## Status

This document tracks the future-looking risk scoring model for Vexer. The calculation below is not active yet; Sprint 7 work will add the required schema fields, policy controls, and services. Until that ships, Vexer emits consensus statuses without numeric scores.

## Scoring model (target state)

**S = Gate(VEX_status) × W_trust(source) × [Severity_base × (1 + α·KEV + β·EPSS)]**

* **Gate(VEX_status)**: `affected`/`under_investigation` → 1, `not_affected`/`fixed` → 0. A trusted “not affected” or “fixed” still zeroes the score.
* **W_trust(source)**: normalized policy weight (baseline 0‒1). Policies may opt into >1 boosts for signed vendor feeds once Phase 1 closes.
* **Severity_base**: canonical numeric severity from Feedser (CVSS or org-defined scale).
* **KEV flag**: 0/1 boost when CISA Known Exploited Vulnerabilities applies.
* **EPSS**: probability [0,1]; bounded multiplier.
* **α, β**: configurable coefficients (default α=0.25, β=0.5) stored in policy.

Safeguards: freeze boosts when product identity is unknown, clamp outputs ≥0, and log every factor in the audit trail.

## Implementation roadmap

| Phase | Scope | Artifacts |
| --- | --- | --- |
| **Phase 1 – Schema foundations** | Extend Vexer consensus/claims and Feedser canonical advisories with severity, KEV, EPSS, and expose α/β + weight ceilings in policy. | Sprint 7 tasks `VEXER-CORE-02-001`, `VEXER-POLICY-02-001`, `VEXER-STORAGE-02-001`, `FEEDCORE-ENGINE-07-001`. |
| **Phase 2 – Deterministic score engine** | Implement a scoring component that executes alongside consensus and persists score envelopes with hashes. | Planned task `VEXER-CORE-02-002` (backlog). |
| **Phase 3 – Surfacing & enforcement** | Expose scores via WebService/CLI, integrate with Feedser noise priors, and enforce policy-based suppressions. | To be scheduled after Phase 2. |

## Data model (after Phase 1)

```json
{
  "vulnerabilityId": "CVE-2025-12345",
  "product": "pkg:name@version",
  "consensus": {
    "status": "affected",
    "policyRevisionId": "rev-12",
    "policyDigest": "0D9AEC…"
  },
  "signals": {
    "severity": {"scheme": "CVSS:3.1", "score": 7.5},
    "kev": true,
    "epss": 0.40
  },
  "policy": {
    "weight": 1.15,
    "alpha": 0.25,
    "beta": 0.5
  },
  "score": {
    "value": 10.8,
    "generatedAt": "2025-11-05T14:12:30Z",
    "audit": [
      "gate:affected",
      "weight:1.15",
      "severity:7.5",
      "kev:1",
      "epss:0.40"
    ]
  }
}
```

## Operational guidance

* **Inputs**: Feedser delivers severity/KEV/EPSS via the advisory event log; Vexer connectors load VEX statements. Policy owns trust tiers and coefficients.
* **Processing**: the scoring engine (Phase 2) runs next to consensus, storing results with deterministic hashes so exports and attestations can reference them.
* **Consumption**: WebService/CLI will return consensus plus score; scanners may suppress findings only when policy-authorized VEX gating and signed score envelopes agree.

## Pseudocode (Phase 2 preview)

```python
def risk_score(gate, weight, severity, kev, epss, alpha, beta, freeze_boosts=False):
    if gate == 0:
        return 0
    if freeze_boosts:
        kev, epss = 0, 0
    boost = 1 + alpha * kev + beta * epss
    return max(0, weight * severity * boost)
```

## FAQ

* **Can operators opt out?** Set α=β=0 or keep weights ≤1.0 via policy.
* **What about missing signals?** Treat them as zero and log the omission.
* **When will this ship?** Phase 1 is planned for Sprint 7; later phases depend on connector coverage and attestation delivery.