@@ -0,0 +1,13 @@
+---
+'@objectstack/plugin-security': patch
+---
+
+fix(plugin-security): a `controlled_by_parent` detail write asks its master the object-level question the master's own write door asks (ADR-0055 §3)
+
+Clause-②: no
+
+ADR-0055 §3 says editing, deleting or creating a detail requires edit access to its master. The master-edit gate's object-level leg asked only for an object-level `update` grant on the master. It now asks what the master's own write door asks of an `update` on the master, with that door's inputs: the master's security posture must resolve, the capabilities the master requires for `update` (ADR-0066 D3) must be held, and the grant is asked under the master's posture (ADR-0066 D2). A principal the master's own write door refuses at its object gate can no longer write a detail under that master.
+
+- **Every door that asks the leg.** An insert, a by-id update and a by-id delete of a detail answer `PERMISSION_DENIED` (403) and write nothing. `checkControlledByParentWrite`, which the comment and attachment parent gates ask, answers `deny` on the `object_permission` leg. A record-grained `security/explain` of a by-id write follows the same leg.
+- **An insert answers before the reference check.** For such a principal, an insert under a master id that exists answers the same refusal as one under an id that exists nowhere. A principal that also cannot read the master, on an on-behalf-of write too, now gets this `PERMISSION_DENIED` on insert where it got the reference check's `VALIDATION_FAILED`. An insert under a master object that does not resolve answers `PERMISSION_DENIED` where it answered `OBJECT_NOT_FOUND` (404).
+- **Unchanged.** A principal the master's own write door admits for an `update` keeps writing its details, and so do a super-user grant and a system context. Under a master that is `public`, resolves and requires no capability for `update`, every principal gets the answer it got before.
@@ -0,0 +1,380 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * [ADR-0055 §3] Writing a `controlled_by_parent` detail requires EDIT access to
+ * its master, and the master's OBJECT-level edit admission is the one its own
+ * write door asks.
+ *
+ * §3: "editing/deleting/creating a detail requires edit access to its master".
+ * The master-edit gate's object-level leg is therefore the master write door's
+ * object gate, asked of an `update` on the master with that door's own inputs:
+ *
+ *  - the master's posture resolves, and an unresolvable one refuses;
+ *  - the capabilities the master requires for `update` are held;
+ *  - the CRUD grant is asked under the master's posture.
+ *
+ * Pinned at every door that asks the leg: an insert, a by-id update and a
+ * by-id delete through the engine, and the security service's master-detail
+ * write check (`checkControlledByParentWrite`) that the parent gates of other
+ * subsystems ask. Each refusal is the leg's own, `object_permission`, and
+ * writes nothing.
+ *
+ * The controls hold the change to exactly that rule: a principal the master's
+ * own door admits for an `update` keeps writing its details, and so does a
+ * super-user grant and a master that carries none of the three inputs.
+ *
+ * Real stack: `ObjectQL` over better-sqlite3 and the real `SecurityPlugin`.
+ */
+
+import { describe, it, expect, vi, beforeAll, afterAll } from 'vitest';
+import { AsyncLocalStorage } from 'node:async_hooks';
+import { ObjectQL } from '@objectstack/objectql';
+import { SqlDriver } from '@objectstack/driver-sql';
+import { PermissionSetSchema } from '@objectstack/spec/security';
+import type { ControlledByParentWriteOutcome } from '@objectstack/spec/contracts';
+import { SysOrganization, SysUser, SysMember } from '@objectstack/platform-objects/identity';
