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score-calculator.ts
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score-calculator.ts
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import { BaseMetric, BaseMetricValue } from './models';
import { parseMetricsAsMap } from './parser';
import { validate } from './validator';
// https://www.first.org/cvss/v3.1/specification-document#7-4-Metric-Values
const baseMetricValueScores: Record<
BaseMetric,
Partial<Record<BaseMetricValue, number>> | null
> = {
[BaseMetric.ATTACK_VECTOR]: { N: 0.85, A: 0.62, L: 0.55, P: 0.2 },
[BaseMetric.ATTACK_COMPLEXITY]: { L: 0.77, H: 0.44 },
[BaseMetric.PRIVILEGES_REQUIRED]: null, // scope-dependent: see getPrivilegesRequiredNumericValue()
[BaseMetric.USER_INTERACTION]: { N: 0.85, R: 0.62 },
[BaseMetric.SCOPE]: { U: 0, C: 0 },
[BaseMetric.CONFIDENTIALITY]: { N: 0, L: 0.22, H: 0.56 },
[BaseMetric.INTEGRITY]: { N: 0, L: 0.22, H: 0.56 },
[BaseMetric.AVAILABILITY]: { N: 0, L: 0.22, H: 0.56 }
};
const getPrivilegesRequiredNumericValue = (
value: BaseMetricValue,
scopeValue: BaseMetricValue
): number => {
if (scopeValue !== 'U' && scopeValue !== 'C') {
throw new Error(`Unknown Scope value: ${scopeValue}`);
}
switch (value) {
case 'N':
return 0.85;
case 'L':
return scopeValue === 'U' ? 0.62 : 0.68;
case 'H':
return scopeValue === 'U' ? 0.27 : 0.5;
default:
throw new Error(`Unknown PrivilegesRequired value: ${value}`);
}
};
const getMetricValue = (
metric: BaseMetric,
metricsMap: Map<BaseMetric, BaseMetricValue>
): BaseMetricValue => {
if (!metricsMap.has(metric)) {
throw new Error(`Missing metric: ${metric}`);
}
return metricsMap.get(metric) as BaseMetricValue;
};
const getMetricNumericValue = (
metric: BaseMetric,
metricsMap: Map<BaseMetric, BaseMetricValue>
): number => {
const value = getMetricValue(metric, metricsMap);
if (metric === BaseMetric.PRIVILEGES_REQUIRED) {
return getPrivilegesRequiredNumericValue(
value,
getMetricValue(BaseMetric.SCOPE, metricsMap)
);
}
const score: Partial<Record<BaseMetricValue, number>> | null =
baseMetricValueScores[metric];
if (!score) {
throw new Error(`Internal error. Missing metric score: ${metric}`);
}
return score[value]!;
};
// ISS = 1 - [ (1 - Confidentiality) × (1 - Integrity) × (1 - Availability) ]
export const calculateIss = (
metricsMap: Map<BaseMetric, BaseMetricValue>
): number => {
const confidentiality = getMetricNumericValue(
BaseMetric.CONFIDENTIALITY,
metricsMap
);
const integrity = getMetricNumericValue(BaseMetric.INTEGRITY, metricsMap);
const availability = getMetricNumericValue(
BaseMetric.AVAILABILITY,
metricsMap
);
return 1 - (1 - confidentiality) * (1 - integrity) * (1 - availability);
};
// https://www.first.org/cvss/v3.1/specification-document#7-1-Base-Metrics-Equations
// Impact =
// If Scope is Unchanged 6.42 × ISS
// If Scope is Changed 7.52 × (ISS - 0.029) - 3.25 × (ISS - 0.02)
export const calculateImpact = (
metricsMap: Map<BaseMetric, BaseMetricValue>,
iss: number
): number =>
metricsMap.get(BaseMetric.SCOPE) === 'U'
? 6.42 * iss
: 7.52 * (iss - 0.029) - 3.25 * Math.pow(iss - 0.02, 15);
// https://www.first.org/cvss/v3.1/specification-document#7-1-Base-Metrics-Equations
// Exploitability = 8.22 × AttackVector × AttackComplexity × PrivilegesRequired × UserInteraction
export const calculateExploitability = (
metricsMap: Map<BaseMetric, BaseMetricValue>
): number =>
8.22 *
getMetricNumericValue(BaseMetric.ATTACK_VECTOR, metricsMap) *
getMetricNumericValue(BaseMetric.ATTACK_COMPLEXITY, metricsMap) *
getMetricNumericValue(BaseMetric.PRIVILEGES_REQUIRED, metricsMap) *
getMetricNumericValue(BaseMetric.USER_INTERACTION, metricsMap);
// https://www.first.org/cvss/v3.1/specification-document#Appendix-A---Floating-Point-Rounding
const roundUp = (input: number): number => {
const intInput = Math.round(input * 100000);
return intInput % 10000 === 0
? intInput / 100000
: (Math.floor(intInput / 10000) + 1) / 10;
};
// https://www.first.org/cvss/v3.1/specification-document#7-1-Base-Metrics-Equations
// If Impact <= 0 => 0; else
// If Scope is Unchanged => Roundup (Minimum [(Impact + Exploitability), 10])
// If Scope is Changed => Roundup (Minimum [1.08 × (Impact + Exploitability), 10])
export const calculateBaseScore = (cvssString: string): number => {
validate(cvssString);
const metricsMap: Map<BaseMetric, BaseMetricValue> = parseMetricsAsMap(
cvssString
);
const iss = calculateIss(metricsMap);
const impact = calculateImpact(metricsMap, iss);
const exploitability = calculateExploitability(metricsMap);
const scopeUnchanged = metricsMap.get(BaseMetric.SCOPE) === 'U';
return impact <= 0
? 0
: scopeUnchanged
? roundUp(Math.min(impact + exploitability, 10))
: roundUp(Math.min(1.08 * (impact + exploitability), 10));
};