diff --git a/backend/src/controllers/customer.controller.ts b/backend/src/controllers/customer.controller.ts
index 059cf6fb..34fe55b1 100644
--- a/backend/src/controllers/customer.controller.ts
+++ b/backend/src/controllers/customer.controller.ts
@@ -24,6 +24,23 @@ import {
 } from '../utils/accessControl.js';
 import { ApiError } from '../utils/apiError.js';
 
+// Mass-Assignment-Schutz für Bankkarten (Pentest R155): nur diese Felder dürfen
+// aus dem Request-Body übernommen werden – NICHT customerId/id/createdAt/… Nur
+// tatsächlich gesendete Keys landen im Ergebnis, damit Partial-Updates und das
+// Änderungs-Audit korrekt bleiben.
+const BANK_CARD_ALLOWED_FIELDS = [
+  'accountHolder', 'iban', 'cardNumber', 'bic', 'bankName', 'expiryDate', 'description', 'isActive',
+] as const;
+
+function pickBankCardFields(body: unknown): Record<string, unknown> {
+  const src = (body && typeof body === 'object') ? (body as Record<string, unknown>) : {};
+  const out: Record<string, unknown> = {};
+  for (const key of BANK_CARD_ALLOWED_FIELDS) {
+    if (key in src) out[key] = src[key];
+  }
+  return out;
+}
+
 // Customer CRUD
 export async function getCustomers(req: AuthRequest, res: Response): Promise<void> {
   try {
@@ -401,7 +418,8 @@ export async function createBankCard(req: AuthRequest, res: Response): Promise<v
   try {
     const customerId = parseInt(req.params.customerId);
     if (!(await canAccessCustomer(req, res, customerId))) return;
-    const card = await customerService.createBankCard(customerId, req.body);
+    const data = pickBankCardFields(req.body);
+    const card = await customerService.createBankCard(customerId, data as any);
     await logChange({
       req, action: 'CREATE', resourceType: 'BankCard',
       resourceId: card.id.toString(),
@@ -421,7 +439,10 @@ export async function updateBankCard(req: AuthRequest, res: Response): Promise<v
   try {
     const cardId = parseInt(req.params.id);
     if (!(await canAccessBankCard(req, res, cardId))) return;
-    const data = req.body;
+    // Mass-Assignment-Schutz (Pentest R155): nur erlaubte Felder aus dem Body,
+    // kein customerId/id/Fremdfeld. Das Audit unten läuft über diese saubere
+    // Menge (keine Phantom-Einträge). Enforcement zusätzlich im Service.
+    const data = pickBankCardFields(req.body);
 
     // Vorherigen Stand laden für Audit
     const before = await prisma.bankCard.findUnique({ where: { id: cardId } });
diff --git a/backend/src/services/customer.service.ts b/backend/src/services/customer.service.ts
index f694cc7b..e4d4859b 100644
--- a/backend/src/services/customer.service.ts
+++ b/backend/src/services/customer.service.ts
@@ -238,10 +238,17 @@ export async function createAddress(
     });
   }
 
+  // Explizite Feld-Whitelist (Mass-Assignment-Schutz, Pentest R155).
   return prisma.address.create({
     data: {
       customerId,
-      ...data,
+      type: data.type,
+      street: data.street,
+      houseNumber: data.houseNumber,
+      postalCode: data.postalCode,
+      city: data.city,
+      country: data.country,
+      isDefault: data.isDefault,
     },
   });
 }
@@ -272,9 +279,18 @@ export async function updateAddress(
     });
   }
 
+  // Explizite Feld-Whitelist (Mass-Assignment-Schutz, Pentest R155).
   return prisma.address.update({
     where: { id },
-    data,
+    data: {
+      type: data.type,
+      street: data.street,
+      houseNumber: data.houseNumber,
+      postalCode: data.postalCode,
+      city: data.city,
+      country: data.country,
+      isDefault: data.isDefault,
+    },
   });
 }
 