+
+import { SysPosition, SysPermissionSet, SysPositionPermissionSet } from './__tests__/legacy-catalog-objects.testkit.js';
+import { SysUserPosition } from './objects/sys-user-position.object.js';
+import { SysUserPermissionSet } from './objects/sys-user-permission-set.object.js';
+import { SecurityPlugin } from './security-plugin.js';
+import { defaultPermissionSets } from './objects/default-permission-sets.js';
+
+const SYS = { context: { isSystem: true } } as never;
+const ORG = 'org_cbp_mow';
+const ME = 'u_cbp_mow_me';
+
+/** A master whose posture its own door asks. */
+const MASTER_POSTURE = 'qa_cbp_mow_master_posture';
+/** A master that requires a capability for an update. */
+const MASTER_CAPABILITY = 'qa_cbp_mow_master_capability';
+/** A master that carries none of the three inputs. */
+const MASTER_PLAIN = 'qa_cbp_mow_master_plain';
+/** A master name no registry resolves. */
+const MASTER_UNRESOLVED = 'qa_cbp_mow_master_unresolved';
+
+const DETAIL_POSTURE = 'qa_cbp_mow_detail_posture';
+const DETAIL_CAPABILITY = 'qa_cbp_mow_detail_capability';
+const DETAIL_PLAIN = 'qa_cbp_mow_detail_plain';
+const DETAIL_UNRESOLVED = 'qa_cbp_mow_detail_unresolved';
+
+const CAPABILITY = 'qa_cbp_mow_capability';
+const CRUD = { allowRead: true, allowCreate: true, allowEdit: true, allowDelete: true } as const;
+
+const SET = {
+  readMasters: 'qa_cbp_mow_read_masters',
+  broad: 'qa_cbp_mow_broad',
+  superUser: 'qa_cbp_mow_super_user',
+  editPostureMaster: 'qa_cbp_mow_edit_posture_master',
+  editCapabilityMaster: 'qa_cbp_mow_edit_capability_master',
+  capabilityHolder: 'qa_cbp_mow_capability_holder',
+} as const;
+
+type Outcome = { kind: 'landed' } | { kind: 'refused'; code?: string; status?: number };
+
+const outcomeOf = (p: Promise<unknown>): Promise<Outcome> =>
+  p.then(
+    () => ({ kind: 'landed' as const }),
+    (e: unknown) => {
+      const x = e as { code?: unknown; status?: unknown; statusCode?: unknown } | null;
+      const status = x?.statusCode ?? x?.status;
+      return {
+        kind: 'refused' as const,
+        code: x?.code == null ? undefined : String(x.code),
+        status: status == null ? undefined : Number(status),
+      };
+    },
+  );
+
+const REFUSED = { kind: 'refused', code: 'PERMISSION_DENIED', status: 403 } as const;
+const LEG_REFUSAL: ControlledByParentWriteOutcome = { outcome: 'deny', leg: 'object_permission' };
+
+/** Release a torn-down rig's async storages (see the explain-enforce-parity suite). */
+const releaseAsyncContexts = (...owners: object[]): void => {
+  for (const owner of owners) {
+    for (const value of Object.values(owner)) if (value instanceof AsyncLocalStorage) value.disable();
+  }
+};
+
+const textField = (name: string) => ({ name, type: 'text' });
+const masterOf = (name: string, extra: Record<string, unknown> = {}) => ({
+  name,
+  label: name,
+  sharingModel: 'public_read_write',
+  ...extra,
+  fields: { id: { name: 'id', type: 'text', primaryKey: true }, name: textField('name') },
+});
+const detailOf = (name: string, master: string) => ({
+  name,
+  label: name,
+  sharingModel: 'controlled_by_parent',
+  fields: {
+    id: { name: 'id', type: 'text', primaryKey: true },
+    note: textField('note'),
+    master: { name: 'master', type: 'master_detail', required: true, reference: master },
+  },
+});
+
+async function boot() {
+  const engine = new ObjectQL();
+  engine.registerDriver(
+    new SqlDriver({ client: 'better-sqlite3', connection: { filename: ':memory:' }, useNullAsDefault: true }) as never,
+    true,
+  );
+  await engine.init();
+  engine.registerApp({
+    id: 'com.objectstack.qa.cbp-master-object-write',
