tegwick
1e1e077b27
TokenReview-based caller identity with audience-scoped tokens and exact resource.system to ServiceAccount bindings, per ops-warden's recommendation. Deletes the unwired tenant-engine live-roles adapter (T03) and adds make verify-posture (T05). Source implements A2; running digest is still A0 until promotion, so tenancy.current.A stays 0. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
65 lines
2.9 KiB
Markdown
65 lines
2.9 KiB
Markdown
# ADR 0004 — authenticate and bind authorization callers
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Status: accepted (source implemented; production promotion pending)
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Date: 2026-08-18
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## Context
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`POST /v1/check` and `/v1/batch_check` accepted an asserted subject, tenant and
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protected-system name from any workload with network reach. NetworkPolicy
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limited reachability but did not establish caller identity. A caller could
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therefore represent another protected system and obtain an authoritative
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decision under the wrong policy package.
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The boundary must authenticate workloads without turning flex-auth into an
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identity issuer, sharing a long-lived secret between services, or coupling
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authorization availability to an unrelated identity-provider round trip.
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## Decision
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Use Kubernetes ServiceAccount tokens with audience `flex-auth`. flex-auth calls
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the Kubernetes TokenReview API through a separately projected reviewer token
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and binds the authenticated ServiceAccount principal to every
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`resource.system` in the request. Both the single and batch endpoints use the
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same choke point; health remains unauthenticated.
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Each deployed policy instance has an explicit, exact binding. For example:
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```text
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tenant-engine=system:serviceaccount:tenant-engine:tenant-engine
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user-engine=system:serviceaccount:user-engine:user-engine
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```
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Unknown systems, missing or invalid tokens, audience mismatch and principal
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mismatch fail closed. TokenReview unavailability returns 503 rather than an
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authorization answer. Tokens and reviewer credentials are re-read rather than
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cached across rotation.
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Three modes support promotion: `disabled`, `warn`, and `enforce`. Warn mode
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records the same authentication failures without logging credentials. It is a
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bounded migration aid, not a conformant steady state. The reviewed desired
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manifests select `enforce`; promotion still follows FLEX-WP-0011 and requires a
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new immutable image digest plus caller rollout evidence.
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## Rejected alternatives
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- NetworkPolicy alone proves network position, not workload identity.
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- A shared header secret has broad replay and rotation blast radius and cannot
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bind a Kubernetes workload principal.
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- Application mTLS would add a separate certificate lifecycle where the
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cluster already has short-lived projected workload identity.
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- Using the caller token itself to invoke TokenReview would grant callers an
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unnecessary API permission. A narrow reviewer ServiceAccount holds only
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`create` on `tokenreviews.authentication.k8s.io`.
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## Consequences
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- Caller identity is bound once at the flex-auth ingress and cannot be swapped
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by changing request JSON.
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- flex-auth depends on the Kubernetes authentication API for uncached checks;
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an outage fails closed with 503.
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- Each new protected system needs an explicit binding and projected caller
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token. There is no wildcard binding.
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- Source and desired state reach A2, while the live declaration remains A0
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until the immutable digest is promoted and probed.
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