@@ -309,10 +325,18 @@ export async function createBankCard(
     description?: string;
   }
 ) {
+  // Explizite Feld-Whitelist statt `...data`-Spread: verhindert Mass-Assignment
+  // (Pentest R155). Sonst könnte roher req.body u.a. `customerId`/`id` setzen.
   return prisma.bankCard.create({
     data: {
       customerId,
-      ...data,
+      accountHolder: data.accountHolder,
+      iban: data.iban,
+      cardNumber: data.cardNumber,
+      bic: data.bic,
+      bankName: data.bankName,
+      expiryDate: data.expiryDate,
+      description: data.description,
       isActive: true,
     },
   });
@@ -331,9 +355,21 @@ export async function updateBankCard(
     isActive?: boolean;
   }
 ) {
+  // Explizite Feld-Whitelist statt `data`-Passthrough: verhindert Mass-
+  // Assignment (Pentest R155 – roher req.body konnte `customerId`/`id`
+  // mutieren). Undefined-Felder ignoriert Prisma → Partial-Update bleibt.
   return prisma.bankCard.update({
     where: { id },
-    data,
+    data: {
+      accountHolder: data.accountHolder,
+      iban: data.iban,
+      cardNumber: data.cardNumber,
+      bic: data.bic,
+      bankName: data.bankName,
+      expiryDate: data.expiryDate,
+      description: data.description,
+      isActive: data.isActive,
+    },
   });
 }
 
@@ -373,10 +409,17 @@ export async function createDocument(
     licenseIssueDate?: Date;
   }
 ) {
+  // Explizite Feld-Whitelist (Mass-Assignment-Schutz, Pentest R155).
   return prisma.identityDocument.create({
     data: {
       customerId,
-      ...data,
+      type: data.type,
+      documentNumber: data.documentNumber,
+      issuingAuthority: data.issuingAuthority,
+      issueDate: data.issueDate,
+      expiryDate: data.expiryDate,
+      licenseClasses: data.licenseClasses,
+      licenseIssueDate: data.licenseIssueDate,
       isActive: true,
     },
   });
@@ -395,9 +438,19 @@ export async function updateDocument(
     isActive?: boolean;
   }
 ) {
+  // Explizite Feld-Whitelist (Mass-Assignment-Schutz, Pentest R155).
   return prisma.identityDocument.update({
     where: { id },
-    data,
+    data: {
+      type: data.type,
+      documentNumber: data.documentNumber,
+      issuingAuthority: data.issuingAuthority,
+      issueDate: data.issueDate,
+      expiryDate: data.expiryDate,
+      licenseClasses: data.licenseClasses,
+      licenseIssueDate: data.licenseIssueDate,
+      isActive: data.isActive,
+    },
   });
 }
 
diff --git a/docs/todo.md b/docs/todo.md
index 59441d5b..e1ea00b0 100644
--- a/docs/todo.md
+++ b/docs/todo.md
@@ -97,6 +97,17 @@ isolierte Instanz (keine Multi-Tenancy im Code), Provisioning + Abrechnung
 
 ## ✅ Erledigt
 
+- [x] **🔒 Mass-Assignment-Schutz: Bankkarte/Adresse/Ausweis (Pentest R155)** (2026-08-13)
+  - Controller reichten rohen `req.body` an Prisma durch → `customerId` (Owner)
+    und `id` (PK) waren per Update mutierbar (staff-only, kein Cross-Tenant, aber
+    Integritätsschwäche – und mit `cardNumber` liegt Finanz-PII drauf).
+  - **Fix:** explizite Feld-Whitelist im Service (create+update) für **BankCard,
+    Address, IdentityDocument** – nur benannte Felder gehen an Prisma, kein
+    `...data`/`req.body`-Spread mehr. Zusätzlich Controller-`pickBankCardFields`
+    für saubere Audit-Logs (keine Phantom-Einträge injizierter Keys).
+  - Verifiziert: Update mit `{customerId:99999, id:88888, bogusField, cardNumber}`
+    → id+customerId **unverändert**, nur cardNumber gesetzt, Fremdfelder ignoriert.
+
 - [x] **🪪 Bankkarte-/Ausweis-Details in Vertrag (Ansicht + Bearbeiten) + Kartennummer** (2026-08-13)
   - **Schema:** neues Feld `BankCard.cardNumber` (String?, optional). Migration
     `20260813100000_bank_card_number` (`ADD COLUMN IF NOT EXISTS`), auf Dev