# Gateway Tenant Auth & ABAC Contract (Web V)

## Status
- Final v1.0 (2025-12-01); aligns with Policy Guild checkpoint for Sprint 0216.

## Decisions (2025-12-01)
- Proof-of-possession: DPoP is **optional** for Web V. If a `DPoP` header is present the gateway verifies it; interactive clients SHOULD send DPoP, service tokens MAY omit it. A cluster flag `Gateway:Auth:RequireDpopForInteractive` can make DPoP mandatory later without changing the contract.
- Scope override header: `X-Stella-Scopes` is accepted only in pre-prod/offline bundles or when `Gateway:Auth:AllowScopeHeader=true`; otherwise the request is rejected with `ERR_SCOPE_HEADER_FORBIDDEN`.
- ABAC overlay: evaluated on every tenant-scoped route after RBAC success; failures are hard denies (no fallback). Attribute sources are frozen for Web V as listed below to keep determism.

## Scope
- Gateway header/claim contract for tenant activation and scope validation across Web V endpoints.
- ABAC overlay hooks with Policy Engine (attributes, evaluation order, failure modes).
- Audit emission requirements for auth decisions (RBAC + ABAC).

## Header & Claim Inputs
| Name | Required | Notes |
| --- | --- | --- |
| `Authorization: Bearer <jwt>` | Yes | RS256/ES256; claims: `iss`, `sub`, `aud`, `exp`, `iat`, `nbf`, `jti`, optional `scp` (space-delimited), `ten` (tenant). DPoP proof verified when `DPoP` header present. |
| `DPoP` | Cond. | Proof-of-possession JWS for interactive clients; validated against `htm`/`htu` and access token `jti`. Ignored for service tokens when absent. |
| `X-Stella-Tenant` | Yes | Tenant slug/UUID; must equal `ten` claim when provided. Missing or mismatch → `ERR_TENANT_MISMATCH` (400). |
| `X-Stella-Project` | Cond. | Required for project-scoped routes; otherwise optional. |
| `X-Stella-Scopes` | Cond. | Only honored when `Gateway:Auth:AllowScopeHeader=true`; rejected with 403 otherwise. Value is space-delimited scopes. |
| `X-Stella-Trace-Id` | Optional | If absent the gateway issues a ULID trace id and propagates downstream. |
| `X-Request-Id` | Optional | Echoed for idempotency diagnostics and response envelopes. |

## Processing Rules
1) Validate JWT signature against offline bundle trust roots; `aud` must be one of `stellaops-web` or `stellaops-gateway`; reject on `exp/nbf` drift > 60s.
2) Resolve tenant: prefer `X-Stella-Tenant`, otherwise `ten` claim. Any mismatch → `ERR_TENANT_MISMATCH` (400).
3) Resolve project: from `X-Stella-Project` when route is project-scoped; otherwise null.
4) Build scope set: start from `scp` claim; if `X-Stella-Scopes` is allowed and present, replace the set with its value.
5) RBAC: check required scopes per route (matrix below). Missing scope → `ERR_SCOPE_MISMATCH` (403).
6) ABAC overlay:
   - Attributes: `subject`, `roles`, `org`, `tenant_id`, `project_id`, route vars (e.g., `finding_id`, `policy_id`), and request body keys explicitly listed in the route contract.
   - Order: RBAC allow → ABAC evaluate → deny overrides → allow.
   - Fail closed: on evaluation error or missing attributes return `ERR_ABAC_DENY` (403) with `reason` + `trace_id`.
7) Determinism: tenant header is mandatory; anonymous/implicit tenants are not allowed. Error codes are stable and surfaced in the response envelope.

## Route Scope Matrix (Web V)
- `/risk/*` → `risk:read` for GET, `risk:write` for POST/PUT; severity events additionally require `notify:emit`.
- `/vuln/*` → `vuln:read` for GET, `vuln:write` for mutations; exports require `vuln:export`.
- `/signals/*` → `signals:read` (GET) / `signals:write` (write APIs).
- `/policy/*` simulation/abac → `policy:simulate` (read) or `policy:abac` (overlay hooks).
- `/vex/consensus*` → `vex:read` (stream/read) or `vex:write` when mutating cache.
- `/audit/decisions`, `/tenant/*` → `tenant:admin`.
- Gateway health/info endpoints remain unauthenticated but include `trace_id`.

## Outputs
- Success: downstream context includes `tenant_id`, `project_id`, `subject`, `scopes`, `abac_result`, `trace_id`, `request_id`.
- Failure envelope (deterministic):
  - 401: `ERR_TOKEN_INVALID`, `ERR_TOKEN_EXPIRED`, `ERR_DPOP_INVALID`.
  - 400: `ERR_TENANT_MISSING`, `ERR_TENANT_MISMATCH`.
  - 403: `ERR_SCOPE_MISMATCH`, `ERR_SCOPE_HEADER_FORBIDDEN`, `ERR_ABAC_DENY`.
  Body: `{ "error": {"code": "ERR_SCOPE_MISMATCH", "message": "scope risk:read required"}, "trace_id": "01HXYZ...", "request_id": "abc" }`.

## Audit & Telemetry
- Emit DSSE-wrapped audit record: `{ tenant_id, project_id, subject, scopes, decision, reason_code, trace_id, request_id, route, ts_utc }`.
- Counters: `gateway.auth.success`, `gateway.auth.denied`, `gateway.auth.abac_denied`, `gateway.auth.tenant_missing`, labeled by route and tenant.

## Examples
### Successful read
```bash
curl -H "Authorization: Bearer $TOKEN" \
     -H "DPoP: $PROOF" \
     -H "X-Stella-Tenant: acme-tenant" \
     -H "X-Stella-Trace-Id: 01HXYZABCD1234567890" \
     https://gateway.stellaops.local/risk/status
```

### Scope/ABAC deny
```json
{
  "error": {"code": "ERR_ABAC_DENY", "message": "project scope mismatch"},
  "trace_id": "01HXYZABCD1234567890",
  "request_id": "req-77c4"
}
```