@@ -0,0 +1,17 @@
+---
+'@objectstack/lint': patch
+---
+
+fix(lint): `security-owd-unset`'s fix line on a writable external object names the ways out `security-external-owner-unmapped` names, instead of recommending `'private'`
+
+Clause-②: no
+Metadata-protocol: none
+
+`security-owd-unset` refuses a custom object with no `sharingModel`, and its fix line recommended `sharingModel: 'private'` for every such object. On a writable external object (`external.writable: true`) whose ownership column no remote row serves, `'private'` is the shape `security-external-owner-unmapped` warns refuses every member edit: the author followed one rule's advice into the other's warning.
+
+- **Now, on a writable binding.** For exactly the objects `security-external-owner-unmapped` judges (`external.writable: true`, with no declared `owner_id`, or `created_by` on an object without `owner_id`), the fix line reads: declare `sharingModel: 'public_read_write'` and decide who may edit in a `beforeUpdate` hook, or declare the ownership column in `fields` and have the driver return the remote owner under that name, and declare `'private'` or `'public_read'`; `'controlled_by_parent'` for master-detail children. Both clauses are that rule's own wording, including its shorter one when an `external.columnMap` entry already targets the column.
+- **Now, on a read-only binding.** An external object that no member writes (`external.writable` not `true`, or a datasource this stack declares with `external.allowWrites: false`) and whose `owner_id` no declared field or `external.columnMap` entry names is told to declare `sharingModel: 'public_read'`: on `'private'` a member reads none of its rows.
+- **Unchanged.** The severity (`error`) and the verdict line. Every other custom object, and an external object whose owner is provable, keeps the `'private'` recommendation.
+- `os explain security-owd-unset` explains both external cases and points at `os explain security-external-owner-unmapped`.
+
+The same fix line reaches the runtime publish gate's 422 issue for an `object` write. That gate sees no `datasources`, so a writable binding on an `allowWrites: false` datasource gets the writable-binding line there; neither door recommends `'private'` for it.
@@ -114,6 +114,18 @@ const SECURITY_OWD_UNSET_EXPLANATION: RuleExplanation = {
       'default); \'public_read\' — everyone reads, the owner writes; \'public_read_write\' — everyone ' +
       'reads and writes; \'controlled_by_parent\' — access derives from the master record (a ' +
       'master-detail child).',
+    'An external object with no provable owner is the exception to that recommendation. On a writable ' +
+      'binding (`external.writable: true`) the remote row carries no `owner_id` or `created_by` the platform can read, so on \'private\' every ' +
+      'member edit is refused, and where the object carries `owner_id` a member reads no row at all ' +
+      '(`os explain security-external-owner-unmapped`). There the fix line names that rule\'s two ways out ' +
+      'instead: \'public_read_write\' with a `beforeUpdate` hook that decides who may edit, or the ownership ' +
+      'column (`owner_id`, or `created_by` on an object without one) declared in `fields` and returned by ' +
+      'the driver, after which \'private\' or \'public_read\' admits edits by the user that column names. ' +
+      'A read-only external binding (`external.writable` not `true`, or a datasource declared with ' +
+      '`external.allowWrites: false`) whose `owner_id` no declared field or `external.columnMap` entry names ' +
+      'is told to declare \'public_read\' instead: no member writes it, and on \'private\' a member reads none ' +
+      'of its rows. An external object whose owner is provable (a declared field, or on a read-only binding ' +
+      'a `columnMap` entry onto `owner_id`) keeps the usual fix line.',
     'System objects (`isSystem: true`, or a name starting with `sys_`) are not judged: the platform ' +
       'owns their posture.',
   ],
 
@@ -4,9 +4,13 @@ import { describe, expect, it } from 'vitest';
 import { defineStack } from '@objectstack/spec';
 import { runAuthoringRules, splitBySeverity } from './authoring-rules.js';
 import {
+  PUBLIC_READ_WRITE_WAY_OUT,
+  READ_ONLY_EXTERNAL_WAY_OUT,
   SECURITY_EXTERNAL_OWNER_UNMAPPED,
+  declareOwnershipColumnWayOut,
   validateExternalOwnerSharing,
 } from './validate-external-owner-sharing.js';
+import { SECURITY_OWD_UNSET, validateSecurityPosture } from './validate-security-posture.js';
 