+    name: 'A controlled_by_parent detail is written only by a principal its master object admits for an update',
+    version: '1.0.0',
+    type: 'plugin',
+    scope: 'system',
+    objects: [
+      SysPosition, SysPermissionSet, SysPositionPermissionSet, SysUserPosition, SysUserPermissionSet,
+      SysOrganization, SysUser, SysMember,
+      masterOf(MASTER_POSTURE, { access: { default: 'private' } }),
+      masterOf(MASTER_CAPABILITY, { requiredPermissions: { update: [CAPABILITY] } }),
+      masterOf(MASTER_PLAIN),
+      detailOf(DETAIL_POSTURE, MASTER_POSTURE),
+      detailOf(DETAIL_CAPABILITY, MASTER_CAPABILITY),
+      detailOf(DETAIL_PLAIN, MASTER_PLAIN),
+      detailOf(DETAIL_UNRESOLVED, MASTER_UNRESOLVED),
+    ],
+  } as never);
+  await engine.syncSchemas();
+
+  const e = engine as unknown as {
+    insert: (o: string, row: Record<string, unknown>, opts: never) => Promise<unknown>;
+    getDriver: (o: string) => { create: (o: string, row: Record<string, unknown>) => Promise<unknown> };
+    logger: { warn: () => void };
+  };
+  await e.insert('sys_organization', { id: ORG, name: ORG, slug: ORG }, SYS);
+  await e.insert('sys_user', { id: ME, name: ME, email: `${ME}@example.test` }, SYS);
+  await e.insert('sys_member', { id: `mem_${ME}`, user_id: ME, organization_id: ORG, role: 'member' }, SYS);
+
+  const sets = [
+    ...defaultPermissionSets,
+    PermissionSetSchema.parse({
+      name: SET.readMasters,
+      objects: {
+        [MASTER_POSTURE]: { allowRead: true },
+        [MASTER_CAPABILITY]: { allowRead: true },
+        [MASTER_PLAIN]: { allowRead: true },
+      },
+    }),
+    PermissionSetSchema.parse({ name: SET.broad, objects: { '*': CRUD } }),
+    PermissionSetSchema.parse({
+      name: SET.superUser,
+      objects: { '*': { ...CRUD, viewAllRecords: true, modifyAllRecords: true } },
+    }),
+    PermissionSetSchema.parse({
+      name: SET.editPostureMaster,
+      objects: { [MASTER_POSTURE]: { allowRead: true, allowEdit: true }, [DETAIL_POSTURE]: CRUD },
+    }),
+    PermissionSetSchema.parse({
+      name: SET.editCapabilityMaster,
+      objects: { [MASTER_CAPABILITY]: { allowRead: true, allowEdit: true }, [DETAIL_CAPABILITY]: CRUD },
+    }),
+    PermissionSetSchema.parse({
+      name: SET.capabilityHolder,
+      objects: { [MASTER_CAPABILITY]: { allowRead: true, allowEdit: true }, [DETAIL_CAPABILITY]: CRUD },
+      systemPermissions: [CAPABILITY],
+    }),
+  ];
+
+  for (const master of [MASTER_POSTURE, MASTER_CAPABILITY, MASTER_PLAIN]) {
+    await e.insert(master, { id: `${master}_row`, name: 'row' }, SYS);
+  }
+
+  let security: { checkControlledByParentWrite: (o: string, id: string, c: unknown) => Promise<ControlledByParentWriteOutcome> } | undefined;
+  const services: Record<string, unknown> = {
+    manifest: { register: vi.fn() },
+    objectql: engine,
+    metadata: {
+      get: async (type: string, name: string) =>
+        type === 'permission' ? sets.find((s: { name?: string }) => s.name === name) ?? null : engine.getSchema(name) ?? null,
+      list: async () => sets,
+    },
+    tenancy: { posture: 'single' },
+  };
+  const ctx = {
+    logger: { info: vi.fn(), warn: vi.fn(), error: vi.fn(), debug: vi.fn() },
+    registerService: (name: string, svc: unknown) => {
+      if (name === 'security') security = svc as typeof security;
+      else services[name] = svc;
+    },
+    hook: () => undefined,
+    getService: (name: string) => {
+      if (!(name in services)) throw new Error(`service not registered: ${name}`);
+      return services[name];
+    },
+  };
+  const plugin = new SecurityPlugin({ fallbackPermissionSet: 'member_default' });
+  await plugin.init(ctx as never);
+  await plugin.start(ctx as never);
+  // The row gates log the writes they refuse.
