diff --git a/backend/__tests__/routes/adminContractsSignedPdfPathTraversal.test.js b/backend/__tests__/routes/adminContractsSignedPdfPathTraversal.test.js
new file mode 100644
index 00000000..ce7c8d29
--- /dev/null
+++ b/backend/__tests__/routes/adminContractsSignedPdfPathTraversal.test.js
@@ -0,0 +1,126 @@
+/**
+ * Same bug class as GHSA-9q5j-vqfw-32hr (fixed in adminEvents/logo.js) —
+ * the signed-PDF upload's multer `filename` callback built the stored
+ * path directly from `req.params.id` with no integer validation:
+ *
+ *   filename: (req, file, cb) => {
+ *     cb(null, `contract-${req.params.id}-${Date.now()}${ext}`);
+ *   }
+ *
+ * `POST /:id/upload-signed-pdf` declares `param('id').isInt({ min: 1 })`,
+ * but express-validator's check only runs inside the route handler via
+ * validateRequest(req) — AFTER multer has already parsed the multipart
+ * body and invoked the filename callback. A traversal payload in the raw
+ * `:id` URL segment reaches multer completely unvalidated.
+ *
+ * Fixed by rejecting any non-positive-integer id before it is used to
+ * build the filename, independent of the declared-but-too-late
+ * express-validator check.
+ */
+const path = require('path');
+const fs = require('fs');
+const os = require('os');
+
+// ALLOWED_MEDIA_TYPES in fileSecurityUtils.js only defines image/video
+// entries, so the route's real fileFilter (validateFileType(..., ['application/pdf']))
+// rejects every PDF upload with "Only PDF files are allowed" — a
+// separate, pre-existing bug unrelated to the path-traversal fix under
+// test here (also present in publicContracts.js, which is why neither
+// suite exercises a successful upload). Stub validateFileType so this
+// suite can drive the full route, including the filename-callback fix,
+// end-to-end.
+jest.mock('../../src/utils/fileSecurityUtils', () => {
+  const actual = jest.requireActual('../../src/utils/fileSecurityUtils');
+  return {
+    ...actual,
+    validateFileType: (filename, mimetype, allowedTypes) => allowedTypes.includes(mimetype),
+  };
+});
+
+process.env.NODE_ENV = 'test';
+process.env.TEST_DATABASE_PATH = path.join(
+  fs.mkdtempSync(path.join(os.tmpdir(), 'picpeak-contracts-signed-pdf-')), 'db.sqlite'
+);
+process.env.JWT_SECRET = process.env.JWT_SECRET || 'admin-contracts-signed-pdf-test-secret';
+
+const request = require('supertest');
+const {
+  bootCrmDb, seedMinimal, assignAdminRole, mintAdminToken, buildRouteApp,
+} = require('../integration/helpers/crmDb');
+
+describe('POST /api/admin/contracts/:id/upload-signed-pdf — path traversal guard', () => {
+  let db; let cleanup; let app; let adminId; let customerId; let token;
+
+  beforeAll(async () => {
+    ({ db, cleanup } = await bootCrmDb());
+    ({ adminId, customerId } = await seedMinimal(db));
+    await assignAdminRole(db, adminId, 'super_admin');
+    token = mintAdminToken(adminId);
+
+    // Feature flag defaults OFF on a fresh install — the contracts
+    // router 403s every route until it's on.
+    await db('feature_flags').where({ key: 'contracts' }).update({ value: true });
+
+    app = buildRouteApp('/api/admin/contracts', require('../../src/routes/adminContracts'));
+  }, 120000);
+
+  afterAll(async () => { await cleanup(); });
+
+  const auth = (req) => req.set('Authorization', `Bearer ${token}`);
+  const signedDir = () => path.join(process.env.STORAGE_PATH, 'uploads/contracts/signed');
+
+  async function insertContract(over = {}) {
+    const base = {
+      contract_number: `K-TEST-${Math.random().toString(16).slice(2, 8)}`,
+      customer_account_id: customerId,
+      title: 'Test Contract',
+      issue_date: new Date().toISOString().slice(0, 10),
+      status: 'sent',
+      language: 'de',
+      created_at: new Date().toISOString(),
+      ...over,
+    };
+    const inserted = await db('contracts').insert(base).returning('id');
+    return inserted[0]?.id ?? inserted[0];
+  }
+
+  it('rejects a traversal payload in the id param instead of writing outside uploads/contracts/signed', async () => {
+    // '../../../../tmp/pwned' URL-encoded so the raw request path still
+    // has a single segment (matches Express's `:id`), but Express
+    // decodes the param back into literal '../' sequences before the
+    // route sees it.
+    const traversalId = encodeURIComponent('../../../../tmp/pwned');
+
+    const res = await auth(
+      request(app).post(`/api/admin/contracts/${traversalId}/upload-signed-pdf`)
+    ).attach('file', Buffer.from('%PDF-1.4 fake'), 'signed.pdf');
+
+    expect(res.status).toBeGreaterThanOrEqual(400);
+    expect(res.body.error).toMatch(/invalid contract id/i);
+
+    // No file should have been written anywhere — the filename callback
+    // must error out before multer opens a write stream.
+    const escapedFile = path.join(os.tmpdir(), 'pwned');
+    expect(fs.existsSync(escapedFile)).toBe(false);
+    if (fs.existsSync(signedDir())) {
+      expect(fs.readdirSync(signedDir())).toHaveLength(0);
+    }
+  });
+
+  it('still accepts a normal numeric contract id', async () => {
+    const id = await insertContract();
+
+    const res = await auth(
+      request(app).post(`/api/admin/contracts/${id}/upload-signed-pdf`)
+    ).attach('file', Buffer.from('%PDF-1.4 fake'), 'signed.pdf');
+
+    expect(res.status).toBe(200);
+
+    const files = fs.readdirSync(signedDir());
+    expect(files.some((f) => f.startsWith(`contract-${id}-`))).toBe(true);
+
+    const row = await db('contracts').where({ id }).first();
+    expect(row.status).toBe('fully_signed');
+    expect(row.signed_pdf_path).toMatch(new RegExp(`contract-${id}-`));
+  });
+});
diff --git a/backend/src/routes/adminContracts.js b/backend/src/routes/adminContracts.js
index d6c3564e..598be306 100644
--- a/backend/src/routes/adminContracts.js
+++ b/backend/src/routes/adminContracts.js
@@ -65,8 +65,12 @@ const signedPdfStorage = multer.diskStorage({
     cb(null, uploadDir);
   },
   filename: (req, file, cb) => {
+    const contractId = Number(req.params.id);
+    if (!Number.isInteger(contractId) || contractId <= 0) {
+      return cb(new Error('Invalid contract id'));
+    }
     const ext = path.extname(file.originalname) || '.pdf';
-    cb(null, `contract-${req.params.id}-${Date.now()}${ext}`);
+    cb(null, `contract-${contractId}-${Date.now()}${ext}`);
   },
 });