Here’s a crisp way to think about “reachability” that makes triage sane and auditable: **treat it like a cryptographic proof**—a minimal, reproducible chain that shows *why* a vuln can (or cannot) hit runtime.

### The idea (plain English)

* **Reachability** asks: “Could data flow from an attacker to the vulnerable code path during real execution?”
* **Proof-carrying reachability** says: “Don’t just say yes/no—hand me a *proof chain* I can re-run.”
  Think: the shortest, lossless breadcrumb trail from entrypoint → sinks, with the exact build + policy context that made it true.

### What the “proof” contains

1. **Scope hash**: content digests for artifact(s) (image layers, SBOM nodes, commit IDs, compiler flags).
2. **Policy hash**: the decision rules used (e.g., “prod disallows unknowns > 0”; “vendor VEX outranks distro unless backport tag present”).
3. **Graph snippet**: the *minimal subgraph* (call/data/control edges) that connects:

   * external entrypoint(s) → user-controlled sources → validators (if any) → vulnerable function(s)/sink(s).
4. **Conditions**: feature flags, env vars, platform guards, version ranges, eBPF-observed edges (if present).
5. **Verdict** (signed): A → {Affected | Not Affected | Under-Constrained} with reason codes.
6. **Replay manifest**: the inputs needed to recompute the same verdict (feeds, rules, versions, hashes).

### Why this helps

* **Auditable**: Every “Not Affected” is defensible (no hand-wavy “scanner says so”).
* **Deterministic**: Same inputs → same verdict (great for change control and regulators).
* **Compact**: You store only the *minimal subgraph*, not the whole monolith.

### Minimal proof example (sketch)

* Artifact: `svc.payments:1.4.7` (image digest `sha256:…`)
* CVE: `CVE-2024-XYZ` in `libyaml 0.2.5`
* Entry: `POST /import`, body → `YamlDeserializer.Parse`
* Guards: none (no schema/whitelist prior to parse)
* Edge chain: `HttpBody → Parse(bytes) → LoadNode() → vulnerable_path()`
* Condition: feature flag `BULK_IMPORT=true`
* Verdict: **Affected**
* Signed DSSE envelope over {scope hash, policy hash, graph snippet JSON, conditions, verdict}.

### How to build it (practical checklist)

* **During build**

  * Emit SBOM (source & binary) with function/file symbols where possible.
  * Capture compiler/linker flags; normalize paths; include feature flags’ default state.
* **During analysis**

  * Static: slice the call graph to the *shortest* source→sink chain; attach type-state facts (e.g., “validated length”).
  * Deps: map CVEs to precise symbol/ABI surfaces (not just package names).
  * Backports: require explicit evidence (patch IDs, symbol presence) before downgrading severity.
* **During runtime (optional but strong)**

  * eBPF trace to confirm edges observed; store hashes of kprobes/uprobes programs and sampling window.
* **During decisioning**

  * Apply merge policy (vendor VEX, distro notes, internal tests) deterministically; hash the policy.
  * Emit one DSSE/attestation per verdict; include replay manifest.

### UI that won’t overwhelm

* **Default card**: Verdict + “Why?” (one-line chain) + “Replay” button.
* **Expand**: shows the 5–10 edge subgraph, conditions, and signed envelope.
* **Compare builds**: side-by-side proof deltas (edges added/removed, policy change, backport flip).

### Operating modes

* **Strict** (prod): Unknowns → fail-closed; proofs required for Not Affected.
* **Lenient** (dev): Unknowns tolerated; proofs optional but encouraged; allow “Under-Constrained”.

### What to measure

* Proof generation rate, median proof size (KB), replay success %, proof dedup ratio, and “unknowns” burn-down.

If you want, I can turn this into a ready-to-ship spec for Stella Ops (attestation schema, JSON examples, API routes, and a tiny .NET verifier).