+  vi.spyOn(e.logger, 'warn').mockImplementation(() => undefined);
+
+  let n = 0;
+  return {
+    engine,
+    /** A stored detail under `master`, written as SYSTEM (seed replay skips the reference check). */
+    seedDetail: async (detail: string, master: string) => {
+      const id = `${detail}_seed_${n++}`;
+      await e.getDriver(detail).create(detail, { id, note: 'seeded', master });
+      return id;
+    },
+    stored: async (object: string, id: string) =>
+      ((await engine.findOne(object, { where: { id }, context: { isSystem: true } } as never)) ?? null) as Record<string, unknown> | null,
+    checkWrite: (detail: string, id: string, context: unknown) =>
+      security!.checkControlledByParentWrite(detail, id, { ...(context as object) }),
+    teardown: async () => {
+      try { await engine.destroy(); } catch { /* noop */ }
+      releaseAsyncContexts(engine, plugin);
+    },
+  };
+}
+
+type Rig = Awaited<ReturnType<typeof boot>>;
+
+const member = (...permissions: string[]) => ({ userId: ME, positions: [], permissions, posture: 'MEMBER' });
+
+let insertSeq = 0;
+
+/** Insert, by-id update and by-id delete of one detail under `masterId`, and what each left stored. */
+async function writeDetail(rig: Rig, detail: string, masterId: string, context: unknown) {
+  const inserted = `${detail}_insert_${insertSeq++}`;
+  const insert = await outcomeOf(
+    rig.engine.insert(detail, { id: inserted, note: '', master: masterId }, { context: { ...(context as object) } } as never),
+  );
+  const updated = await rig.seedDetail(detail, masterId);
+  const update = await outcomeOf(
+    rig.engine.update(detail, { note: 'written' }, { where: { id: updated }, context: { ...(context as object) } } as never),
+  );
+  const deleted = await rig.seedDetail(detail, masterId);
+  const del = await outcomeOf(rig.engine.delete(detail, { where: { id: deleted }, context: { ...(context as object) } } as never));
+  return {
+    insert,
+    update,
+    delete: del,
+    insertedStored: (await rig.stored(detail, inserted)) !== null,
+    updatedNote: (await rig.stored(detail, updated))?.note,
+    deletedStored: (await rig.stored(detail, deleted)) !== null,
+    check: await rig.checkWrite(detail, await rig.seedDetail(detail, masterId), context),
+  };
+}
+
+const masterUpdate = (rig: Rig, master: string, context: unknown) =>
+  outcomeOf(
+    rig.engine.update(master, { name: 'written' }, { where: { id: `${master}_row` }, context: { ...(context as object) } } as never),
+  );
+
+describe("a controlled_by_parent detail is not written by a principal its master's own write door refuses at the object gate", () => {
+  let rig: Rig;
+  beforeAll(async () => { rig = await boot(); });
+  afterAll(async () => { await rig.teardown(); });
+
+  describe("on the master's posture", () => {
+    const context = member(SET.readMasters, SET.broad);
+
+    it("the master's own write door refuses the update, and the principal reads the master (the precondition)", async () => {
+      expect(await masterUpdate(rig, MASTER_POSTURE, context)).toMatchObject(REFUSED);
+      const read = await rig.engine.findOne(MASTER_POSTURE, { where: { id: `${MASTER_POSTURE}_row` }, context: { ...context } } as never);
+      expect(read).toBeTruthy();
+    });
+
+    it('insert, by-id update and by-id delete are refused, and nothing is written', async () => {
+      const got = await writeDetail(rig, DETAIL_POSTURE, `${MASTER_POSTURE}_row`, context);
+      expect(got.insert).toMatchObject(REFUSED);
+      expect(got.update).toMatchObject(REFUSED);
+      expect(got.delete).toMatchObject(REFUSED);
+      expect(got.insertedStored).toBe(false);
+      expect(got.updatedNote).toBe('seeded');
+      expect(got.deletedStored).toBe(true);
+    });
+
+    it('the master-detail write check refuses on the object_permission leg', async () => {
+      const id = await rig.seedDetail(DETAIL_POSTURE, `${MASTER_POSTURE}_row`);
+      expect(await rig.checkWrite(DETAIL_POSTURE, id, context)).toEqual(LEG_REFUSAL);
+    });
+
