@@ -0,0 +1,9 @@
+---
+'@objectstack/core': patch
+---
+
+A session whose active organization is no longer one the user belongs to now resolves with no active organization instead of that one's data.
+
+Under a wall-enforcing tenancy posture (`isolated` / `group`), `resolveAuthzContext` took a browser session's stored `activeOrganizationId` as the request tenant without ever comparing it to the user's current memberships — the framework's only such comparison was gated on an API-key principal. A session whose owner had been removed from an organization therefore kept reading that organization's rows and writing into it until the session expired on its own (7 days by default), including when the removal went through the product's own offboarding path.
+
+That claim is now vetted: if it is not in the caller's `accessible_org_ids`, it is dropped and the context resolves with no active organization at all, which the tenant wall already fails closed on (reads resolve to nothing; a tenant-scoped write is refused by ADR-0123 D2). The principal is **not** refused — a session is a person who may hold memberships elsewhere, so they stay signed in and can switch to an organization they are actually in. The API-key arm is unchanged: a key is its organization binding and is still refused outright. The wire is unchanged; the drop is reported to the operator as a single server-side `warn`.
@@ -1543,3 +1543,243 @@ describe('the in-memory ObjectQL double honours `limit` (#10978)', () => {
     expect(found.every((r: any) => r.organization_id === 'o1')).toBe(true);
   });
 });
+
+
+// ── [#15409] The SESSION arm of the organization claim ─────────────────────
+
+/**
+ * [#15409 — maintainer ruling 2026-09-05, option B] A browser session whose
+ * `activeOrganizationId` names an organization its owner has LEFT.
+ *
+ * ## What was measured, before the guard existed
+ *
+ * On a live `isolated` boot with the real cloud-private `Organizations` plugin
+ * and a file-backed sqlite store, a session whose owner had been removed
+ * through better-auth's OWN `/organization/remove-member` — driven by the org
+ * owner, 200, the `sys_member` row really deleted — went on READING that
+ * organization's rows (`GET 200`, total 4, every row `@org_alpha`) and WRITING
+ * into it (`POST 201`), the written row read back out of the sqlite file with
+ * the server stopped carrying `organization_id: org_alpha` and the removed
+ * user as `created_by`. Nothing revoked, nothing expired: `revoked_at` was
+ * still `null` and the default session lifetime is 7 days.
+ *
+ * The resolver already HELD the fact. `get-session` returned positions
+ * `["user","org_member"]` while the membership was intact and `["user"]` on
+ * the very next request — off the same `sys_member` read that builds
+ * `accessible_org_ids` — while still serving that organization's rows. The
+ * only tenant-claim-vs-membership comparison in the framework was
+ * `keyPrincipal`-gated, so a session never reached it.
+ *
+ * ## What is pinned here, and what is deliberately NOT
+ *
+ * Option B, ruled: the claim is DROPPED, the principal is NOT refused. So the
+ * assertions below are about `ctx.tenantId` going away while `ctx.userId`
+ * STAYS — the one pair that would go red if someone later "simplified" B into
+ * the API-key arm's option A. Refusing the whole credential is right for an API
+ * key, which IS its organization binding; a session is a person who may hold
+ * legitimate memberships elsewhere.
+ *
+ * ⛔ No new refusal mechanism is pinned because none was added: with no active
+ * organization, Layer 0 already fails closed — `!organizationId ⇒
+ * RLS_DENY_FILTER` in `plugin-security/src/tenant-layer.ts` for reads, and
+ * ADR-0123 D2's write refusal for writes. Those two are pinned where they live.
+ * The end-to-end consequence — reads nothing, write does not land, read back
+ * from the STORE — is pinned in
+ * `packages/rest/src/single-kernel-isolated-session-org-claim-matrix.test.ts`.
+ */
+describe('[#15409] a session organization claim that no membership backs', () => {
+  /** The `sys_session` token — the credential. It must never reach a log line. */
+  const SESSION_TOKEN = 'sess_token_never_log_me';
+
+  /**
+   * A session as better-auth returns it: the ROW `id` and the `token` are
+   * separate columns, which is why the log line can name one and never the
+   * other.
