diff --git a/backend/src/BigRegister.Api/Contracts/Mappers.cs b/backend/src/BigRegister.Api/Contracts/Mappers.cs
index d4af6fb..ae32125 100644
--- a/backend/src/BigRegister.Api/Contracts/Mappers.cs
+++ b/backend/src/BigRegister.Api/Contracts/Mappers.cs
@@ -70,8 +70,12 @@ public static class Mappers
       a.CreatedAt.ToString("o"), a.UpdatedAt.ToString("o"), SubmittedAtOf(a));
 
   /// Admin summary — same shape plus the owner (WP-36; the user-facing list leaves Owner null).
+  /// The owner is a BSN, and both consumers of this mapper are cross-owner lists read by
+  /// someone who is not the subject (`/admin/cases`, `/werkvoorraad`), so it goes out masked
+  /// (RB-03/BIO-003). Masking here rather than at each endpoint means a third cross-owner
+  /// list cannot be added that forgets to.
   public static ApplicationSummaryDto ToAdminSummaryDto(this Aanvraag a, DateTimeOffset now) =>
-      a.ToSummaryDto(now) with { Owner = a.Owner };
+      a.ToSummaryDto(now) with { Owner = Pii.MaskTail(a.Owner, 3) };
 
   public static ApplicationDetailDto ToDetailDto(this Aanvraag a, DateTimeOffset now) => new(
       a.Id, a.Type, a.ToStatusDto(now), DraftOf(a), a.DocumentIds,
diff --git a/backend/src/BigRegister.Api/Domain/People/Pii.cs b/backend/src/BigRegister.Api/Domain/People/Pii.cs
new file mode 100644
index 0000000..c98b620
--- /dev/null
+++ b/backend/src/BigRegister.Api/Domain/People/Pii.cs
@@ -0,0 +1,18 @@
+namespace BigRegister.Domain.People;
+
+/// <summary>
+/// One redaction rule for identifiers that must not leave the server in full (BSN,
+/// BIG-nummer). Lives in <c>Domain/</c> because three layers need it — the DTO mappers
+/// (<c>Contracts/Mappers.cs</c>), the audit writes (<c>Data/DocumentStore.cs</c>) and the
+/// endpoints themselves — and a second hand-rolled copy is exactly how one of them drifts
+/// into leaking. Mirrors the FE <c>maskTail</c> (<c>libs/shared/src/ui/debug-state/mask.ts</c>)
+/// so wire redaction and the dev panel agree on what a masked value looks like.
+/// </summary>
+public static class Pii
+{
+  /// Keep the last <paramref name="keep"/> characters, mask the rest. Idempotent: masking an
+  /// already-masked value is a no-op, so a defence-in-depth second call is harmless.
+  public static string MaskTail(string value, int keep) =>
+      value.Length <= keep ? new string('*', value.Length)
+                           : new string('*', value.Length - keep) + value[^keep..];
+}
diff --git a/backend/src/BigRegister.Api/Program.cs b/backend/src/BigRegister.Api/Program.cs
index 20e3ce4..bc7e98f 100644
--- a/backend/src/BigRegister.Api/Program.cs
+++ b/backend/src/BigRegister.Api/Program.cs
@@ -12,6 +12,7 @@ using BigRegister.Domain.Documents;
 using BigRegister.Domain.Features;
 using BigRegister.Domain.Intake;
 using BigRegister.Domain.Letters;
+using BigRegister.Domain.People;
 using BigRegister.Domain.Registrations;
 using BigRegister.Domain.Submissions;
 using BigRegister.Api.Zgw;
@@ -460,7 +461,10 @@ api.MapGet("/beoordeling/{id}", (string id, HttpContext ctx, IZaakSource zaken)
       if (c is null || c.Status.Tag == "Concept") return Results.NotFound();
       var docs = DocumentStore.ByIds(c.DocumentIds)
           .Select(d => new BeoordelingDocumentDto(d.DocumentId, d.CategoryId, d.FileName)).ToList();
-      var masked = c with { Owner = MaskTail(c.Owner!, 3) };
+      // Belt and braces: ToAdminSummaryDto already masks the local source (RB-03) and
+      // MaskTail is idempotent, but IZaakSource has a second implementation whose Owner
+      // is mapped from OpenZaak, so this stays as the guarantee for this response.
+      var masked = c with { Owner = Pii.MaskTail(c.Owner!, 3) };
       // WP-68 (F3): non-throwing — c.Status.Tag crosses the IZaakSource wire boundary, so an
       // unrecognised tag degrades to "cannot decide" instead of a 500.
       var canBesluiten = Enum.TryParse<AanvraagStatusTag>(c.Status.Tag, out var tag) && BeoordelingRules.CanDecide(tag);
@@ -869,12 +873,6 @@ void RecordZgwDivergence(HttpContext ctx, string id, string referentie, Exceptio
   AuthzAuditStore.Record("zgw:divergence", referentie, allowed: false, Authz.ResolvePrincipal(ctx).Role.ToString(), cid);
 }
 