+    it('an insert naming a master id that exists nowhere answers the same refusal', async () => {
+      expect(
+        await outcomeOf(
+          rig.engine.insert(DETAIL_POSTURE, { id: 'qa_cbp_mow_dangling', note: '', master: 'qa_cbp_mow_no_such_master' }, { context: { ...context } } as never),
+        ),
+      ).toMatchObject(REFUSED);
+    });
+  });
+
+  describe('on a capability the master requires for an update', () => {
+    const context = member(SET.editCapabilityMaster);
+
+    it("the master's own write door refuses the update (the precondition)", async () => {
+      expect(await masterUpdate(rig, MASTER_CAPABILITY, context)).toMatchObject(REFUSED);
+    });
+
+    it('insert, by-id update and by-id delete are refused, and nothing is written', async () => {
+      const got = await writeDetail(rig, DETAIL_CAPABILITY, `${MASTER_CAPABILITY}_row`, context);
+      expect(got.insert).toMatchObject(REFUSED);
+      expect(got.update).toMatchObject(REFUSED);
+      expect(got.delete).toMatchObject(REFUSED);
+      expect(got.insertedStored).toBe(false);
+      expect(got.updatedNote).toBe('seeded');
+      expect(got.deletedStored).toBe(true);
+      expect(got.check).toEqual(LEG_REFUSAL);
+    });
+  });
+
+  describe("on a master whose posture does not resolve", () => {
+    const context = member(SET.broad);
+
+    it('an insert is refused, and nothing is written', async () => {
+      const got = await outcomeOf(
+        rig.engine.insert(DETAIL_UNRESOLVED, { id: 'qa_cbp_mow_unresolved_insert', note: '', master: 'qa_cbp_mow_any' }, { context: { ...context } } as never),
+      );
+      expect(got).toMatchObject(REFUSED);
+      expect(await rig.stored(DETAIL_UNRESOLVED, 'qa_cbp_mow_unresolved_insert')).toBeNull();
+    });
+
+    it('the master-detail write check refuses on the object_permission leg', async () => {
+      const id = await rig.seedDetail(DETAIL_UNRESOLVED, 'qa_cbp_mow_any');
+      expect(await rig.checkWrite(DETAIL_UNRESOLVED, id, context)).toEqual(LEG_REFUSAL);
+    });
+  });
+});
+
+describe("a principal its master's own write door admits for an update keeps writing the master's details", () => {
+  let rig: Rig;
+  beforeAll(async () => { rig = await boot(); });
+  afterAll(async () => { await rig.teardown(); });
+
+  const LANDS = { insert: { kind: 'landed' }, update: { kind: 'landed' }, delete: { kind: 'landed' } } as const;
+  const WRITTEN = { insertedStored: true, updatedNote: 'written', deletedStored: false, check: { outcome: 'allow' } } as const;
+
+  const CONTROLS = [
+    { label: 'an object-level edit grant on the master', context: member(SET.editPostureMaster), master: MASTER_POSTURE, detail: DETAIL_POSTURE },
+    { label: 'a super-user grant', context: member(SET.superUser), master: MASTER_POSTURE, detail: DETAIL_POSTURE },
+    { label: 'the required capability with an edit grant', context: member(SET.capabilityHolder), master: MASTER_CAPABILITY, detail: DETAIL_CAPABILITY },
+    { label: 'a master that carries none of the three inputs', context: member(SET.readMasters, SET.broad), master: MASTER_PLAIN, detail: DETAIL_PLAIN },
+  ] as const;
+
+  for (const { label, context, master, detail } of CONTROLS) {
+    it(`${label}: the master's own door admits the update, and insert, by-id update and by-id delete of a detail land`, async () => {
+      expect(await masterUpdate(rig, master, context)).toEqual({ kind: 'landed' });
+      const got = await writeDetail(rig, detail, `${master}_row`, context);
+      expect(got).toMatchObject({ ...LANDS, ...WRITTEN });
+    });
+  }
+
+  it('a system context writes a detail under every master', async () => {
+    for (const [detail, master] of [
+      [DETAIL_POSTURE, `${MASTER_POSTURE}_row`],
+      [DETAIL_CAPABILITY, `${MASTER_CAPABILITY}_row`],
+      [DETAIL_UNRESOLVED, 'qa_cbp_mow_any'],
+    ] as const) {
+      const id = `${detail}_system`;
+      expect(await outcomeOf(rig.engine.insert(detail, { id, note: '', master }, SYS))).toEqual({ kind: 'landed' });
+      expect(await rig.stored(detail, id)).toBeTruthy();
+    }
+  });
+});