+   */
+  const sessionAs = (userId: string, org: string | null, id = 'ses_probe') =>
+    async () => ({
+      user: { id: userId, email: `${userId}@x.com` },
+      session: { id, token: SESSION_TOKEN, userId, activeOrganizationId: org },
+    });
+
+  /**
+   * `u_ex` was removed from `org_alpha` and is still a member of `org_beta` —
+   * the shape that makes "still authenticated elsewhere" measurable rather than
+   * asserted. `u_ex2` is a fellow member of `org_alpha`, seeded so the
+   * fellow-org peer list has something to leak if the drop is incomplete.
+   */
+  const tables = () => ({
+    sys_user: [{ id: 'u_ex', email: 'u_ex@x.com' }, { id: 'u_ex2', email: 'u_ex2@x.com' }],
+    sys_member: [
+      { user_id: 'u_ex', organization_id: 'org_beta', role: 'member' },
+      { user_id: 'u_ex2', organization_id: 'org_alpha', role: 'member' },
+    ],
+    sys_user_position: [],
+    sys_user_permission_set: [],
+  });
+
+  let warnSpy: ReturnType<typeof vi.spyOn>;
+  beforeEach(() => { warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {}); });
+  afterEach(() => { warnSpy.mockRestore(); });
+  const dropLines = () =>
+    warnSpy.mock.calls
+      .map((c: unknown[]) => c.map(String).join(' '))
+      .filter((l: string) => l.includes('Session organization claim dropped'));
+
+  it('CONTROL · a BACKED claim is adopted untouched — the guard is not a blanket', async () => {
+    const ql = makeQl(tables());
+    const ctx = await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', 'org_beta'), tenancyPosture: 'isolated',
+    });
+    expect(ctx.userId).toBe('u_ex');
+    expect(ctx.tenantId).toBe('org_beta');
+    expect(dropLines()).toHaveLength(0);
+  });
+
+  it('THE SUBJECT: an UNBACKED claim resolves with NO active organization', async () => {
+    const ql = makeQl(tables());
+    const ctx = await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', 'org_alpha'), tenancyPosture: 'isolated',
+    });
+    expect(ctx.tenantId).toBeUndefined();
+    // …and the key is GONE, not present-and-undefined: a downstream
+    // `'tenantId' in ctx` must read the same as a session that never picked one.
+    expect(Object.prototype.hasOwnProperty.call(ctx, 'tenantId')).toBe(false);
+  });
+
+  /**
+   * ⭐ B, NOT A. The assertion that goes red if the session arm is ever
+   * "simplified" into the API-key arm's refusal: the principal is still
+   * authenticated, still carries its own identity and its real membership set.
+   */
+  it('B, not A: the PRINCIPAL survives — still authenticated, still a member where it really is', async () => {
+    const ql = makeQl(tables());
+    const ctx = await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', 'org_alpha'), tenancyPosture: 'isolated',
+    });
+    expect(ctx.userId).toBe('u_ex');
+    expect(ctx.email).toBe('u_ex@x.com');
+    // The organization they ARE in is still reachable — this is what lets them
+    // switch to it instead of being signed out of everything.
+    expect(ctx.accessible_org_ids).toEqual(['org_beta']);
+    // ⛔ And it is NOT the API-key refusal: no principal was refused.
+    expect(ctx.authRefusal).toBeUndefined();
+  });
+
+  /**
+   * The drop is not a field edit — the envelope is RE-RESOLVED with no tenant.
+   * `org_user_ids` is the one that would give it away: computed under the
+   * rejected claim it enumerates the members of the organization the user left,
+   * and Layer 1 scopes identity tables with it.
+   */
+  it('the envelope carries no residue of the rejected claim — the fellow-org peer list is not the left org\'s', async () => {
+    const ql = makeQl(tables());
+    const ctx = await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', 'org_alpha'), tenancyPosture: 'isolated',
+    });
+    expect(ctx.org_user_ids).toEqual(['u_ex']);
+    expect(ctx.org_user_ids).not.toContain('u_ex2');
+  });
+
+  it('the same claim under `group` is dropped too — the wall is membership-derived there as well', async () => {
+    const ql = makeQl(tables());
+    const ctx = await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', 'org_alpha'), tenancyPosture: 'group',
+    });
+    expect(ctx.userId).toBe('u_ex');
+    expect(ctx.tenantId).toBeUndefined();
+    expect(dropLines()).toHaveLength(1);
+  });
+
+  /**
+   * Under `single` there is no organization boundary to cross, and a deployment
+   * with no membership rows at all would otherwise lose every session's active
+   * organization. Same scoping the API-key arm carries, same reason.
