@@ -0,0 +1,19 @@
+---
+'@objectstack/plugin-security': patch
+---
+
+fix(plugin-security)!: security explain refuses a `userId` that no `sys_user` record has with `404 RECORD_NOT_FOUND`, naming the id, instead of explaining the baseline principal the resolver builds for any id
+
+Clause-②: no (narrowing) — `POST` / `GET /security/explain` and the in-process `explain()` refuse a request naming another user whose id no `sys_user` record has, which they answered with a verdict before. Nothing is widened.
+Metadata-protocol: none
+
+<!-- adr-0087: not-required (no-migration-prescription) No key of any schema is added, removed, renamed or re-shaped, and no export or type changes: the explain request's userId keeps its shape, and an id that names nobody is now refused instead of explained. A verdict about nobody had no meaning to keep, so there is nothing for a migration to rewrite. -->
+
+**BREAKING** accept-set narrowing at the access-explanation door (`@objectstack/plugin-security`), served at `/security/explain` on both transports.
+
+- **Before.** An administrator explaining another user by an id that no `sys_user` record has (a typo, a deleted user, an id from another deployment) was answered `200` with a full verdict, usually `allowed: true`: the authorization resolver gives any id the baseline position and permission sets. `GET /data/sys_user/ID` answered 404 for the same id.
+- **Now.** That request is refused `404 RECORD_NOT_FOUND`, and the message names the id. The id is read through the same `sys_user` existence read the on-behalf-of delegator path uses.
+- **Unchanged.** The order of checks: an anonymous caller is refused 401, and a caller who may not explain others gets the same 403 `PERMISSION_DENIED` whether the id exists or not, before any existence read. Explaining yourself (no `userId`, or your own) and explaining a user who exists answer as before.
+- **Who sees it.** A caller holding `manage_users` (or a system caller) who names an id that does not exist. An organization administrator or a delegated administrator naming such an id still gets the 403: no membership or scope covers a user who does not exist.
+
+**The remedy.** None needed: pick the user from the directory, or check the id.
@@ -96,6 +96,11 @@ rule:
   `adminScope` whose business-unit subtree covers that user (ADR-0090 D12):
   an administrator who can rewire a user's grants may read why they resolve
   the way they do — and *only* a covering administrator. Anything else → 403.
+- **A `userId` no user has** — once the caller may explain that user, an id
+  with no `sys_user` record is refused `404 RECORD_NOT_FOUND`, naming the id,
+  instead of being explained as a baseline member. The 403 above comes first,
+  so a caller who may not explain others cannot use explain to learn which ids
+  exist.
 
 A deployment without `@objectstack/plugin-security` answers 501.
 
 
@@ -341,6 +341,45 @@ export class MaskedValueWriteError extends Error {
   }
 }
 
+/**
+ * [#26641] `explain` was asked about ANOTHER user, by an id no `sys_user` row
+ * has → `404 RECORD_NOT_FOUND`, naming the id.
+ *
+ * The explainer builds the explained principal through the authorization
+ * resolver, which synthesises the baseline anchor and sets for ANY id. So an
+ * id that names nobody (a typo, a deleted user, an id from another deployment)
+ * was explained as a baseline member and answered `200 allowed: true`: a
+ * verdict about a person who does not exist. As with an unknown object
+ * ({@link ExplainObjectNotFoundError}), nothing can be evaluated, so the honest
+ * answer is a refusal, not a verdict.
+ *
+ * `RECORD_NOT_FOUND` at 404 is the standard catalog's pair for "the record with
+ * this id does not exist", the answer `GET /data/sys_user/ID` gives for the
+ * same id, so it needs no ledger row. The REST door serves it through its
+ * classified-refusal arm, which keeps a declared 4xx `code` and `status`.
+ *
+ * Only a caller already admitted to explain someone else reaches this check,
+ * so it tells nobody new which ids exist: an anonymous caller is refused 401
+ * at the door and a caller who may not explain others gets their 403 before
+ * any existence read. The prefix is not the `[Security] Access denied`
+ * matcher, and the class carries both `status` and `statusCode`, for the
+ * reasons the classes above record.
+ */
+export class ExplainUserNotFoundError extends Error {
+  readonly code = 'RECORD_NOT_FOUND';
+  readonly status = 404;
+  readonly statusCode = 404;
+  readonly userId: string;
+  constructor(userId: string) {
+    super(
+      `[Security] Unknown user: explain was asked about user '${userId}', but no sys_user record has that id. `
+      + `No principal was evaluated, so this is NOT an access decision.`,
+    );
+    this.name = 'ExplainUserNotFoundError';
+    this.userId = userId;
+  }
+}
+
 /**
  * [#18253] `explain` was asked about an object name that no declaration on this
  * runtime answers to → `404 OBJECT_NOT_FOUND`.
 
