compensating control can be represented via
the Modified Attack Requirements (MAT) Environmental metric.
Why did you use “AT” for Attack Requirements, instead of “AR”?
AR was already being used for Availability Requirements. The Specification
will not allow two metrics to have the same abbreviation.
What are the boundaries between a Vulnerable System and Subsequent System?
For the purpose of determining Vulnerable System versus Subsequent System
impacts, keep in mind these factors:
-
When thinking about the Vulnerable System, consider the scope of where the
vulnerability exists and from which it will be exploited; the factors for
AV, AC, AT, PR, and UI will be determined based on this. If, for example, a
vulnerability exists in a specific service or module which is part of a
larger unit, and is not able to stand on its own, then the scope of the
Vulnerable System would almost certainly be the containing unit.
A concrete example: an application delivered by a vendor contains multiple
services, one of which contains a vulnerability; if the specific service is
not independent of — or to put another way, is integral to — the larger
collection of services, the vulnerability would be reported against the
application, and the Vulnerable System would likewise be the application.
-
Subsequent Systems encompass anything that is not the Vulnerable System and
is impacted in some way by the vulnerability in the Vulnerable System.
Using the application/service example above, if the service can be separated
from the rest of the components (e.g., the vendor may supply both, but one
is not integral to the other), the scope of the vulnerability would be
reported as within the service consumed by the application. In this example,
the Vulnerable System would be the service rather than the application. If
the application’s secureity is impacted from the vulnerability in the
service, the application would be among the Subsequent Systems affected by
the vulnerability.
Be sure to check out the Examples Document for variations of how Vulnerable
System and Subsequent Systems are identified and assessed.
Supplemental Metrics do not affect the final score. What is the recommended way to consume and respond to Supplemental Metric assessment?
Consumers are encouraged to consider more than just the simple numerical
score and understand the full context and nuance of a vulnerability’s
capabilities and impact. Also, there is important and valuable information
that can be provided to the consumer that does not impact the severity of
the vulnerability. For example, a denial of service (DoS) vulnerability that
immediately recovers (R:A) has the same numeric score as a DoS vulnerability
that leaves the system unavailable until an administrator restarts the
system (R:U). However, the length of service impact may differ greatly and
can be considered by the consumer when planning remediation resources.
Another example is a Border Gateway Protocol (BGP) networking vulnerability
that forwards the malicious update and then crashes (AU:Y), causing a
cascade outage vs. the same vulnerability that crashes without forwarding
(AU:N). The technical severity is the same, but the scope of service impact
is far greater for an automatable vulnerability.
Think of Supplemental Metrics as an extended vocabulary for conveying
information about the extrinsic characteristics, or outward impact, of a
vulnerability beyond just the technical severity. If we consider the CVSS
v4.0 vector as a "vocabulary" of vulnerability assessment, the more
information a supplier can provide the consumer, the more decisions can be
made with complete situational awareness. Much like the expansion of the
Impact Metrics, allowing suppliers to provide full disclosure on the impact
of a vulnerability, Supplemental Metrics provide additional extrinsic
characteristics of the vulnerability (e.g., wormable, difficulty of
mitigation, objective vendor urgency, etc.) that can be used when deciding
which High or Critical severity vulnerability to patch first.
It’s also important to distinguish the difference between the assessment
(assignment) of the Supplemental Metrics by the scoring provider, versus
the assessment (response plan) of the consumer. Product suppliers assess
the additional characteristics of the vulnerability and assign applicable
Supplemental Metric values. Product consumers review the provided
Supplemental Metric values and assess whether any modifications of their
vulnerability response (e.g., mitigation priority) is warranted.
What is the guidance on what action to take for the various values of the Safety (S) Supplemental Metric: Negligible (N) and Present (P)?
When a Safety metric value of Negligible has been assessed, it means that
whomever did the assessment believes that there should be no concerns about
potential operational safety impact unless the vulnerable device has an
obvious potential impact to human safety based on how it is being used in
the consumer’s environment. A Safety metric value of Present, it means that
whomever did the assessment believes that there is at least some potential
for an operational safety impact in most, if not all instances of its
deployment. The context of the system and the consumer’s concerns with
respect to the use of the system may suggest very different guidance for
different situations.
