diff --git a/.env.example b/.env.example
index 67097b71045..f259d4d854a 100644
--- a/.env.example
+++ b/.env.example
@@ -3070,6 +3070,11 @@ QUOTA_STORE_DRIVER=sqlite
 # Telegram Mini App bridge. The update endpoint remains disabled while the bot
 # token is unset. Used by: src/lib/telegram/* and src/app/api/telegram/update/route.ts.
 # TELEGRAM_BOT_TOKEN=
+# Shared secret registered with setWebhook and echoed back by Telegram as the
+# X-Telegram-Bot-Api-Secret-Token header. REQUIRED for the webhook path: without
+# it the webhook is rejected with 503, because an unauthenticated update lets any
+# caller mint API keys and spend upstream quota. The Mini App path does not use it.
+# TELEGRAM_WEBHOOK_SECRET=
 # TELEGRAM_DEFAULT_MODEL=auto/chat
 # TELEGRAM_BOT_API_BASE=https://api.telegram.org
 # TELEGRAM_WEBHOOK_TIMEOUT_MS=60000
diff --git a/changelog.d/fixes/13172-telegram-webhook-secret.md b/changelog.d/fixes/13172-telegram-webhook-secret.md
new file mode 100644
index 00000000000..49f14b55fb7
--- /dev/null
+++ b/changelog.d/fixes/13172-telegram-webhook-secret.md
@@ -0,0 +1 @@
+- **fix(telegram):** authenticate webhook deliveries with Telegram's `secret_token` so an unauthenticated caller can no longer mint API keys or spend upstream quota ([#13172](https://github.com/diegosouzapw/OmniRoute/issues/13172))
diff --git a/docs/reference/ENVIRONMENT.md b/docs/reference/ENVIRONMENT.md
index f357f735840..13b70dd4e3b 100644
--- a/docs/reference/ENVIRONMENT.md
+++ b/docs/reference/ENVIRONMENT.md
@@ -1623,6 +1623,7 @@ These settings were introduced after the previous environment-contract snapshot.
 | `ADOBE_FIREFLY_CHROME_HEADLESS` | `0` | `open-sse/services/adobeFireflyBrowserLogin.ts` | Debug-only true-headless mode; Adobe colligo normally rejects the resulting risk session. |
 | `CHROME_PATH` | auto-detect | `open-sse/executors/cloudflare-playground.ts`, `open-sse/executors/chatgpt-web-codex.ts` | Optional absolute Chrome executable used by the browser-driven executors when platform auto-detection is insufficient. |
 | `TELEGRAM_BOT_TOKEN` | _(unset)_ | `src/lib/telegram/config.ts` | BotFather token that enables the inbound webhook and signs Mini App `initData`. |
+| `TELEGRAM_WEBHOOK_SECRET` | _(unset)_ | `src/lib/telegram/config.ts` | Shared secret registered via `setWebhook` and verified against the `X-Telegram-Bot-Api-Secret-Token` header on every webhook delivery. Required for the webhook path; unset means webhook deliveries are refused with 503. |
 | `TELEGRAM_DEFAULT_MODEL` | `auto/chat` | `src/lib/telegram/chatProxy.ts` | Model used for Telegram chat replies. |
 | `TELEGRAM_BOT_API_BASE` | `https://api.telegram.org` | `src/lib/telegram/config.ts` | Bot API base URL override for proxies or self-hosted Bot API servers. |
 | `TELEGRAM_WEBHOOK_TIMEOUT_MS` | `60000` | `src/lib/telegram/config.ts` | Timeout in milliseconds for outbound Bot API calls. |
diff --git a/src/app/api/telegram/update/route.ts b/src/app/api/telegram/update/route.ts
index 1fd3582da99..c2194a58955 100644
--- a/src/app/api/telegram/update/route.ts
+++ b/src/app/api/telegram/update/route.ts
@@ -13,12 +13,18 @@
  *   3. Handles /start (returns the Mini App deep link) and everything else
  *      as a chat prompt proxied through the OmniRoute pipeline.
  */
+import { timingSafeEqual } from "node:crypto";
 import { NextResponse } from "next/server";
 import { z } from "zod";
 import { validateBody, isValidationFailure } from "@/shared/validation/helpers";
 import type { TelegramUpdate } from "@/lib/telegram/botApi";
 import { extractChatMessage, sendTelegramMessage } from "@/lib/telegram/botApi";
-import { getTelegramBotToken, isTelegramEnabled } from "@/lib/telegram/config";
+import {
+  getTelegramBotToken,
+  getTelegramWebhookSecret,
+  isTelegramEnabled,
+  isTelegramWebhookSecretConfigured,
+} from "@/lib/telegram/config";
 import { verifyInitData, parseInitData } from "@/lib/telegram/initData";
 import { proxyChat } from "@/lib/telegram/chatProxy";
 import { formatTelegramGatewayError } from "@/lib/telegram/errorMessage";
@@ -33,7 +39,12 @@ import { resolveOmniRouteBaseUrl } from "@/shared/utils/resolveOmniRouteBaseUrl"
 const telegramBodySchema = z
   .object({
     initData: z.string().optional(),
-    message: z.string().optional(),
+    // `message` is a STRING on the Mini App path ({ initData, message }) and an
+    // OBJECT on the webhook path (a Telegram update). Constraining it to a
+    // string rejected every real webhook delivery with 400 before any auth or
+    // routing ran, so accept either shape here and let each branch validate the
+    // shape it actually needs.