 /**
  * The two external objects of `packages/runtime/src/plugin-driver-external-object.spike.test.ts`,
@@ -220,3 +224,88 @@ describe('security-external-owner-unmapped — the fix line', () => {
     }
   });
 });
+
+// [#27472] `security-owd-unset` refuses a custom object with no sharingModel, and its fix line used
+// to recommend 'private' everywhere: on a writable external object that is the shape this rule warns
+// refuses every member edit. Its fix line for the objects this rule judges is built from this
+// rule's own two ways out; every other object keeps the 'private' recommendation.
+describe("security-owd-unset's fix line on an external object", () => {
+  const owdUnsetHint = (stack: Record<string, unknown>, where: string) => {
+    const findings = validateSecurityPosture(stack).filter((f) => f.rule === SECURITY_OWD_UNSET && f.where === where);
+    expect(findings, where).toHaveLength(1);
+    return findings[0]!.hint ?? '';
+  };
+  const OLD_HINT =
+    "declare sharingModel: 'private' (owner + shares; recommended), 'public_read', " +
+    "'public_read_write', or 'controlled_by_parent' (master-detail children)";
+
+  it("spike_owned gets this rule's ways out and no 'private' recommendation", () => {
+    const hint = owdUnsetHint(SPIKE_STACK, 'object "spike_owned"');
+    expect(hint).toContain(PUBLIC_READ_WRITE_WAY_OUT);
+    expect(hint).toContain(declareOwnershipColumnWayOut('owner_id'));
+    expect(hint).not.toContain('recommended');
+    expect(hint).not.toContain('\n');
+    // Severity and verdict are the rule's own and do not move.
+    const [finding] = validateSecurityPosture(SPIKE_STACK).filter((f) => f.where === 'object "spike_owned"');
+    expect(finding).toMatchObject({ severity: 'error', path: 'objects[1].sharingModel' });
+    expect(finding!.message).toBe(
+      "custom object declares no sharingModel (OWD); the runtime falls back to 'private', " +
+        'but the baseline must be an authored decision',
+    );
+  });
+
+  it('a managed custom object keeps the private recommendation', () => {
+    expect(owdUnsetHint({ objects: [{ name: 'leave_request', label: 'Leave Request' }] }, 'object "leave_request"')).toBe(
+      OLD_HINT,
+    );
+  });
+
+  it('names created_by on an external object without owner_id', () => {
+    const hint = owdUnsetHint({ objects: [ext({ ownership: 'none' })] }, 'object "remote_row"');
+    expect(hint).toContain(declareOwnershipColumnWayOut('created_by'));
+    expect(hint).not.toContain('recommended');
+  });
+
+  it('words the mapped case as this rule does: only the declaration is missing', () => {
+    const hint = owdUnsetHint({ objects: [ext({}, { columnMap: { assignee: 'owner_id' } })] }, 'object "remote_row"');
+    expect(hint).toContain(declareOwnershipColumnWayOut('owner_id', true));
+    expect(hint).toContain(PUBLIC_READ_WRITE_WAY_OUT);
+    expect(hint).not.toContain('recommended');
+  });
+
+  // [#27425] On 'private', an external object whose owner_id is the platform's unprovable anchor
