diff --git a/.changeset/cel-pushdown-membership-null-member-fail-closed.md b/.changeset/cel-pushdown-membership-null-member-fail-closed.md
new file mode 100644
index 0000000000..1e56d5dcf9
--- /dev/null
+++ b/.changeset/cel-pushdown-membership-null-member-fail-closed.md
@@ -0,0 +1,43 @@
+---
+"@objectstack/formula": patch
+---
+
+fix(formula): fail closed on a null MEMBER of a resolved membership array in the CEL pushdown (#13496)
+
+`compileCelToFilter` already fails closed when a `current_user.*` variable
+resolves to `undefined`/`null` — the module's docblock calls it "the no active
+org fail-closed path" and it is pinned for the SCALAR case. `lowerMembership`
+did not apply the same discipline one level in: it checked only
+`Array.isArray(value)` and emitted the list verbatim, so a null MEMBER of a
+resolved membership array went straight into a security `$in`. The one shape
+that IS a permission predicate was the one shape that did not fail closed.
+
+`lowerMembership` now refuses a `null`/`undefined` member of a **variable-resolved**
+membership array with the same `unresolved-variable` reason the scalar path
+uses, which the RLS path already turns into the deny sentinel.
+
+Maintainer ruling, 2026-08-31 (quoted unchanged): 「membership 数组中的 null
+**成员**触发与 null 标量同款处置——`unresolved-variable` / deny sentinel,⛔ 不
+strip、不静默清洗。」
+
+**Why refuse rather than strip.** Stripping the unresolved member is safe in
+POSITIVE polarity only. `not in` is a supported, pinned member of the pushdown
+subset (`!(x in y)` lowers to `$not` wrapping `$in`), and `$in: []` matches
+nothing on every backend — so `$not { $in: [] }` matches the WHOLE table.
+Stripping therefore inverts into fail-OPEN exactly where the predicate is a
+blocklist, and it silently deletes a blocklist entry in the mixed
+`['u1', null]` case. Refusing needs no polarity awareness at all: it throws
+before any `$not` wrapper is built. Both polarities are pinned.
+
+**No shipped behaviour changes.** No first-party provider puts a null into a
+membership array — `resolve-authz-context.ts` filters non-strings out of
+`org_user_ids`, and the kernel spec declares `org_user_ids: z.array(z.string())`
+— so the refused shape was never a declared-valid input. This makes the
+implementation match the declaration rather than narrowing it. A fully resolved
+list, an empty list (`$in: []`, a legitimate declared predicate) and the
+authoring-time `isPushdownableCel` shape gate are all unchanged and pinned so.
+
+Out of scope, deliberately: an AUTHORED literal null inside a list
+(`record.status in ['lost', null]`) is a declared predicate, not an unresolved
+variable, and what such a filter should select is a separate open question. It
+is untouched, with a pin recording that.
diff --git a/packages/formula/src/cel-to-filter.test.ts b/packages/formula/src/cel-to-filter.test.ts
index 7b775604d1..22369884dd 100644
--- a/packages/formula/src/cel-to-filter.test.ts
+++ b/packages/formula/src/cel-to-filter.test.ts
@@ -216,3 +216,106 @@ describe('compileCelToFilter — input shapes', () => {
     expect(r.ok && r.filter).toEqual({ dept: 'sales' });
   });
 });
+
+/**
+ * A null/undefined MEMBER of a resolved membership array fails closed, like the
+ * null SCALAR variable already pinned above (maintainer ruling, 2026-08-31).
+ *
+ * BOTH POLARITIES are pinned, and that is the point of the suite rather than a
+ * completeness flourish. The alternative repair — stripping the unresolved member
+ * — is safe in POSITIVE polarity (`$in` over the surviving members never grants
+ * more than those members grant) and INVERTS under the supported `not in` form:
+ * `!(x in y)` lowers to `$not` wrapping `$in`, and `$in: []` matching nothing makes
+ * `$not { $in: [] }` match the whole table. A positive-only suite is green for both
+ * repairs and therefore pins nothing about the one that was ruled on.
