diff --git a/src/lib/auth/__tests__/profile-update.test.ts b/src/lib/auth/__tests__/profile-update.test.ts
new file mode 100644
index 000000000..ccb0b7a9b
--- /dev/null
+++ b/src/lib/auth/__tests__/profile-update.test.ts
@@ -0,0 +1,76 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    user: {
+      findUnique: vi.fn(),
+      update: vi.fn(),
+    },
+  },
+}));
+
+vi.mock("@/lib/auth/audit", () => ({
+  auditLog: vi.fn().mockResolvedValue(undefined),
+}));
+
+import { applyProfileUpdate } from "../profile-update";
+import { prisma } from "@/lib/db";
+
+const USER_ID = "user-1";
+
+const STUB_USER = {
+  id: USER_ID,
+  username: "marc",
+  displayName: null,
+  email: null,
+  role: "USER",
+  heightCm: null,
+  dateOfBirth: null,
+  gender: null,
+  timezone: "Europe/Berlin",
+  locale: null,
+};
+
+beforeEach(() => {
+  vi.resetAllMocks();
+  vi.mocked(prisma.user.update).mockResolvedValue(STUB_USER as never);
+});
+
+describe("applyProfileUpdate timezone validation", () => {
+  it("accepts a valid IANA zone", async () => {
+    const result = await applyProfileUpdate(USER_ID, {
+      timezone: "Europe/Berlin",
+    });
+    expect(result.ok).toBe(true);
+    expect(prisma.user.update).toHaveBeenCalledTimes(1);
+  });
+
+  it("rejects an unknown zone with 422", async () => {
+    const result = await applyProfileUpdate(USER_ID, {
+      timezone: "Mars/Tharsis",
+    });
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.status).toBe(422);
+      expect(result.message).toMatch(/Invalid IANA timezone/i);
+    }
+    expect(prisma.user.update).not.toHaveBeenCalled();
+  });
+
+  it("rejects an empty-string timezone", async () => {
+    const result = await applyProfileUpdate(USER_ID, { timezone: "" });
+    expect(result.ok).toBe(false);
+    expect(prisma.user.update).not.toHaveBeenCalled();
+  });
+
+  it("rejects an injection-shaped string", async () => {
+    const result = await applyProfileUpdate(USER_ID, {
+      timezone: "Europe/Berlin' OR 1=1",
+    });
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.status).toBe(422);
+    }
+    expect(prisma.user.update).not.toHaveBeenCalled();
+  });
+});