-// Keep the last `keep` characters, mask the rest — mirrors the FE maskTail
-// (src/app/shared/ui/debug-state/mask.ts) so wire redaction and the dev panel agree.
-static string MaskTail(string value, int keep) =>
-    value.Length <= keep ? new string('*', value.Length)
-                         : new string('*', value.Length - keep) + value[^keep..];
-
 static string Now() => DateTimeOffset.UtcNow.ToString("o");
 
 BriefViewDto ToView(HttpContext ctx, BriefEntity e) => new(
@@ -888,7 +886,7 @@ BriefViewDto ToView(HttpContext ctx, BriefEntity e) => new(
     // behandel scherm can show whom/what it concerns without brief/ importing registratie.
     // The BIG-nummer ships MASKED by default (PRD-0002 §5c, field-level PII); the reveal
     // endpoint returns the full value, gated + audited.
-    new CaseContextDto(SeedData.Registration.Naam, MaskTail(SeedData.Registration.BigNummer, 3), e.Beroep, BriefSeed.AanvraagReferentie));
+    new CaseContextDto(SeedData.Registration.Naam, Pii.MaskTail(SeedData.Registration.BigNummer, 3), e.Beroep, BriefSeed.AanvraagReferentie));
 
 // Emit (decision flags, via ToView) and enforce (Forbidden/Conflict below) both run
 // through Authz — see BriefStore.Review and Authz.CanActOn — so they cannot drift.
diff --git a/backend/tests/BigRegister.Tests/AdminCasesTests.cs b/backend/tests/BigRegister.Tests/AdminCasesTests.cs
index 9bfc9dc..475778a 100644
--- a/backend/tests/BigRegister.Tests/AdminCasesTests.cs
+++ b/backend/tests/BigRegister.Tests/AdminCasesTests.cs
@@ -1,6 +1,7 @@
 using System.Net;
 using System.Net.Http.Json;
 using BigRegister.Api.Contracts;
+using BigRegister.Api.Data;
 using Microsoft.AspNetCore.Mvc.Testing;
 