+ */
+describe('compileCelToFilter — a null MEMBER of a membership array fails closed', () => {
+  const vars = (org_user_ids: unknown[]) => ({ current_user: { id: 'u_me', org_user_ids } });
+  /**
+   * `ok` above pins its second argument to the exact shape of the module-level
+   * `VARS`, so it cannot take these partial contexts. Same assertion, same throw
+   * on an unexpected refusal, widened only where this suite needs it.
+   */
+  const filterOf = (src: string, v: Record<string, unknown>) => {
+    const r = compileCelToFilter(src, { variables: v });
+    if (!r.ok) throw new Error(`expected ok for "${src}" but got ${r.reason}: ${r.detail}`);
+    return r.filter;
+  };
+  const expectUnresolved = (r: ReturnType<typeof compileCelToFilter>, path = 'current_user.org_user_ids') => {
+    expect(r.ok).toBe(false);
+    if (r.ok) return;
+    expect(r.reason).toBe('unresolved-variable');
+    expect(r.detail).toContain(path);
+    expect(r.detail).toContain('unresolved member');
+  };
+
+  // ---- POSITIVE polarity: `x in y` -> $in ---------------------------------
+  it('positive: null among resolved members → unresolved-variable (no $in emitted)', () => {
+    expectUnresolved(compileCelToFilter('id in current_user.org_user_ids', { variables: vars(['u_me', null]) }));
+  });
+  it('positive: a lone null member → unresolved-variable', () => {
+    expectUnresolved(compileCelToFilter('id in current_user.org_user_ids', { variables: vars([null]) }));
+  });
+  it('positive: an undefined member fails closed too (the scalar path refuses both)', () => {
+    expectUnresolved(compileCelToFilter('id in current_user.org_user_ids', { variables: vars(['u_me', undefined]) }));
+  });
+
+  // ---- NEGATIVE polarity: `!(x in y)` -> $not wrapping $in ----------------
+  // This is where stripping inverted into allow-all; refusing must reach the SAME
+  // result here as in positive polarity, with no polarity threading in the lowerer.
+  it('negated `not in`: null among resolved members → unresolved-variable (no $not{$in} emitted)', () => {
+    expectUnresolved(compileCelToFilter('!(id in current_user.org_user_ids)', { variables: vars(['u_me', null]) }));
+  });
+  it('negated `not in`: a lone null member → unresolved-variable, NOT $not{$in:[]} (whole table)', () => {
+    expectUnresolved(compileCelToFilter('!(id in current_user.org_user_ids)', { variables: vars([null]) }));
+  });
+  it('negated inside a disjunction: the surviving disjunct does not rescue the compile', () => {
+    expectUnresolved(
+      compileCelToFilter("!(id in current_user.org_user_ids) || owner == current_user.id", {
+        variables: vars([null]),
+      }),
+    );
+  });
+  it('negated inside a conjunction fails closed as well', () => {
+    expectUnresolved(
+      compileCelToFilter("!(id in current_user.org_user_ids) && owner == current_user.id", {
+        variables: vars(['u_me', null]),
+      }),
+    );
+  });
+
+  // ---- the two polarities agree, which is the ruling's "same treatment" ----
+  it('both polarities and the null SCALAR variable yield the identical reason', () => {
+    const scalar = compileCelToFilter('record.organization_id == current_user.organization_id', {
+      variables: { current_user: { organization_id: null } },
+    });
+    const positive = compileCelToFilter('id in current_user.org_user_ids', { variables: vars([null]) });
+    const negated = compileCelToFilter('!(id in current_user.org_user_ids)', { variables: vars([null]) });
+    const reasons = [scalar, positive, negated].map((r) => (r.ok ? 'ok' : r.reason));
+    expect(reasons).toEqual(['unresolved-variable', 'unresolved-variable', 'unresolved-variable']);
+  });
+
+  // ---- shapes this guard must NOT move --------------------------------------
+  it('a fully resolved membership array still compiles, in both polarities', () => {
+    expect(filterOf('id in current_user.org_user_ids', vars(['u_me', 'u_peer']))).toEqual({
+      id: { $in: ['u_me', 'u_peer'] },
+    });
+    expect(filterOf('!(id in current_user.org_user_ids)', vars(['u_me', 'u_peer']))).toEqual({
+      $not: { id: { $in: ['u_me', 'u_peer'] } },
+    });
+  });
+  it('an EMPTY membership array still compiles to $in:[] in both polarities (a declared predicate, unchanged here)', () => {
+    expect(filterOf('id in current_user.org_user_ids', vars([]))).toEqual({ id: { $in: [] } });
+    expect(filterOf('!(id in current_user.org_user_ids)', vars([]))).toEqual({ $not: { id: { $in: [] } } });
+  });
+  it('an AUTHORED literal null in a list is not an unresolved variable — out of this guard scope', () => {
+    // A declared predicate, the same way `record.x == null` lowers to `$null` rather
+    // than failing closed. What such a filter SELECTS is a separate open question;
+    // this pin records only that the compiler still lowers it, unchanged.