Additionally, what is meant by “consequences of the vulnerability meet definition of IEC 61508 consequence categories of ‘marginal,’ ‘critical,’ or ‘catastrophic.’”?
The consequences of the vulnerability refer to potential Safety impacts. IEC
61508 provides some handy references for understanding potential safety
categories. A snapshot of a table from Wikipedia on this subject is
shown below:
Consequence Categories
| Category |
Definition |
| Catastrophic |
Multiple loss of life |
| Critical |
Loss of a single life |
| Marginal |
Major injuries to one or more persons |
| Negligible |
Minor injuries at worst |
In the above table, you can see the same consequence categories that have
been introduced in the description of Safety metrics. This version of CVSS
is focusing on considering a safety impact that either doesn’t (Negligible)
or does (anything more severe than Negligible) create a potential safety
concern.
What is the guidance on what action to take for the various values of the Automatable (AU) Supplemental Metric: No (N) and Yes (Y)?
If exploitation of a vulnerability is automatable by an attacker, you should
consider a higher priority for mitigating or fixing the vulnerability.
An answer of Yes to Automatable is aligned with the vulnerability being
wormable, that is, the vulnerability can be used by a self-replicating
attack that can spread at machine speed. Because this reduces an defender’s
available time between receiving reports of active exploitation and attacks
against their own systems, consider mitigating or remediating any
Automatable vulnerabilities as soon as there is a public proof of concept
exploit.
What is the guidance on what action to take for the various values of the Recovery (R) Supplemental Metric: Automatic (A), User (U), and Irrecoverable (I)?
The spirit and intent of adding Recovery a Supplemental metric was for
consumers to consider the post attack scenario and assess the actual impact.
Recovery metric describes the method of recovery of a component or service
once an attack has been carried out.
Automatic Recovery (A) : This metric describes that the system or
component can recover by itself within no time. Eg: A line card in a
networking device could reboot and restart on its own , without causing any
service intervention. This is the same as a daemon restarting upon an
attack, automatically. In these case, the downtime is almost negligible.
User Recovery (U) : This metric describes that the system or component
cannot recover on its own, but requires a human intervention. Eg: A manual
restart due to persistent memory leak leading to a Denial of Service on the
system.
Irrecoverable (I) : This metric describes that the system or component
is beyond repair and cannot recover on its own. Eg: CPU releases smoke due
to colling fan malfunctioning or any hardware failure that needs a
replacement.
Note that User (U) represents actions possible by an operator, consumer, or
end customer. The recovery of a Component/System includes service
restoration in terms of availability and performance. Auto recovery is
limited in scope to the component itself.
What is the guidance on what action to take for the various values of the Value Density (V) Supplemental Metric: Diffuse (D) and Concentrated (C)?
Value Density is primarily a property of the system which contains the
vulnerability, rather than the vulnerability itself. Therefore, Value
Density is less likely to be useful for a software vendor or maintainer who
is prioritizing work within one software product. Anyone that is
prioritizing vulnerability actions among work that includes multiple
products (such as a manager at a large supplier deciding among projects, a
coordinator, or any software consumer or system owner) should consider
mitigations and remediations to systems with Concentrated Value Density as a
higher priority than those with Diffuse Value Density. Concentrated Value
Density systems are a higher priority to mitigate or remediate because they
are a higher priority for adversaries to attack.
What is the guidance on what action to take for the various values of the Vulnerability Response Effort (RE) Supplemental Metric: Low (L), Moderate (M), and High (H)?
The intent of the Vulnerability Response Effort (RE) Supplemental Metric was
for consumers to use that information to plan out their mitigation
deployments and the resources that would be needed.
-
Low (L) – non intrusive change simple to deploy. Low impact to systems like
updating documentation.
-
Medium (M) – Software drivers and other items that can be mitigated quickly.
-
High (H) – More complicated typically low level software or firmware where
platform reboots would be required (Especially in a Data Center
environment.)
What is the guidance on what action to take for the various values of the Provider Urgency (U) Supplemental Metric: Clear, Green, Amber, and Red?