@@ -790,6 +790,24 @@ export async function buildContextForUser(
   };
 }
 
+/**
+ * [#26641] The `sys_user` row a user id names, read under the system context,
+ * or `null` when no row has that id. The one existence read of a user the
+ * explainer makes: {@link resolveDelegatorContext} asks it of a delegator, and
+ * the explain API (`explainAccessForCaller`) asks it of the user it was asked
+ * to explain. Each needs it for the same reason: {@link buildContextForUser}
+ * resolves a principal for ANY id, because the resolver synthesises the
+ * baseline anchor and sets for an id no row has.
+ *
+ * A read that throws is not absence, so this function does not catch: each
+ * caller decides what a failed read means. The delegator path fails closed and
+ * reads it as `missing`; the explain API lets it propagate as a fault rather
+ * than answer that the user does not exist.
+ */
+export async function readSysUserRow(ql: any, userId: string): Promise<any | null> {
+  return (await ql.findOne('sys_user', { where: { id: userId }, context: SYSTEM_CTX })) ?? null;
+}
+
 /**
  * [ADR-0090 D10] Result of resolving the delegator behind an on-behalf-of
  * principal. `none` = no delegation link; `missing` = the link names a user that
@@ -864,7 +882,7 @@ export async function resolveDelegatorContext(
   if (!oboId) return { kind: 'none' };
   let user: any = null;
   try {
-    user = await ql.findOne('sys_user', { where: { id: oboId }, context: SYSTEM_CTX });
+    user = await readSysUserRow(ql, oboId);
   } catch {
     user = null;
   }
 
@@ -247,6 +247,11 @@ describe('[ADR-0090 D12, amended 2026-10-17] the organization administrator assi
       it('the delegable-scope report offers the org admin its in-boundary positions only, and explain admits it for its member', async () => {
         const h = await boot(posture);
         const security = h.plugin as any;
+        // [#26641] Explain answers only for a user a `sys_user` row carries, so
+        // the explained member gets one here (the boot leaves the table
+        // unprovisioned for the write cases above).
+        await h.engine.syncObjectSchema(SysUser.name);
+        await h.engine.insert('sys_user', { id: 'u_target', name: 'Target', email: 'u_target@example.test' }, { context: { isSystem: true } } as any);
         for (const context of [CALLERS.orgAdmin, CALLERS.orgAdminNoBypass]) {
           const sets = await security.resolvePermissionSetsForContext(context);
           const report = await security.delegatedAdminGate.describeDelegableScope(sets, context);
 
@@ -72,14 +72,15 @@ import { OWNERSHIP_QUEUE_SERVICE, type AuthoredRowWriteOperation, type IOwnershi
 import type { ExplainDecision, ExplainOperation, PermissionSet } from '@objectstack/spec/security';
 import type { SecurityPlugin } from './security-plugin.js';
 import { absentUnderCallerRead, requiredCapsForOperation, unreadableUnderCallerRead } from './security-plugin.js';
-import { PermissionDeniedError } from './errors.js';
+import { ExplainUserNotFoundError, PermissionDeniedError } from './errors.js';
 import { grantRowPolicySpeaks } from './external-owd-grant-stamp.js';
 import { rlsOperationForVerb } from './lifecycle-verb-rls-operation.js';
 import {
   buildContextForUser,
   delegatedOrganizationRefusal,
   explainAccessForAsker,
   intersectDelegatedScope,
+  readSysUserRow,
   resolveDelegatorContext,
 } from './explain-engine.js';
 
@@ -164,6 +165,15 @@ export async function explainAccessForCaller(
         }
       }
     }
+    // [#26641] ...and only a user who exists is explained. The resolver
+    // synthesises the baseline anchor and sets for ANY id, so an id no
+    // `sys_user` row has was explained as a baseline member, `allowed: true`.
+    // Asked here, after the caller is admitted and never before, so a caller
+    // who may not explain others learns nothing about which ids exist; read
+    // through the delegator path's own existence read (`readSysUserRow`).
+    if (!(await readSysUserRow(this.ql, request.userId))) {
+      throw new ExplainUserNotFoundError(request.userId);
+    }
     // [#20515] Resolved in the CALLER's organization — the one the explain
     // right above was checked in. Grants scoped to any other organization do
     // not apply there, and with no active organization only global grants do.
 
