feat(security): reject secret last-applied annotations (CUST-WP-0073)

Assistant: codex
Assistant-Model: gpt-6-astra
Assistant-Session: 01a0e7fb-275d-7db0-b1df-39ed943427da

argocd/platform-addons/secret-annotation-guard/README.md

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+# Secret annotation guard
+
+Implemented under the existing CUST-WP-0073-T03; no new platform workplan.
+Native admission policy denies the last-applied annotation on Secret CREATE
+and UPDATE, including an empty value. No new workload or controller.
+
+Before the first manual sync, run `scripts/secret_annotation_maintenance.py`
+on railiance01 through the supervised admin path, first without arguments,
+then with `--clean`. It removes only the duplicate annotation; it never prints
+kubectl output or Secret values. Concurrent Secret churn can stop cleanup;
+inspect the receipt before retrying. Preserve only names, counts and booleans.
+
+Declare the policy through the pinned `secret-annotation-guard` Argo Application;
+automated sync and prune are off. Platform-addons AppProject must allow both
+admission kinds. Sync policy first and inspect typeChecking; bind only after
+cleanup. Test clean CREATE/UPDATE and denied annotated CREATE/UPDATE using
+synthetic data in whitehat; verify client-side apply is denied, then delete
+only the test fixture. Secret writers must use server-side apply or replace.
+
+Rollback is a manual Argo sync of a reviewed revision without the binding
+(with explicit resource-scoped pruning), or attended break-glass deletion of
+the binding followed by Git reconciliation. Removing the binding reopens this
+leak path. The policy does not rotate credentials or isolate agent accounts.

argocd/platform-addons/secret-annotation-guard/kustomization.yaml

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+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - policy.yaml

argocd/platform-addons/secret-annotation-guard/policy.yaml

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+# CUST-WP-0073-T03: Secret annotation guard; manual ArgoCD sync.
+# Owner: railiance-platform. Clean existing annotations in an attended session
+# before binding; otherwise subsequent updates to those Secrets are rejected.
+apiVersion: admissionregistration.k8s.io/v1
+kind: ValidatingAdmissionPolicy
+metadata:
+  name: reject-secret-last-applied
+spec:
+  failurePolicy: Fail
+  matchConstraints:
+    resourceRules:
+      - apiGroups: [""]
+        apiVersions: ["v1"]
+        operations: ["CREATE", "UPDATE"]
+        resources: ["secrets"]
+        scope: "*"
+  validations:
+    - expression: '!has(object.metadata.annotations) || !("kubectl.kubernetes.io/last-applied-configuration" in object.metadata.annotations)'
+      message: "Secret last-applied annotations are forbidden; use server-side apply or replace."
+---
+apiVersion: admissionregistration.k8s.io/v1
+kind: ValidatingAdmissionPolicyBinding
+metadata:
+  name: reject-secret-last-applied
+spec:
+  policyName: reject-secret-last-applied
+  validationActions: [Deny]