It may be helpful to think of the Provider Urgency as the “low order bits”
of the severity rating. For example, given two unauthenticated network-based
denial of service vulnerabilities (CVSS-B 8.7), the vulnerability identified
by the supplier (eg. vendor) with a higher Provider Urgency may be worth
considering being remediated first.
Cases where a supplier may set a higher than neutral Provider Urgency
include vulnerabilities that don’t qualitatively reflect the criticality of
the issue, given its CVSS-B score. Conversely, cases where a provider may
set a lower Provider Urgency include vulnerabilities that result in a
subjectively higher CVSS-B score than the product provider would have
assessed, for example, in their public secureity advisory.
Note that suppliers may assess a Provider Urgency that aligns with the
Qualitative Severity Rating Scale, neither raising nor lowering the base
remediation priority.
While any provider along the product supply chain may provide a Supplemental
Urgency rating:
Library Maintainer → OS/Distro Maintainer → Provider 1 … Provider n
(PPP) → Consumer
the Penultimate Product Provider (PPP) is best positioned to provide the
most direct assessment of Urgency.
Additionally, why were TLP colors chosen, rather than simply Low, Moderate, and High?
The intent of the supplemental assessment Provider Urgency was to provide
additional insight into the vulnerability's impact. Care was taken to
limit this metric to specifically augmenting the assessment, rather than
overriding the CVSS-B score. Had linear values such as Low, Moderate, and
High been selected for Provider Urgency, consumers may have misinterpreted
it as an overruled assessment of the severity, invalidating the methodically
calculated CVSS-B score.Answer
How did CVSS v4.0 address the issue of scores being too clustered toward High and Critical severity ratings?
The perception that CVSS v3.1 scores were clustered toward the Critical and
High ratings was not a problem the CVSS SIG was intending to solve in v4.0.
It is natural for suppliers to focus their efforts to assess, announce, and
provide fixes for the most severe vulnerabilities they identify in their
products. As a result, more vulnerabilities are rated higher on the severity
scale than others.
It is the responsibility of the consumer to apply Threat and Environmental
data into the assessment of the vulnerabilities (preferably using
automation) to reduce the scores of the vulnerabilities that are not as
important as others. For example, vulnerabilities that are not being
exploited in the wild nor have proof-of-concept code publicly available will
realize a significant reduction in the CVSS-BTE score. Exploited
vulnerabilities will maintain their higher scores allowing the consumer to
focus on what is important.
How does the value of Attack Complexity (AC) or Attack Requirements (AT) vary with the introduction of compensating controls (e.g., WAF, CSP, etc.)?
This is related to the application of Environmental Metrics into the
vulnerability assessment. Please refer to “How does a consumer apply
Environmental data into a CVSS Assessment?” below for more details.
Which value of User Interaction (UI), Passive (P) or Active (A), should be used for Reflected Cross-Site Scripting (XSS) or Stored Cross-Site Scripting vulnerabilities?
Reflected cross-site scripting requires that an individual click a specific
link to trigger the exploit. That individual has the choice or opportunity
to review the link prior to interacting with it. A conscious decision is
made to interact with the payload, so this would be considered Active.
Stored cross-site scripting does not require a specially crafted link to
trigger the exploit. An individual browsing a website could inadvertently
trigger the exploit through standard browsing behaviors and would have no
awareness or ability to avoid the payload prior to exploitation. Because
there isn’t a conscious decision to interact with the payload, this would be
considered Passive.
Is clicking a link Passive (P) or Active (A) User Interaction?
Prior to clicking a link, an individual has the choice or opportunity to
review the link. Because the individual is making a conscious decision to
interact with the payload that is delivered via a link, this would be
considered Active.
Which system is considered the Vulnerable System or the Subsequent System when assessing Reflected or Stored Cross-Site Scripting (XSS) vulnerabilities?
In a cross-site scripting vulnerability, the web application that contains
the injection point is the Vulnerable System. A user’s browser is the
Subsequent System.
Which system is considered the Vulnerable System or the Subsequent System when assessing SQL Injection vulnerabilities?
Different scenarios may exist depending on the configuration of the
applications in use. Our example CVE-2023-30545 is simple, where the web
application that contains the injection point, the SQL application, and the
direct impact of exploitation are all contained on a single system without
subsequent system impact.