@@ -3621,6 +3621,68 @@ describe('explainAccessForCaller (ADR-0090 D6/D12)', () => {
     expect(self.principal.userId).toBe('u_east_1');
   });
 
+  // [#26641] Explaining ANOTHER user by an id no `sys_user` row has is refused
+  // `404 RECORD_NOT_FOUND`, naming the id: the resolver synthesises a baseline
+  // principal for any id, so it used to answer a verdict about nobody. The
+  // existence read runs only after the caller is admitted to explain others.
+  describe('[#26641] an id no sys_user row has', () => {
+    const GHOST = 'u_no_such_user_26641';
+    /** The ids `sys_user` was asked about, in order. */
+    const userReads = (plugin: any): string[] => {
+      const seen: string[] = [];
+      const ql = (plugin as any).ql;
+      const findOne = ql.findOne.bind(ql);
+      ql.findOne = async (object: string, opts: any) => {
+        if (object === 'sys_user') seen.push(String(opts?.where?.id));
+        return findOne(object, opts);
+      };
+      return seen;
+    };
+
+    it('a caller holding manage_users is refused 404 RECORD_NOT_FOUND naming the id', async () => {
+      const { plugin } = await boot();
+      const caller = { userId: 'u_hr', positions: [], permissions: ['hr_admin'] };
+      for (const operation of ['read', 'create']) {
+        const err: any = await plugin
+          .explainAccessForCaller({ object: 'task', operation, userId: GHOST }, caller)
+          .then(() => null, (e) => e);
+        expect(err, `${operation}: refused, not answered`).toMatchObject({
+          name: 'ExplainUserNotFoundError', code: 'RECORD_NOT_FOUND', status: 404, statusCode: 404, userId: GHOST,
+        });
+        expect(err.message).toContain(`'${GHOST}'`);
+      }
+    });
+
+    it('a caller who may not explain others gets their 403 first, and no existence read is made', async () => {
+      const { plugin } = await boot();
+      const seen = userReads(plugin);
+      for (const caller of [
+        { userId: 'u_east_1', positions: [], permissions: [] },
+        { userId: 'u_delegate', positions: [], permissions: ['sub_admin'] },
+      ]) {
+        await expect(
+          plugin.explainAccessForCaller({ object: 'task', operation: 'read', userId: GHOST }, caller),
+          `${caller.userId}: the same refusal an existing user outside their reach gets`,
+        ).rejects.toMatchObject({ name: 'PermissionDeniedError', code: 'PERMISSION_DENIED' });
+      }
+      expect(seen, 'the unknown id is never looked up for a caller refused the right').not.toContain(GHOST);
+    });
+
+    it('an existing user is still explained, and a self-explanation is never refused for its id', async () => {
+      const { plugin } = await boot();
+      const hr = { userId: 'u_hr', positions: [], permissions: ['hr_admin'] };
+      const other = await plugin.explainAccessForCaller({ object: 'task', operation: 'read', userId: 'u_west_1' }, hr);
+      expect(other.principal.userId).toBe('u_west_1');
+      // Explaining yourself is unchanged: no `userId`, or your own, explains the
+      // request's own context, whether or not a row carries its id.
+      const ghostSelf = { userId: GHOST, positions: [], permissions: [] };
+      for (const request of [{}, { userId: GHOST }]) {
+        const self = await plugin.explainAccessForCaller({ object: 'task', operation: 'read', ...request }, ghostSelf);
+        expect(self.principal.userId).toBe(GHOST);
+      }
+    });
+  });
+
   // [#20515] Explaining ANOTHER user resolves that user's grants in the CALLER's
   // organization — the one the explain right was checked in. A grant scoped to
   // an organization applies only while that organization is the one resolved
 