+   */
+  it('does NOT apply under `single` — no wall, nothing to justify a claim against', async () => {
+    const ql = makeQl(tables());
+    const ctx = await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', 'org_alpha'), tenancyPosture: 'single',
+    });
+    expect(ctx.tenantId).toBe('org_alpha');
+    expect(dropLines()).toHaveLength(0);
+  });
+
+  /**
+   * An unwired caller is never made WORSE, only less strict — the doctrine
+   * `ResolveAuthzInput.tenancyPosture` already states for the two API-key
+   * refusals. A transport that supplies no posture keeps its old behaviour.
+   */
+  it('does NOT apply when the transport supplies no posture', async () => {
+    const ql = makeQl(tables());
+    const ctx = await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', 'org_alpha'),
+    });
+    expect(ctx.tenantId).toBe('org_alpha');
+    expect(dropLines()).toHaveLength(0);
+  });
+
+  it('a session with NO claim at all is not a drop — nothing was claimed', async () => {
+    const ql = makeQl(tables());
+    const ctx = await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', null), tenancyPosture: 'isolated',
+    });
+    expect(ctx.userId).toBe('u_ex');
+    expect(ctx.tenantId).toBeUndefined();
+    expect(dropLines()).toHaveLength(0);
+  });
+
+  it('a membership whose ADR-0091 window has lapsed does not back a claim', async () => {
+    const ql = makeQl({
+      ...tables(),
+      sys_member: [{ user_id: 'u_ex', organization_id: 'org_alpha', role: 'member', valid_until: '2000-01-01T00:00:00Z' }],
+    });
+    const ctx = await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', 'org_alpha'), tenancyPosture: 'isolated',
+    });
+    expect(ctx.userId).toBe('u_ex');
+    expect(ctx.tenantId).toBeUndefined();
+    expect(dropLines()).toHaveLength(1);
+  });
+
+  /**
+   * [#15256 / 2A, mirrored] The operator's only exit. The wire is unchanged, so
+   * without this line an offboarded member's live session naming the
+   * organization they left is invisible to everyone.
+   */
+  it('says it OUT LOUD, once, naming session / principal / organization / reason — and ⛔ never the token', async () => {
+    const ql = makeQl(tables());
+    await resolveAuthzContext({
+      ql, headers: H(), getSession: sessionAs('u_ex', 'org_alpha', 'ses_victim'), tenancyPosture: 'isolated',
+    });
+    const lines = dropLines();
+    expect(lines).toHaveLength(1);
+    expect(lines[0]).toContain('organization_membership_ended');
+    expect(lines[0]).toContain('session=ses_victim');
+    expect(lines[0]).toContain('principal=u_ex');
+    expect(lines[0]).toContain('organization=org_alpha');
+    // ⛔ THE CREDENTIAL. `sys_session.token` is replay-proven impersonation —
+    // its own field comment says so. It must never appear in a log line.
+    expect(lines[0]).not.toContain(SESSION_TOKEN);
+  });
+
+  /**
+   * The API-key arm is UNCHANGED (#15256 decision 1A stands): a key IS its
+   * organization binding, so it keeps refusing the principal outright. The
+   * session line must not fire for it — one credential, one decision point.
+   */
+  it('the API-key arm is untouched: an ex-member KEY is still refused, and says so in ITS own words', async () => {
+    const raw = 'osk_15409_exmember';
+    const ql = makeQl({
+      ...tables(),
+      sys_api_key: [{ id: 'key_ex', key: hashApiKey(raw), revoked: false, user_id: 'u_ex', active_organization_id: 'org_alpha' }],
+    });
+    const ctx = await resolveAuthzContext({
+      ql, headers: { 'x-api-key': raw }, tenancyPosture: 'isolated',
+    });
+    expect(ctx.userId).toBeUndefined();
+    expect(ctx.authRefusal?.reason).toBe('organization_membership_ended');
+    // ⛔ Not degraded into the session's drop: the key is REFUSED, not trimmed.
+    expect(dropLines()).toHaveLength(0);
+  });
+});