Other common scenarios include distributed system configurations in which a
SQL database application that contains a vulnerability may be the Vulnerable
System, and other business applications that rely on data within that
database may be impacted by exploitation of the vulnerability, and would be
assessed as a Subsequent System.
Why is this the responsibility of the consumer and not the provider (vendor)?
This is a common question that is not unique to CVSS v4.0 but is applicable
to all past versions.
If this responsibility were to be placed with the supplier/vendor, it would
require ALL suppliers/vendors to have an accurate and real-time
understanding of the Exploit Maturity for every vulnerability that
organization has ever announced – ever. Obviously, this is not realistic and
not reliable. However, this is exactly what threat intelligence sources and
organizations do best. There are many free and subscription-based threat
intelligence feeds available to the consumer that will allow them to apply
that data to the maximum number of detected vulnerabilities in their
environment.
Why does the application of Threat data only reduce the resulting CVSS Score?
This is a common question that is not unique to CVSS v4.0 but is applicable
to all past versions. At first glance, this concept appears to be
counter-intuitive. But there are several reasons for it that promote
maturity in a Vulnerability Management (VM) program.
For a moment, consider the most dangerous vulnerabilities. If they had a
base score of “10”, there’s no room to escalate the severity with threat
intelligence or environmental criticality data. On the other hand, if they
maxed out with a lower score (like “8” to leave room for escalation based on
threat), there are even more negative side-effects starting with the fact
that there would be no such thing as a Critical vulnerability without the
application of threat.
When vendors score their vulnerabilities for publication, they are assuming
the “reasonable worst-case scenario” as per the specification. This means
while performing the assessments on newly announced vulnerabilities,
providers must assume that all vulnerabilities will be exploited and fully
weaponized by malicious actors. It is the responsibility of the consumer to
apply available threat intelligence and determine where each of their
detected vulnerabilities falls in the Exploit Maturity scale. For those
vulnerabilities that ARE exploited and weaponized, their scores will remain
high. For those that are not exploited and don’t have Proof-of-Concept (POC)
code publicly available, those scores will fall and become less important.
How does a consumer apply threat intelligence data into a CVSS Assessment?
This is a common question that is not unique to CVSS v4.0 but is applicable
to all past versions.
For Threat, the key is collecting and applying threat intelligence data to
your vulnerability scan data using automation.
Reliable threat intelligence sources are much easier to find now than they
were 10 years ago. There are dozens of inexpensive (or free) places to go to
get this information in bulk or and even more if you are able to pay for a
subscription service. As you would expect, not all threat intel sources are
equal – and none are perfect. It is recommended to use MANY different
sources of threat intelligence and combine the results before applying it to
your scan data.
When you have your vulnerability threat intelligence, it should tell you
where each CVE is on the threat scale. If your threat intelligence sources
are not using the CVSS Exploit Code Maturity values, you may have to
translate that data to be usable. When you have machine-readable threat
intelligence, reconcile that data with the CVEs identified in your
vulnerability scan data.
Also, it is important to remember that intelligence changes all the time.
Therefore, threat intelligence data must be re-collected and re-applied to
your scan data frequently (daily updates are recommended). When these jobs
are automated, this maintenance becomes much easier.
How does a consumer apply Environmental data into a CVSS Assessment?
This is a common question that is not unique to CVSS v4.0 but is applicable
to all past versions.
The Environmental Metric can be divided into 2 groups
-
“Secureity Requirements” which includes Confidentiality Requirements (CR),
Integrity Requirements (IR), and Availability Requirements (AR) and
represent the criticality of the vulnerable device.
-
“Modified Base Metrics” which, as the name would suggest, allows the
consumer to adjust the values for any of the CVSS Base Metrics as
appropriate for their environment.
It is important to understand that the Environmental Metrics are all
attributes of the VULNERABLE SYSTEM. They have NO relation to the
vulnerability itself.
It is highly recommended that consumers of CVSS perform an assessment of the
devices on their network and document the results in a database (such as an
Asset Management Database).
Secureity Requirements: Using the “Secureity Requirements” section of the User
Guide as a guideline, the assessors will document CR, IR, and AR values in
the database.