@@ -0,0 +1,122 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * #26641 — `POST /security/explain` naming a `userId` that no `sys_user` row
+ * has is refused `404 RECORD_NOT_FOUND`, naming the id, on the showcase boot.
+ *
+ * ## The defect
+ *
+ * The explainer builds the explained principal through the authorization
+ * resolver, which synthesises the baseline anchor and sets for ANY id. So the
+ * seed admin explaining `no_such_user_qa_123` was answered `200 allowed: true`
+ * with principal `positions: ["everyone"]` and the member baseline sets, while
+ * `GET /data/sys_user/no_such_user_qa_123` answered 404: a verdict about a
+ * person who does not exist.
+ *
+ * ## The order of checks, pinned with the refusal
+ *
+ * The existence read runs only after the caller is admitted to explain someone
+ * else, so the explain door cannot be used to learn which ids exist:
+ *
+ *  - `[anonymous]` no session → 401, whatever id the body names.
+ *  - `[unprivileged]` a plain member naming the unknown id gets the same 403
+ *    `PERMISSION_DENIED` it gets naming a user who exists.
+ *  - `[unknown]` the seed admin naming the unknown id → 404 `RECORD_NOT_FOUND`
+ *    naming it, for `read` and `create`, consistent with the data door's 404.
+ *  - `[existing]` the seed admin explaining the member who exists is answered
+ *    as before (200, the principal is that member), and explaining yourself is
+ *    unchanged.
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import { type VerifyStack } from '@objectstack/verify';
+import { bootShowcase } from './showcase-boot.js';
+import { showcaseAppDefaultSecurity } from './showcase-security.js';
+
+const SYS = { isSystem: true } as const;
+const OBJECT = 'showcase_task';
+const GHOST = 'no_such_user_qa_123';
+const MEMBER_EMAIL = 'explain-unknown-user.member.26641@verify.test';
+
+describe('#26641: security explain refuses a userId no sys_user row has', () => {
+  let stack: VerifyStack;
+  let adminTok = '';
+  let memberTok = '';
+  let adminId = '';
+  let memberId = '';
+
+  /** `POST /security/explain` as whoever holds `token` (`null` = no session). */
+  const explain = async (token: string | null, body: Record<string, unknown>) => {
+    const res = token === null
+      ? await stack.api('/security/explain', {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify(body),
+      })
+      : await stack.apiAs(token, 'POST', '/security/explain', body);
+    const text = await res.text();
+    let json: any = null;
+    try { json = JSON.parse(text); } catch { /* the status still reports */ }
+    return { status: res.status, text, json };
+  };
+
+  beforeAll(async () => {
+    stack = await bootShowcase({ security: showcaseAppDefaultSecurity() });
+    adminTok = await stack.signIn(); // the seeded admin first, so the sign-up below is a plain member
+    memberTok = await stack.signUp(MEMBER_EMAIL);
+    const ql: any = await stack.kernel.getServiceAsync('objectql');
+    memberId = String((await ql.findOne('sys_user', { where: { email: MEMBER_EMAIL }, context: SYS }))?.id ?? '');
+    expect(memberId, 'the signed-up member exists').toBeTruthy();
+    expect(await ql.findOne('sys_user', { where: { id: GHOST }, context: SYS }), 'no row carries the probe id').toBeFalsy();
+    const me = await explain(adminTok, { object: OBJECT, operation: 'read' });
+    expect(me.status, `admin self-explain: ${me.text}`).toBe(200);
+    adminId = String(me.json?.principal?.userId ?? '');
+    expect(adminId, 'the admin self-explanation names its principal').toBeTruthy();
+  }, 300_000);
+
+  afterAll(async () => {
+    await stack?.stop?.();
+  });
+
+  it('[control] the data door answers 404 for the same id', async () => {
+    const res = await stack.apiAs(adminTok, 'GET', `/data/sys_user/${GHOST}`);
+    expect(res.status).toBe(404);
+  });
+
+  it('[unknown] the seed admin is refused 404 RECORD_NOT_FOUND naming the id, for read and create', async () => {
+    for (const operation of ['read', 'create']) {
+      const r = await explain(adminTok, { object: OBJECT, operation, userId: GHOST });
+      expect(r.status, `${operation}: ${r.text}`).toBe(404);
+      expect(r.json?.success, r.text).toBe(false);
+      expect(r.json?.error?.code, r.text).toBe('RECORD_NOT_FOUND');
+      expect(String(r.json?.error?.message ?? ''), r.text).toContain(GHOST);
+      expect(r.json?.allowed, 'no verdict is answered').toBeUndefined();
+    }
+  });
+
+  it('[anonymous] no session is refused 401 before anything about the id', async () => {
+    const r = await explain(null, { object: OBJECT, operation: 'read', userId: GHOST });
+    expect(r.status, r.text).toBe(401);
+  });
+
+  it('[unprivileged] a plain member gets the same 403 for the unknown id as for a user who exists', async () => {
+    const unknown = await explain(memberTok, { object: OBJECT, operation: 'read', userId: GHOST });
+    const existing = await explain(memberTok, { object: OBJECT, operation: 'read', userId: adminId });
+    for (const [label, r] of [['unknown id', unknown], ['existing user', existing]] as const) {
+      expect(r.status, `${label}: ${r.text}`).toBe(403);
+      expect(r.json?.error?.code, `${label}: ${r.text}`).toBe('PERMISSION_DENIED');
+    }
+  });
+
+  it('[existing] the seed admin explaining the member who exists, and anyone explaining themselves, is answered', async () => {
+    const other = await explain(adminTok, { object: OBJECT, operation: 'read', userId: memberId });
+    expect(other.status, other.text).toBe(200);
+    expect(other.json?.principal?.userId).toBe(memberId);
+    expect(typeof other.json?.allowed).toBe('boolean');
+    for (const body of [{ object: OBJECT, operation: 'read' }, { object: OBJECT, operation: 'read', userId: memberId }]) {
+      const self = await explain(memberTok, body);
+      expect(self.status, self.text).toBe(200);
+      expect(self.json?.principal?.userId).toBe(memberId);
+    }
+  });
+});