+    message: z.union([z.string(), z.record(z.string(), z.unknown())]).optional(),
     update_id: z.number().optional(),
     // allow unknown update fields
   })
@@ -103,6 +114,21 @@ export async function POST(request: Request) {
   }
 
   // ── Bot webhook path: TelegramUpdate ─────────────────────────────────────
+  // Unlike the Mini App branch above (which verifies the initData HMAC), a
+  // webhook body carries no proof of origin: `chat.id` is attacker-chosen and
+  // reaches proxyChat(), which mints a real API key and spends upstream quota.
+  // Telegram's `secret_token` echo is the only authentication available here.
+  if (!isTelegramWebhookSecretConfigured()) {
+    return NextResponse.json(
+      { ok: false, error: "Telegram webhook secret not configured" },
+      { status: 503 }
+    );
+  }
+  const presentedSecret = request.headers.get("x-telegram-bot-api-secret-token") || "";
+  if (!webhookSecretMatches(presentedSecret, getTelegramWebhookSecret())) {
+    return NextResponse.json({ ok: false, error: "Unauthorized" }, { status: 401 });
+  }
+
   const update = body as unknown as TelegramUpdate;
   const chat = extractChatMessage(update);
   if (!chat) {
@@ -117,6 +143,22 @@ export async function POST(request: Request) {
   return NextResponse.json({ ok: true });
 }
 
+/**
+ * Constant-time comparison of the presented webhook secret against the
+ * configured one. A plain `===` short-circuits on the first differing byte and
+ * leaks the shared-prefix length through response timing; `timingSafeEqual`
+ * does not. It requires equal-length buffers, so a length mismatch is rejected
+ * up front (the length itself is not secret).
+ *
+ * Exported as a test seam only — not part of the route contract.
+ */
+export function webhookSecretMatches(presented: string, expected: string): boolean {
+  const a = Buffer.from(presented);
+  const b = Buffer.from(expected);
+  if (a.length !== b.length) return false;
+  return timingSafeEqual(a, b);
+}
+
 async function handleAndReply(chatId: number, text: string, messageId?: number): Promise<void> {
   try {
     const trimmed = text.trim();
diff --git a/src/lib/telegram/botApi.ts b/src/lib/telegram/botApi.ts
index 4bdc50071a5..c1a0e5e0489 100644
--- a/src/lib/telegram/botApi.ts
+++ b/src/lib/telegram/botApi.ts
@@ -5,7 +5,12 @@
  * replies and setWebhook for webhook registration. Streaming is emulated
  * by the caller via progressive edits (sendMessage / editMessageText).
  */
-import { getTelegramBotApiBase, getTelegramBotToken, getTelegramWebhookTimeoutMs } from "./config";
+import {
+  getTelegramBotApiBase,
+  getTelegramBotToken,
+  getTelegramWebhookTimeoutMs,
+  getTelegramWebhookSecret,
+} from "./config";
 
 export interface TelegramSendMessageParams {
   chat_id: number | string;
@@ -92,7 +97,15 @@ export async function setTelegramWebhook(
   opts: { dropPending?: boolean } = {}
 ): Promise<{ url: string; pending_update_count?: number }> {
   if (url) {
-    return botFetch("setWebhook", { url, drop_pending_updates: opts.dropPending ?? true });
+    // Register the shared secret so Telegram echoes it back as
+    // X-Telegram-Bot-Api-Secret-Token on every delivery; the webhook route
+    // rejects deliveries that do not carry it (#13172).
+    const secret = getTelegramWebhookSecret();
+    return botFetch("setWebhook", {
+      url,
+      drop_pending_updates: opts.dropPending ?? true,
+      ...(secret ? { secret_token: secret } : {}),
+    });
   }
   return botFetch("deleteWebhook", { drop_pending_updates: opts.dropPending ?? true });
 }
diff --git a/src/lib/telegram/config.ts b/src/lib/telegram/config.ts
index 421739ef5ed..817641e9be9 100644
--- a/src/lib/telegram/config.ts
+++ b/src/lib/telegram/config.ts
@@ -25,6 +25,30 @@ export function getTelegramWebhookTimeoutMs(): number {
   return Number.isInteger(parsed) && parsed > 0 ? parsed : DEFAULT_WEBHOOK_TIMEOUT_MS;
 }
 
+/**
+ * Shared secret for authenticating Telegram webhook deliveries.
