diff --git a/src/lib/server/captcha.test.ts b/src/lib/server/captcha.test.ts
new file mode 100644
index 0000000..cce7dea
--- /dev/null
+++ b/src/lib/server/captcha.test.ts
@@ -0,0 +1,49 @@
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+process.env.WEBUI_AUTH_SECRET = "test-secret-with-at-least-32-characters";
+
+const { createCaptcha, verifyCaptcha } = await import("./captcha");
+
+afterEach(() => {
+  vi.useRealTimers();
+});
+
+describe("captcha", () => {
+  it("round-trips: correct answer verifies", () => {
+    const { id, question } = createCaptcha();
+    const match = question.match(/^(\d+) \+ (\d+) = \?$/);
+    expect(match).not.toBeNull();
+    const answer = String(Number(match![1]) + Number(match![2]));
+    expect(verifyCaptcha(id, answer)).toBe(true);
+  });
+
+  it("rejects a wrong answer", () => {
+    const { id, question } = createCaptcha();
+    const match = question.match(/^(\d+) \+ (\d+) = \?/)!;
+    const wrong = String(Number(match[1]) + Number(match[2]) + 1);
+    expect(verifyCaptcha(id, wrong)).toBe(false);
+  });
+
+  it("rejects a tampered payload", () => {
+    const { id } = createCaptcha();
+    const [payload] = id.split(".");
+    const forged = `${payload}.${"a".repeat(43)}`;
+    expect(verifyCaptcha(forged, "2")).toBe(false);
+  });
+
+  it("rejects an expired captcha", () => {
+    vi.useFakeTimers();
+    vi.setSystemTime(new Date("2026-08-08T00:00:00Z"));
+    const { id, question } = createCaptcha();
+    const match = question.match(/^(\d+) \+ (\d+) = \?/)!;
+    const answer = String(Number(match[1]) + Number(match[2]));
+    vi.setSystemTime(new Date("2026-08-08T00:06:00Z"));
+    expect(verifyCaptcha(id, answer)).toBe(false);
+  });
+
+  it("rejects malformed inputs", () => {
+    expect(verifyCaptcha(undefined, "2")).toBe(false);
+    expect(verifyCaptcha("not-a-token", "2")).toBe(false);
+    expect(verifyCaptcha(123, "2")).toBe(false);
+  });
+});
diff --git a/src/lib/server/captcha.ts b/src/lib/server/captcha.ts
new file mode 100644
index 0000000..574217a
--- /dev/null
+++ b/src/lib/server/captcha.ts
@@ -0,0 +1,58 @@
+import { createHmac, randomInt, timingSafeEqual } from "node:crypto";
+import { authSecret } from "@/lib/auth";
+
+const CAPTCHA_TTL_MS = 5 * 60 * 1000;
+
+interface CaptchaPayload {
+  a: number;
+  b: number;
+  exp: number;
+}
+
+function sign(encoded: string, secret: string): string {
+  return createHmac("sha256", secret).update(encoded).digest("base64url");
+}
+
+export function createCaptcha(): { id: string; question: string } {
+  const secret = authSecret();
+  if (!secret) throw new Error("WEBUI_AUTH_SECRET is not configured.");
+  const a = randomInt(1, 10);
+  const b = randomInt(1, 10);
+  const payload: CaptchaPayload = { a, b, exp: Date.now() + CAPTCHA_TTL_MS };
+  const encoded = Buffer.from(JSON.stringify(payload)).toString("base64url");
+  return {
+    id: `${encoded}.${sign(encoded, secret)}`,
+    question: `${a} + ${b} = ?`,
+  };
+}
+
+export function verifyCaptcha(id: unknown, answer: unknown): boolean {
+  const secret = authSecret();
+  if (!secret || typeof id !== "string" || typeof answer !== "string") {
+    return false;
+  }
+  const [encoded, signature] = id.split(".");
+  if (!encoded || !signature) return false;
+  const expected = Buffer.from(sign(encoded, secret));
+  const provided = Buffer.from(signature);
+  if (provided.length !== expected.length || !timingSafeEqual(provided, expected)) {
+    return false;
+  }
+  let payload: Partial<CaptchaPayload>;
+  try {
+    payload = JSON.parse(
+      Buffer.from(encoded, "base64url").toString("utf8")
+    ) as Partial<CaptchaPayload>;
+  } catch {
+    return false;
+  }
+  if (
+    typeof payload.a !== "number" ||
+    typeof payload.b !== "number" ||
+    typeof payload.exp !== "number" ||
+    payload.exp < Date.now()
+  ) {
+    return false;
+  }
+  return answer.trim() === String(payload.a + payload.b);
+}