diff --git a/backend/src/middleware/__tests__/requireDeploymentAuth.test.ts b/backend/src/middleware/__tests__/requireDeploymentAuth.test.ts
new file mode 100644
index 00000000..859d8736
--- /dev/null
+++ b/backend/src/middleware/__tests__/requireDeploymentAuth.test.ts
@@ -0,0 +1,63 @@
+import { describe, expect, it, vi, beforeEach } from 'vitest';
+
+const getById = vi.fn();
+const verifyApiKey = vi.fn();
+
+vi.mock('../../repositories/deploymentRepository.js', () => ({
+  createDeploymentRepository: () => ({ getById }),
+  verifyApiKey: (...args: unknown[]) => verifyApiKey(...args),
+}));
+
+vi.mock('../../repositories/projectRepository.js', () => ({
+  getProjectRepository: () => ({}),
+}));
+
+vi.mock('../../services/authService.js', () => ({
+  authService: { verifyAccessToken: vi.fn() },
+}));
+
+vi.mock('../resourceOwnership.js', () => ({
+  verifyProjectOwnership: vi.fn(),
+}));
+
+import { requireDeploymentAuth } from '../requireDeploymentAuth.js';
+
+function mockRes() {
+  const res: any = {};
+  res.status = vi.fn().mockReturnValue(res);
+  res.json = vi.fn().mockReturnValue(res);
+  return res;
+}
+
+describe('requireDeploymentAuth enumeration resistance', () => {
+  beforeEach(() => {
+    getById.mockReset();
+    verifyApiKey.mockReset();
+  });
+
+  it('returns 404 Not found for missing deployment', async () => {
+    getById.mockResolvedValue(null);
+    const req: any = { params: { deploymentId: 'missing' }, headers: {} };
+    const res = mockRes();
+    const next = vi.fn();
+    await requireDeploymentAuth(req, res, next);
+    expect(res.status).toHaveBeenCalledWith(404);
+    expect(res.json).toHaveBeenCalledWith({ error: 'Not found' });
+    expect(next).not.toHaveBeenCalled();
+  });
+
+  it('returns the same 404 Not found for a bad API key', async () => {
+    getById.mockResolvedValue({ deploymentId: 'dep-1', projectId: 'p1' });
+    verifyApiKey.mockResolvedValue(null);
+    const req: any = {
+      params: { deploymentId: 'dep-1' },
+      headers: { 'x-api-key': 'bad-key' },
+    };
+    const res = mockRes();
+    const next = vi.fn();
+    await requireDeploymentAuth(req, res, next);
+    expect(res.status).toHaveBeenCalledWith(404);
+    expect(res.json).toHaveBeenCalledWith({ error: 'Not found' });
+    expect(next).not.toHaveBeenCalled();
+  });
+});
diff --git a/backend/src/middleware/requireDeploymentAuth.ts b/backend/src/middleware/requireDeploymentAuth.ts
index 8854ed80..f547b615 100644
--- a/backend/src/middleware/requireDeploymentAuth.ts
+++ b/backend/src/middleware/requireDeploymentAuth.ts
@@ -45,8 +45,9 @@ export async function requireDeploymentAuth(
   const apiKey = req.headers['x-api-key'] as string | undefined;
   if (apiKey) {
     const keyRecord = await verifyApiKey(apiKey, deploymentRepo);
+    // Same 404 body as a missing deployment so API keys cannot enumerate IDs.
     if (!keyRecord || keyRecord.deploymentId !== deployment.deploymentId) {
-      res.status(403).json({ error: 'Forbidden' });
+      res.status(404).json({ error: 'Not found' });
       return;
     }
     req.deployment = deployment;