 namespace BigRegister.Tests;
@@ -34,7 +35,10 @@ public class AdminCasesTests(TestWebApplicationFactory factory) : IClassFixture<
       list.EnsureSuccessStatusCode();
       var cases = (await list.Content.ReadFromJsonAsync<List<ApplicationSummaryDto>>())!;
       var mine = cases.Single(x => x.Id == a.Id);
-      Assert.False(string.IsNullOrEmpty(mine.Owner)); // admin list carries the owner
+      // RB-03/BIO-003: the owner is carried, but masked — it is a BSN, and this list is
+      // read by someone who is not the subject.
+      Assert.Equal("******782", mine.Owner);
+      Assert.DoesNotContain(DocumentStore.DemoOwner, mine.Owner);
     }
     finally
     {
diff --git a/backend/tests/BigRegister.Tests/WerkvoorraadTests.cs b/backend/tests/BigRegister.Tests/WerkvoorraadTests.cs
index dc76a16..c665f08 100644
--- a/backend/tests/BigRegister.Tests/WerkvoorraadTests.cs
+++ b/backend/tests/BigRegister.Tests/WerkvoorraadTests.cs
@@ -38,7 +38,8 @@ public class WerkvoorraadTests(TestWebApplicationFactory factory) : IClassFixtur
       var queue = (await res.Content.ReadFromJsonAsync<List<ApplicationSummaryDto>>())!;
       var mine = queue.Single(x => x.Id == a.Id);
       Assert.Equal("InBehandeling", mine.Status.Tag);
-      Assert.False(string.IsNullOrEmpty(mine.Owner)); // cross-owner, like /admin/cases
+      // RB-03/BIO-003: masked, like /admin/cases — both inherit ToAdminSummaryDto.
+      Assert.Equal("******782", mine.Owner);
     }
     finally
     {
diff --git a/docs/project/refactor-backlog-setup/refactor-backlog/implementation/rb-03.md b/docs/project/refactor-backlog-setup/refactor-backlog/implementation/rb-03.md
new file mode 100644
index 0000000..3cb43a1
--- /dev/null
+++ b/docs/project/refactor-backlog-setup/refactor-backlog/implementation/rb-03.md
@@ -0,0 +1,45 @@
+# RB-03 — mask the owner BSN on the cross-owner case lists
+
+Status: **implemented** · 2026-08-27 · Source findings: `07-bio2-compliance.md` BIO-003 · `99-backlog.md` RB-03
+
+## What was wrong
+
+`Mappers.ToAdminSummaryDto` set `Owner = a.Owner` — the raw BSN. Two endpoints consume it,
+both cross-owner lists read by someone who is **not** the subject:
+
+- `GET /admin/cases` (`cases:manage`)
+- `GET /werkvoorraad` (`aanvraag:beoordelen`)
+
+`GET /beoordeling/{id}` — the *detail* view of the same data — already masked. So the
+detail screen showed `******782` while the list one click earlier showed the whole BSN.
+
+## What changed
+
+| File                        | Change                                                                            |
+| --------------------------- | ----------------------------------------------------------------------------------- |
+| `Domain/People/Pii.cs`      | **new** — `Pii.MaskTail`, moved out of `Program.cs`                               |
+| `Contracts/Mappers.cs`      | `Owner = Pii.MaskTail(a.Owner, 3)`                                                |
+| `Program.cs`                | local `MaskTail` deleted; two call sites point at `Pii.MaskTail`                  |
+| `AdminCasesTests.cs`        | asserts the masked value and that `DemoOwner` does not appear                     |
+| `WerkvoorraadTests.cs`      | same assertion, replacing the `IsNullOrEmpty` one                                 |
+
+**Masked in the mapper, not at the endpoints.** The point of the ticket is that both
+lists *inherit* it, so a third cross-owner list cannot be added that forgets to mask.
+
+**`MaskTail` moved to `Domain/People/Pii.cs`** because it now has three callers across
+three folders (`Contracts`, `Program.cs`, and `Data` once **RB-04** lands), and a second
+hand-rolled copy is how one of them drifts into leaking. It is documented as idempotent,
+which is what lets `/beoordeling/{id}` keep its own call: `IZaakSource` has a second
+implementation (`OpenZaakZaakSource` → `ZgwZaakMapper`, which maps `Owner` from the zaak
+`identificatie`), so that endpoint's guarantee should not depend on which source answered.
+
+## Blast radius on the frontend — none
+
+Both consumers use the value for display only (`admin-cases.page.ts:101`,
+`beoordeling-view.ts:40`, `werkvoorraad-item-view.ts:28`); the `parse*` boundaries require
+a non-empty string, which a masked BSN still is. Nothing keys, filters or looks up by owner.
+
+## Verification
+
+`dotnet format --verify-no-changes` clean. `dotnet test`: **251 passed, 1 failed** — the
+pre-existing `OpenZaakIntegrationTests.Admin_cases_…`, which needs a live container.