+  // reads no row for a member, whatever its writability: a read-only binding is told 'public_read'.
+  it("recommends 'public_read' on a read-only binding with an unprovable owner", () => {
+    const readOnlyDatasource = { name: 'remote', schemaMode: 'external', external: { allowWrites: false } };
+    const { writable: _writable, ...noWritable } = ext().external;
+    for (const [label, stack] of [
+      ['writable: false', { objects: [ext({}, { writable: false })] }],
+      ['writable unset', { objects: [{ ...ext(), external: noWritable }] }],
+      ['allowWrites: false', { datasources: [readOnlyDatasource], objects: [ext()] }],
+    ] as const) {
+      const hint = owdUnsetHint(stack, 'object "remote_row"');
+      expect(hint, label).toBe(READ_ONLY_EXTERNAL_WAY_OUT);
+      expect(hint, label).not.toContain('recommended');
+    }
+  });
+
+  it('keeps the private recommendation where the owner is provable', () => {
+    const ownerField = { ...NAME_FIELD, owner_id: { type: 'lookup', reference: 'sys_user', label: 'Owner' } };
+    for (const [label, stack] of [
+      ['declared owner_id, writable', { objects: [ext({ fields: ownerField })] }],
+      ['declared owner_id, read-only', { objects: [ext({ fields: ownerField }, { writable: false })] }],
+      ['columnMap onto owner_id, read-only', { objects: [ext({}, { writable: false, columnMap: { assignee: 'owner_id' } })] }],
+    ] as const) {
+      expect(owdUnsetHint(stack, 'object "remote_row"'), label).toBe(OLD_HINT);
+    }
+  });
+
+  it('either way out it names, once taken, clears this rule too', () => {
+    const ownerField = { ...NAME_FIELD, owner_id: { type: 'lookup', reference: 'sys_user', label: 'Owner' } };
+    for (const obj of [ext({ sharingModel: 'public_read_write' }), ext({ sharingModel: 'private', fields: ownerField })]) {
+      const stack = { objects: [obj] };
+      expect(validateSecurityPosture(stack).map((f) => f.rule)).toEqual([]);
+      expect(validateExternalOwnerSharing(stack)).toEqual([]);
+    }
+  });
+});
@@ -37,9 +37,11 @@
  * column to `owner_id` changes nothing, and the driver must return the field
  * itself. So the author's vouch this rule reads is a DECLARED field — the same
  * provenance the runtime guards read (`resolveInjectedColumnProvenance`,
- * `@objectstack/spec/data`) — never a `columnMap` entry alone. A declared
- * `owner_id` also scopes reads by owner; the platform's own injected one does
- * not (plugin-sharing treats it as unstored and leaves reads unscoped).
+ * `@objectstack/spec/data`) — never a `columnMap` entry alone. Reads follow
+ * the same anchor: a declared `owner_id` scopes them by owner, while the
+ * platform's own injected one, with no `columnMap` entry naming it, proves no
+ * owner, so on an owner-scoped model a member's read scope admits no row
+ * (plugin-sharing `hasPhantomOwnerAnchor`, #27425) and an edit answers 404.
  *
  * ## The two arms
  *
@@ -99,9 +101,37 @@ type AnyRec = Record<string, unknown>;
 const OWNER_COLUMN = 'owner_id';
 const CREATOR_COLUMN = 'created_by';
 