+    expect(ok("record.status in ['lost', null]")).toEqual({ status: { $in: ['lost', null] } });
+  });
+  it('isPushdownableCel is untouched: the authoring gate resolves no variables', () => {
+    expect(isPushdownableCel('id in current_user.org_user_ids').ok).toBe(true);
+    expect(isPushdownableCel('!(id in current_user.org_user_ids)').ok).toBe(true);
+  });
+});
diff --git a/packages/formula/src/cel-to-filter.ts b/packages/formula/src/cel-to-filter.ts
index 44d77cbeb4..b87311f1d2 100644
--- a/packages/formula/src/cel-to-filter.ts
+++ b/packages/formula/src/cel-to-filter.ts
@@ -39,7 +39,10 @@
  *   `current_user.org_user_ids` → a pre-resolved membership array for `$in`
  *   (honours ADR-0055: the runtime pre-resolves the set; the compiler never emits
  *   a subquery). A variable that resolves to `undefined`/`null` yields
- *   `unresolved-variable` (the "no active org" fail-closed path).
+ *   `unresolved-variable` (the "no active org" fail-closed path) — and so does a
+ *   null/undefined MEMBER of a resolved membership array, which is the same
+ *   unresolved value one level in. See {@link lowerMembership} for why the member
+ *   is refused rather than dropped.
  */
 
 import type { ASTNode } from '@marcbachmann/cel-js';
@@ -366,6 +369,35 @@ function lowerMembership(elemNode: ASTNode, containerNode: ASTNode, ctx: Ctx): F
   if (value !== SHAPE_VALUE && !Array.isArray(value)) {
     throw new CompileError('unsupported', `\`in\` requires an array/list on the right`);
   }
+  // A null/undefined MEMBER of a RESOLVED membership variable fails closed, exactly
+  // as the scalar `resolveValue` path does one level up: the same unresolved value,
+  // the same `unresolved-variable` reason, the same deny sentinel downstream. Until
+  // this guard the member was emitted verbatim into a security `$in`, so the one
+  // shape that IS a permission predicate was the one shape that did not fail closed.
+  //
+  // Refused, never dropped. Stripping the member was measured to INVERT under the
+  // supported `not in` form (`!(x in y)` → `$not` wrapping `$in`): `$in: []` matches
+  // nothing, so `$not { $in: [] }` matches the WHOLE table. "Matches nothing" is
+  // fail-closed in POSITIVE polarity only, which makes stripping fail-OPEN precisely
+  // where the predicate is a blocklist. Refusing here needs no polarity awareness at
+  // all — it throws before any `$not` wrapper is built, so every enclosing shape
+  // (`!`, `&&`, `||`) collapses to the single `unresolved-variable` result.
+  //
+  // Deliberately NOT this guard's business: an AUTHORED literal null inside a list
+  // (`record.status in ['lost', null]`). That is a declared predicate rather than an
+  // unresolved variable — the same distinction `== null` already draws, where a
+  // literal null lowers to `$null` instead of failing closed — and what such a
+  // filter should SELECT is a separate open question this compiler does not answer.
+  if (container.kind === 'var' && Array.isArray(value)) {
+    const idx = value.findIndex((member) => member === null || member === undefined);
+    if (idx !== -1) {
+      throw new CompileError(
+        'unresolved-variable',
+        `variable "${container.path.join('.')}" has an unresolved member at index ${idx} ` +
+          `(${String(value[idx])}); a membership array must resolve every member`,
+      );
+    }
+  }
   return { [(elem as { path: string }).path]: { $in: value } } as FilterCondition;
 }