Update documentation to include expanded Infrastructure Admin permissions

docs/infra/infra-admin-permissions.md

@@ -5,6 +5,8 @@ account needs various roles assigned in the appropriate Microsoft Entra ID
 tenant. There are alternate permission setups possible, but this is what has
 been tested.
 
+## Option 1 — Broad Access (Tested)
+
 Scoped to relevant Subscription(s), without conditions limiting their
 application:
 - Owner
@@ -16,4 +18,70 @@ Scoped to the Microsoft Entra ID tenant itself:
 - Cloud Application Administrator, to register the GitHub Actions identity. This
   requirement could be removed by future work[1].
 
+## Option 2 — Least-Privilege with ABAC Conditions
+
+For a more security-conscious setup, the `Owner` and `Role Based Access Control
+Administrator` roles can be granted with an **ABAC condition** that restricts
+which roles the admin can assign to others.
+
+Scoped to relevant Subscription(s), with an ABAC condition limiting role
+assignment to only the roles Terraform requires:
+- Owner *(with ABAC condition, see below)*
+- Key Vault Administrator
+- Role Based Access Control Administrator *(with ABAC condition, see below)*
+- Storage Blob Data Contributor
+
+Scoped to the Microsoft Entra ID tenant itself:
+- Cloud Application Administrator, to register the GitHub Actions identity. This
+  requirement could be removed by future work[1].
+
+### Required ABAC Condition
+
+The ABAC condition must allow assigning only the following roles to service
+principals and managed identities:
+
+| Role | GUID |
+|---|---|
+| `Contributor` | `b24988ac-6180-42a0-ab88-20f7382dd24c` |
+| `Key Vault Secrets Officer` | `b86a8fe4-44ce-4948-aee5-eccb2c155cd7` |
+| `Key Vault Certificates Officer` | `a4417e6f-fecd-4de8-b567-7b0420556985` |
+| `Role Based Access Control Administrator` | `f58310d9-a9f6-439a-9e8d-f62e7b41a168` |
+| `Storage Blob Data Contributor` | `ba92f5b4-2d11-453d-a403-e96b0029c9fe` |
+| `AcrPull` | `7f951dda-4ed3-4680-a7ca-43fe172d538d` |
+| `Key Vault Secrets User` | `4633458b-17de-408a-b874-0445c86b69e6` |
+| `Key Vault Crypto Service Encryption User` | `e147488a-f6f5-4113-8e2d-b22465e65bf6` |
+
+To configure the condition in the Azure Portal:
+> Subscriptions → \<your subscription\> → IAM → Role assignments → find the
+> admin's Owner entry → **Edit condition** → add the role GUIDs above to the
+> allowed list.
+
+### Important: ABAC Condition Must Include All Required Roles
+
+If the ABAC condition is more restrictive than the list above, Terraform will
+fail with a `403 AuthorizationFailed` error during `make infra-set-up-account`.
+In that case, a user with an **unconditional Owner** must either:
+
+- Pre-assign the missing roles to the GitHub Actions service principal manually
+  before running Terraform:
+
+  ```bash
+  SP_OBJECT_ID="<github-actions-service-principal-object-id>"
+  SCOPE="/subscriptions/<subscription-id>"
+
+  for ROLE in \
+    "Contributor" \
+    "Key Vault Secrets Officer" \
+    "Key Vault Certificates Officer" \
+    "Role Based Access Control Administrator" \
+    "Storage Blob Data Contributor"; do
+    az role assignment create \
+      --role "$ROLE" \
+      --assignee "$SP_OBJECT_ID" \
+      --scope "$SCOPE"
+  done
+  ```
+
+- Or broaden the ABAC condition to include all roles in the table above.
+
 [1]: https://github.com/navapbc/template-infra-azure/issues/17.