105 lines
4.5 KiB
Markdown
105 lines
4.5 KiB
Markdown
## Status
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This document tracks the future-looking risk scoring model for Excititor. The calculation below is not active yet; Sprint 7 work will add the required schema fields, policy controls, and services. Until that ships, Excititor emits consensus statuses without numeric scores.
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## Scoring model (target state)
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**S = Gate(VEX_status) × W_trust(source) × [Severity_base × (1 + α·KEV + β·EPSS)]**
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* **Gate(VEX_status)**: `affected`/`under_investigation` → 1, `not_affected`/`fixed` → 0. A trusted “not affected” or “fixed” still zeroes the score.
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* **W_trust(source)**: normalized policy weight (baseline 0‒1). Policies may opt into >1 boosts for signed vendor feeds once Phase 1 closes.
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* **Severity_base**: canonical numeric severity from Concelier (CVSS or org-defined scale).
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* **KEV flag**: 0/1 boost when CISA Known Exploited Vulnerabilities applies.
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* **EPSS**: probability [0,1]; bounded multiplier.
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* **α, β**: configurable coefficients (default α=0.25, β=0.5) stored in policy.
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Safeguards: freeze boosts when product identity is unknown, clamp outputs ≥0, and log every factor in the audit trail.
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## Implementation roadmap
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| Phase | Scope | Artifacts |
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| --- | --- | --- |
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| **Phase 1 – Schema foundations** | Extend Excititor consensus/claims and Concelier canonical advisories with severity, KEV, EPSS, and expose α/β + weight ceilings in policy. | Sprint 7 tasks `EXCITITOR-CORE-02-001`, `EXCITITOR-POLICY-02-001`, `EXCITITOR-STORAGE-02-001`, `FEEDCORE-ENGINE-07-001`. |
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| **Phase 2 – Deterministic score engine** | Implement a scoring component that executes alongside consensus and persists score envelopes with hashes. | Planned task `EXCITITOR-CORE-02-002` (backlog). |
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| **Phase 3 – Surfacing & enforcement** | Expose scores via WebService/CLI, integrate with Concelier noise priors, and enforce policy-based suppressions. | To be scheduled after Phase 2. |
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## Policy controls (Phase 1)
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Operators tune scoring inputs through the Excititor policy document:
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```yaml
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excititor:
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policy:
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weights:
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vendor: 1.10 # per-tier weight
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ceiling: 1.40 # max clamp applied to tiers and overrides (1.0‒5.0)
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providerOverrides:
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trusted.vendor: 1.35
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scoring:
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alpha: 0.30 # KEV boost coefficient (defaults to 0.25)
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beta: 0.60 # EPSS boost coefficient (defaults to 0.50)
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```
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* All weights (tiers + overrides) are clamped to `[0, weights.ceiling]` with structured warnings when a value is out of range or not a finite number.
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* `weights.ceiling` itself is constrained to `[1.0, 5.0]`, preserving prior behaviour when omitted.
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* `scoring.alpha` / `scoring.beta` accept non-negative values up to 5.0; values outside the range fall back to defaults and surface diagnostics to operators.
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## Data model (after Phase 1)
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```json
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{
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"vulnerabilityId": "CVE-2025-12345",
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"product": "pkg:name@version",
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"consensus": {
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"status": "affected",
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"policyRevisionId": "rev-12",
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"policyDigest": "0D9AEC…"
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},
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"signals": {
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"severity": {"scheme": "CVSS:3.1", "score": 7.5},
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"kev": true,
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"epss": 0.40
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},
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"policy": {
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"weight": 1.15,
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"alpha": 0.25,
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"beta": 0.5
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},
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"score": {
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"value": 10.8,
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"generatedAt": "2025-11-05T14:12:30Z",
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"audit": [
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"gate:affected",
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"weight:1.15",
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"severity:7.5",
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"kev:1",
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"epss:0.40"
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]
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}
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}
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```
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## Operational guidance
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* **Inputs**: Concelier delivers severity/KEV/EPSS via the advisory event log; Excititor connectors load VEX statements. Policy owns trust tiers and coefficients.
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* **Processing**: the scoring engine (Phase 2) runs next to consensus, storing results with deterministic hashes so exports and attestations can reference them.
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* **Consumption**: WebService/CLI will return consensus plus score; scanners may suppress findings only when policy-authorized VEX gating and signed score envelopes agree.
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## Pseudocode (Phase 2 preview)
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```python
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def risk_score(gate, weight, severity, kev, epss, alpha, beta, freeze_boosts=False):
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if gate == 0:
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return 0
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if freeze_boosts:
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kev, epss = 0, 0
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boost = 1 + alpha * kev + beta * epss
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return max(0, weight * severity * boost)
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```
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## FAQ
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* **Can operators opt out?** Set α=β=0 or keep weights ≤1.0 via policy.
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* **What about missing signals?** Treat them as zero and log the omission.
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* **When will this ship?** Phase 1 is planned for Sprint 7; later phases depend on connector coverage and attestation delivery.
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