-const PUBLIC_READ_WRITE_WAY_OUT =
+/**
+ * The two ways out this rule names, as fix-line clauses. `security-owd-unset`
+ * words its fix line for the same objects from these, so an author following
+ * either rule's advice lands on a shape the other does not refuse.
+ */
+export const PUBLIC_READ_WRITE_WAY_OUT =
   "declare sharingModel: 'public_read_write' and decide who may edit in a beforeUpdate hook";
 
+/**
+ * The declaration way out, for the column {@link externalOwnershipGap} names.
+ * `mapped`: an `external.columnMap` entry already targets the column, so what
+ * is missing is only the declared field.
+ */
+export function declareOwnershipColumnWayOut(column: string, mapped = false): string {
+  const role = column === OWNER_COLUMN ? 'owner' : 'creator';
+  if (mapped) return `declare ${column} in fields (a plugin driver must return the remote ${role} under that name itself)`;
+  return (
+    `declare ${column} in fields and have the driver return the remote ${role} under that name ` +
+    '(a built-in SQL driver does it through external.columnMap; a plugin driver must return it itself)'
+  );
+}
+
+/**
+ * The fix-line clause `security-owd-unset` gives a read-only external binding
+ * whose owner anchor is unprovable ({@link hasUnprovableOwnerAnchor}): no
+ * member writes it, and on 'private' a member reads none of its rows.
+ */
+export const READ_ONLY_EXTERNAL_WAY_OUT =
+  "declare sharingModel: 'public_read' (no member writes a read-only binding, and on 'private' a member " +
+  'reads no row: no declared field or external.columnMap entry names the remote owner_id)';
+
 /** Does `external.columnMap` (remote column → local field) map some remote column to `field`? */
 function columnMapTargets(external: AnyRec, field: string): boolean {
   const map = external.columnMap;
@@ -129,50 +159,83 @@ function undeclaredOwnershipColumn(obj: AnyRec): { column: string; injected: boo
  * declares no ownership column a remote row serves. Returns the findings
  * (empty = clean); the CLI folds them in as advisory warnings.
  */
-export function validateExternalOwnerSharing(stack: AnyRec): ExternalOwnerSharingFinding[] {
-  const out: ExternalOwnerSharingFinding[] = [];
-  if (!stack || typeof stack !== 'object') return out;
-  const objects = recordsOf(stack.objects);
-  // ADR-0015's datasource half of the double write opt-in: a datasource this
-  // stack declares with `external.allowWrites: false` (also the parsed
-  // default) lets no write through, so no ownership check is what refuses.
-  const readOnlyDatasources = new Set(
+/**
+ * ADR-0015's datasource half of the double write opt-in: the datasources this
+ * stack declares with `external.allowWrites: false` (also the parsed default)
+ * let no write through, so no ownership check is what refuses.
+ */
+export function readOnlyExternalDatasources(stack: AnyRec): ReadonlySet<unknown> {
+  return new Set(
     recordsOf(stack.datasources)
       .filter((ds) => {
         const ext = ds.external;
         return !!ext && typeof ext === 'object' && (ext as AnyRec).allowWrites === false;
       })
       .map((ds) => ds.name),
   );
+}
+
+/**
+ * The ownership column whose missing storage refuses every member write on
+ * this object under an owner-keyed sharing model — or `undefined` when the
+ * object is not a writable external binding, sits on a datasource that lets
+ * no write through, or declares the column the check reads. `mapped`: an
+ * `external.columnMap` entry targets the column, so only the declaration is
+ * missing. The sharing model itself is the caller's to judge.
+ */
+export function externalOwnershipGap(
+  obj: AnyRec,
+  readOnlyDatasources: ReadonlySet<unknown>,
+): { column: string; injected: boolean; mapped: boolean } | undefined {
+  const external = obj.external;
+  if (!external || typeof external !== 'object' || Array.isArray(external)) return undefined;
+  if ((external as AnyRec).writable !== true) return undefined;
+  if (readOnlyDatasources.has(obj.datasource)) return undefined;
+  const undeclared = undeclaredOwnershipColumn(obj);
+  if (!undeclared) return undefined;
+  return { ...undeclared, mapped: columnMapTargets(external as AnyRec, undeclared.column) };
+}
+
+/**
+ * [#27425] The lint's reading of plugin-sharing's `hasPhantomOwnerAnchor`: an
+ * external object whose `owner_id` is the platform's injected anchor (no
+ * declared field) and no `external.columnMap` entry targets it. No owner is
+ * provable for such an object, so record sharing admits no row by ownership on
+ * an owner-scoped model, whatever the binding's writability.
+ */
+export function hasUnprovableOwnerAnchor(obj: AnyRec): boolean {
+  const external = obj.external;
+  if (!external || typeof external !== 'object' || Array.isArray(external)) return false;
+  if (columnMapTargets(external as AnyRec, OWNER_COLUMN)) return false;
+  return resolveInjectedColumnProvenance(obj, OWNER_COLUMN) === 'injected-unprovisioned';
+}
+
+export function validateExternalOwnerSharing(stack: AnyRec): ExternalOwnerSharingFinding[] {
+  const out: ExternalOwnerSharingFinding[] = [];
+  if (!stack || typeof stack !== 'object') return out;
+  const objects = recordsOf(stack.objects);
+  const readOnlyDatasources = readOnlyExternalDatasources(stack);
 