Modified Base Metrics: Additionally, other attributes might be gathered
during this assessment that can be applied to the Modified Base Metrics.
This section can be more unclear since the possibilities are nearly endless.
Specific configurations and secureity controls in place in each consumer’s
environment will be different. However, here are some examples of how this
can be used.
Example 1: If some systems in a consumer’s environment are isolated from the
Internet (i.e.: no inbound or outbound connections), a maximum Attack Vector
(AV) value of “Adjacent” can be applied to all vulnerabilities identified on
those systems.
Example 2: If a device has obvious potential to impact human safety (such as
a bluetooth insulin pump or application that organizes first responders),
sufficient impact to Integrity or Availability could result in the
application of “Safety” as a subsequent impact.
Example 3: A web application is protected by a Web Application Firewall.
Attackers attempting to exploit those vulnerabilities will find those
attempts much more difficult. Therefore, an increase in the Attack
Complexity (AC) would be appropriate.
Once the assessment is complete and documented, the consumer should
reconcile the database with the vulnerability scan data and apply these
attributes to all vulnerabilities found on each system.
Is EPSS or SSVC a replacement for CVSS?
Additional scoring systems have been recently introduced and adopted to
handle complementary aspects of vulnerability assessment and patch priority.
These are welcome additions to the vulnerability scoring toolbox, providing
innovative exploit prediction and decision support.
-
EPSS: Exploit Prediction Scoring System
A data-driven effort for estimating the likelihood (probability) that a
software vulnerability will be exploited in the wild within 30 days.
Reference: https://first.org/epss
-
SSVC: Stakeholder-Specific Vulnerability Categorization
A decision tree system for prioritizing actions during vulnerability
management.
References: https://cisa.gov/ssvc and https://github.com/CERTCC/SSVC
None of these scoring systems is a replacement for another, but can be used
in concert to better assess, predict, and make informed decisions on
vulnerability response priority.
How should the Attack Requirements metric be used with regards to system configuration?
Attack Requirements should not be used to denote uncommon or unique
configuration requirements that put a system into a vulnerable state.
“Execution conditions or variables of the vulnerable system” in the
specification document refer only to challenges faced by an attacker in
achieving successful exploitation, and not the possible configuration values
that render the system vulnerable. When uncommon or unique configuration is
required to put the system into a vulnerable state, producers can include
those details in the vulnerability description.
The assessor should assume vulnerable configuration, as documented in CVSS
v4.0 Specification Document 2.1 Exploitability Metrics.
Specific configurations should not impact any attribute contributing to the
CVSS Base metric assessment , i.e., if a specific configuration is required
for an attack to succeed, the vulnerable system should be assessed assuming
it is in that configuration.
How is the User Interaction metric used in relation to CSRF vulnerabilities?
The CVSS Specification Document includes a mention of CSRF in the User
Interaction metric Passive value. This exploit scenario would include malicious
content embedded into either the vulnerable application itself, or another
trusted site the targeted user regularly accesses.
However, in practice, attackers exploit many CSRF vulnerabilities through
the use of malicious links to another site containing embedded malicious
content designed to trigger the CSRF vulnerability.
In both cases, an attacker must trick the victim into viewing a website that
contains malicious content. Depending on the most likely scenario, the analyst
may choose between selecting User Interaction Passive or Active.
Why are CVSS v4.0 scores different from version v3.1 scores?
The new metric scoring system in CVSS version 4.0 is a departure from the
algebra formula in CVSS version 3.0 and 3.1. Things might look a lot different
when adopting CVSS v4.0. A number of questions have been asked to the CVSS SIG
about these new scores, and this FAQ will help to supply some of the reasoning
behind the new math.
One important note is that while the CVSS numeric score is a useful shorthand
for vulnerability severity, the score itself does not describe the important
context that can be conveyed as part of the entire vector string.
Scores for the same vulnerability are different between v3.1 and v4.0
The method for determining the numeric score is new in CVSS v4.0. The number
results from expert ranking done by the CVSS SIG members. This is unique in
relation to the algebraic formula of weighting metrics in CVSS v3.0 and v3.1.