+ *
+ * Telegram echoes the `secret_token` passed to `setWebhook` back on every
+ * delivery in the `X-Telegram-Bot-Api-Secret-Token` header, which is the only
+ * way to prove a webhook POST actually came from Telegram. Kept in the
+ * environment alongside the bot token so it is never stored in the DB.
+ */
+export function getTelegramWebhookSecret(): string {
+  return process.env.TELEGRAM_WEBHOOK_SECRET || "";
+}
+
+/**
+ * Whether webhook deliveries are authenticated.
+ *
+ * When no secret is configured the webhook path is rejected outright rather
+ * than served unauthenticated: an open path mints API keys and spends upstream
+ * quota for any caller (see #13172). The Mini App path is unaffected — it
+ * authenticates with the initData HMAC and does not use this secret.
+ */
+export function isTelegramWebhookSecretConfigured(): boolean {
+  return getTelegramWebhookSecret().length > 0;
+}
+
 export function getTelegramBotApiBase(): string {
   return process.env.TELEGRAM_BOT_API_BASE || "https://api.telegram.org";
 }
diff --git a/tests/unit/telegram-webhook-secret-13172.test.ts b/tests/unit/telegram-webhook-secret-13172.test.ts
new file mode 100644
index 00000000000..f525ee745a5
--- /dev/null
+++ b/tests/unit/telegram-webhook-secret-13172.test.ts
@@ -0,0 +1,72 @@
+/**
+ * Regression test for #13172: the Telegram webhook path must authenticate.
+ *
+ * Telegram echoes the `secret_token` given to `setWebhook` back on every
+ * delivery as `X-Telegram-Bot-Api-Secret-Token`. Without checking it, any
+ * caller can POST a synthetic update with an arbitrary `chat.id`, which reaches
+ * proxyChat() and mints a real API key plus upstream spend.
+ *
+ * The Mini App branch authenticates separately (initData HMAC) and must keep
+ * working without a webhook secret.
+ */
+import { describe, test, before, after } from "node:test";
+import assert from "node:assert/strict";
+
+const BOT_TOKEN = "123456:AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA";
+const SECRET = "s3cret-webhook-token";
+
+let POST: (req: Request) => Promise<Response>;
+let webhookSecretMatches: (a: string, b: string) => boolean;
+const proxied: number[] = [];
+
+before(async () => {
+  process.env.TELEGRAM_BOT_TOKEN = BOT_TOKEN;
+  process.env.TELEGRAM_WEBHOOK_SECRET = SECRET;
+
+  const mod = await import("../../src/app/api/telegram/update/route.ts");
+  POST = mod.POST as typeof POST;
+  webhookSecretMatches = mod.webhookSecretMatches as typeof webhookSecretMatches;
+});
+
+after(() => {
+  delete process.env.TELEGRAM_WEBHOOK_SECRET;
+});
+
+function webhookRequest(headers: Record<string, string> = {}): Request {
+  return new Request("https://example.test/api/telegram/update", {
+    method: "POST",
+    headers: { "content-type": "application/json", ...headers },
+    // A realistic Telegram update: `message` is an object here, whereas the
+    // Mini App path sends it as a string. Both shapes must reach their branch.
+    body: JSON.stringify({
+      update_id: 1,
+      message: { chat: { id: 999 }, text: "hi", message_id: 5 },
+    }),
+  });
+}
+
+describe("telegram webhook authentication (#13172)", () => {
+  test("rejects a delivery with no secret header", async () => {
+    const res = await POST(webhookRequest());
+    assert.equal(res.status, 401, "unauthenticated webhook must be rejected");
+    assert.deepEqual(proxied, [], "no chat should be proxied");
+  });
+
+  test("rejects a delivery with a wrong secret", async () => {
+    const res = await POST(
+      webhookRequest({ "x-telegram-bot-api-secret-token": "wrong-token-value" })
+    );
+    assert.equal(res.status, 401, "a mismatched secret must be rejected");
+  });
+
+  test("accepts a delivery carrying the configured secret", async () => {
+    const res = await POST(webhookRequest({ "x-telegram-bot-api-secret-token": SECRET }));
+    assert.equal(res.status, 200, "a correctly authenticated delivery must be accepted");
+  });
+
+  test("comparison is length-safe and value-correct", () => {
+    assert.equal(webhookSecretMatches(SECRET, SECRET), true);
+    assert.equal(webhookSecretMatches("short", SECRET), false, "length mismatch must not throw");
+    assert.equal(webhookSecretMatches("", ""), true, "equal empties compare equal");
+  });
+});