   objects.forEach((obj, i) => {
-    const external = obj.external;
-    if (!external || typeof external !== 'object' || Array.isArray(external)) return;
-    if ((external as AnyRec).writable !== true) return;
-    if (readOnlyDatasources.has(obj.datasource)) return;
     if (effectiveSharingModelOf(obj) === 'public') return;
-    const undeclared = undeclaredOwnershipColumn(obj);
+    const undeclared = externalOwnershipGap(obj, readOnlyDatasources);
     if (!undeclared) return;
 
-    const { column } = undeclared;
+    const { column, mapped } = undeclared;
     const name = typeof obj.name === 'string' ? obj.name : `(object ${i})`;
     const model = typeof obj.sharingModel === 'string' ? `'${obj.sharingModel}'` : "'private' (the default)";
     const check =
       column === OWNER_COLUMN
         ? 'record sharing reads owner_id'
         : 'with no owner_id, the ownership floor reads created_by == the caller';
-    const mapped = columnMapTargets(external as AnyRec, column);
-    const role = column === OWNER_COLUMN ? 'owner' : 'creator';
     // [#27425] On `private`, record sharing also scopes READS by owner_id, and
     // an owner it cannot read admits no row: the edit's pre-image read finds
     // none, so the refusal is a 404 rather than the write check's 403.
     const refusal =
       column === OWNER_COLUMN && effectiveSharingModelOf(obj) === 'private'
         ? "(404: the member's read scope admits no row)"
         : '(403)';
-    const declareWayOut =
-      `declare ${column} in fields and have the driver return the remote ${role} under that name ` +
-      '(a built-in SQL driver does it through external.columnMap; a plugin driver must return it itself)';
+    const declareWayOut = declareOwnershipColumnWayOut(column, mapped);
 
     out.push({
       severity: 'warning',
@@ -191,8 +254,7 @@ export function validateExternalOwnerSharing(stack: AnyRec): ExternalOwnerSharin
           `${check} before every edit and delete, so every member write is refused ${refusal} unless an app-authored ` +
           'row-level policy admits the row',
       hint: mapped
-        ? `Declare ${column} in fields (a plugin driver must return the remote ${role} under that name itself), ` +
-          `or ${PUBLIC_READ_WRITE_WAY_OUT}`
+        ? `D${declareWayOut.slice(1)}, or ${PUBLIC_READ_WRITE_WAY_OUT}`
         : `Either ${PUBLIC_READ_WRITE_WAY_OUT}, or ${declareWayOut}`,
     });
   });
 
@@ -145,6 +145,10 @@ describe('validateSecurityPosture (ADR-0090 D7)', () => {
     const text = explanation!.paragraphs.join('\n');
     expect(text).toContain('ADR-0090 D1');
     expect(text).toContain("read and edit every other user's records");
+    // [#27472] The writable-external exception to the 'private' recommendation, and the rule it defers to.
+    expect(text).toContain('external.writable: true');
+    expect(text).toContain('os explain security-external-owner-unmapped');
+    expect(text).toContain("is told to declare 'public_read' instead");
     expect(finding.message).not.toContain('incident');
   });
 
@@ -1792,6 +1796,8 @@ describe('validateSecurityPosture — reads only keys the spec declares (meta-te
       'cbpDetails', // #26591: the resolving details by name — JS Map methods.
       'policies', // one set's `rowLevelSecurity` records — `.length`, a JS property.
       'predicate', // a policy's `using` / `check` text, a string — `.trim`.
+      // #27472: `externalOwnershipGap`'s answer — `.column`, a field of that helper's own return value.
+      'ownershipGap',
     ]);
     expect(receivers.filter((r) => !tabled.has(r) && !PLUMBING.has(r))).toEqual([]);
   });