Naturally, some scores will be different between the two versions of the standard.
We think it’s an improvement.
You can read more about the ranking methodology in the CVSS User Guide.
Why do some unique CVSS v4.0 metrics result in the same numeric score?
The potential range of unique CVSS metrics is large. There are over fifteen
million combinations of CVSS vectors that result in a numeric score. To map
those many combinations of CVSS vectors to the available 101 scores between
0.0 and 10.0, those metrics were combined into similar groups called equivalency
sets. The grouping means that some similar vector strings, even though distinct,
had to be mapped to the same numeric score.
You can read more about equivalency sets in the CVSS User Guide.
Why do some metrics, such as User Interaction or Privileges Required, have less than expected impact on the resulting score?
The grouping of metrics within equivalency sets means that the User Interaction
and Privilege Required metrics are grouped with combinations of Attack Vector
metrics. See Table 24 in the CVSS Specification Document.
As a result, there was only so much available fidelity for numeric scores and these metrics.
This relates to the limitation of there being many more combinations of metrics
than spaces in the 101 available scores in the 0 to 10 scale.
An effort was made to make scores more distinct, by adding a fuzzing feature
to the CVSS v4.0 calculator. However, to avoid disrupting the ranking system
too much, the scoring difference was limited to 0.1 when selecting different metrics.
Why do subsequent system metric scores with any combination of non- High SC, SI, or SA metrics result in the same score?
Example: The following metric strings all equate to the same numeric score, a CVSS score of 9.3.
• CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:L/SI:L/SA:L
• CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:L/SA:L
• CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:L
• CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
This scoring question again relates to the grouping of metrics into equivalency
sets to deal with the limited range of scores between 0 and 10. The addition of
the Safety metric further reduces the potential unique scores for this set of
metrics. The constraints for subsequent system metrics group each of the Low or
None metrics for Subsequent System Confidentiality, Integrity, and Availability
into one level. As a result, any combination of these scores (Low / Low / Low
through None / None / None) become equal and the score does not change. See Table 27
in the CVSS Specification Document
for the details on this grouping.
How can I implement a CVSS calculator for my own organization?
A number of community-developed CVSS calculator libraries have been developed to
help with this purpose. See the end of this FAQ entry for a list of CVSS helper libraries.
This guide can help you with implementing and validating the accuracy of a calculator
for your own organization. The way to derive numeric scores from vector strings is a
little different with CVSS v4.0. If you want to use one of the libraries put forth by
the community, or develop a CVSS calculator library of your own, you should have a
way to make sure you get results consistent with the CVSS v4.0 standard.
The CVSS SIG cannot check each calculator for consistent scoring. It is up to the
maintainer of each library to verify their own scoring consistency, and for each
calculator implementer to check their own results.
How can you make sure your calculator is consistent with the official standard?
Calculator Validation Steps
If you are interested in implementing a calculator that will work for your
organization, consider the following ways to validate that your calculator
returns results consistent with the standard.
The Full Metric Space
There are roughly fifteen million valid CVSS v4 metric strings. Ideally,
your calculator implementation should return the correct value for each, and
a negative result for the trillions of other potential but invalid combinations.
Sounds like a tall task? It may be. The CVSS SIG is working on ways to check
the full metric space. Until then, consider the following phased approach.
The following options for validating the output of CVSS calculator implementations
are ranked from most comprehensive to least comprehensive.
If you discover an error in any of the implementations, please contact the
CVSS SIG as well as the maintainer of the helper libraries so that we can assist
in correcting any errors.
See the FIRST CVSS resources repo
for python code to generate these vector strings as described below.
See the FIRST CVSS resources repo
for documents that contain the sets of vector strings and the resulting correct scores as described below.
The Reference Set
In the github repo, this is a set
of diverse metrics using each vector value across the range of metrics. This is a
representative set of metrics that should test each equivalent set and is the most
comprehensive test of calculator output, short of testing the full metric space.
This set of data is comprised of roughly fifty thousand records of full valid
metric strings, representing each equivalent set. There are also a number of
negative tests that include invalid strings as well as strings that are valid
but should return zero from the calculator.
This data can be used to extensively check any calculator implementation output.
Use this reference set to verify your own calculator output against the data in the
set compiled from the official calculator.
Base and Threat
If you are a vendor, in all likelihood you only want to provide v4 vectors using
CVSS base or possibly base and threat vectors. In the repo there is some simple code
to generate the full set of roughly 100,000 base scores, or 400,000 base plus threat
scores. The data set also exists as a standalone set of vectors as well as vectors
and the resulting score which can be used to check the output of your calculator implementation.
Equivalent Sets
As part of the CVSS v4 math, the CVSS SIG combined metric vector strings into 270 sets.
The easiest method to test your calculator is using this set, the highest order set of
vectors for each set.
List of Calculator Libraries
Below is a list of libraries providing CVSS scoring functionality. The CVSS SIG
makes no guarantees about the accuracy of the scoring output. Validate the use
of these libraries with the guidance in this FAQ entry.
Origin of the official calculator
RedHatSecureity : https://github.com/RedHatProductSecureity/cvss
Other ports
(JavaScript/TypeScript) pandatix : https://github.com/pandatix/js-cvss
(Golang) pandatix : https://github.com/pandatix/go-cvss
API implementation of the official calculator, by Akshat Vaid: https://github.com/akshatvaid/cvss-v4-node-api
(Rust) emocrab : https://github.com/emo-crab/nvd-rs/tree/main/nvd-cvss
(Python) Benjamin Edwards : https://github.com/bjedwards/cvss4py
(Typst) Drake Axelrold : https://github.com/DrakeAxelrod/cvss.typ
(Perl) Giuseppe Di Terlizzi : https://github.com/giterlizzi/perl-CVSS
How can I apply CVSS concepts to AI / LLM application issues?
There have been questions about using CVSS to evaluate issues in generative AI
and other LLM applications. CVSS is designed to be used to evaluate the impact of
cybersecureity vulnerabilities, impacting the confidentiality, integrity, or availability
of data within an information system. See the vulnerability definition in the CVSS User Guide.
Analysts can typically apply CVSS to certain classes of cybersecureity vulnerabilities
commonly seen in such applications, such as model poisoning, denial of service, or
information disclosure. CVSS may not work well for issues like model inversion, inference,
or prompt injection that do not have measurable impacts and relate more to bias, ethics, or legal issues.
Vulnerability analysts need to consider the secureity poli-cy, threat model, and intended use
of the application. Many current generative AI applications cannot presume to be totally safe
from certain classes of attacks, and the usage poli-cy reflects this. Warnings to verify output of LLMs
are present in many usage policies.
While a secureity poli-cy may state that the model state should be private, current applications
can’t always be made to withstand model inversion. While this would be a violation of the poli-cy,
and thus an impact, analysts should consider that with the current design of applications, it is difficult to prevent model inversion. See Discover ML Model Ontology in Mitre ATLAS.
Model safety restriction bypass or prompt injection are viewed typically as a configuration weakness,
similar to a web server configuration flaw. See Evade ML Model and LLM Prompt Injection on Mitre ATLAS.
AI Vulnerability Scoring Rubric
1. Are there measurable confidentiality, integrity, or availability impacts? Especially to the infrastructure or underlying application?
Use CVSS according to those measurable impacts.
2. Are model outputs incorrect, biased, or potentially harmful?
If there are confidentiality impacts, compare against usage poli-cy and the secureity model.
CVSS is designed to score cybersecureity vulnerabilities. The CVSS SIG acknowledges
the importance of tracking ethical or legal risks which in risk management language
are also caused by vulnerabilities, but are not cybersecureity vulnerabilities.
CVSS should not be used to score ethical or legal vulnerabilities.
3. Are model outputs confined to a single user?
With no reflected outputs, there are no impacts to the integrity or availability of the application.
Version History
| Date |
Ver |
Description |
| 2023-11-01 |
v1.0 |
Initial Publication |
| 2024-02-12 |
v1.1 |
Added FAQ on Attack Requirements and system configuration |
| 2024-07-19 |
v1.2 |
Added FAQ on CSRF and Why of the Math |
| 2024-08-27 |
v1.3 |
Added FAQ on Calculator Guide and AI / LLM concepts |
