fix(telegram): require TELEGRAM_WEBHOOK_SECRET in webhook mode (#13527)

When TELEGRAM_WEBHOOK_URL was set but TELEGRAM_WEBHOOK_SECRET was not,
python-telegram-bot received secret_token=None and the webhook endpoint
accepted any HTTP POST. Anyone who could reach the listener could inject
forged updates — spoofed user IDs, spoofed chat IDs, attacker-controlled
message text — and trigger handlers as if Telegram delivered them.

The fix refuses to start the adapter in webhook mode without the secret.
Polling mode (default, no webhook URL) is unaffected — polling is
authenticated by the bot token directly.

BREAKING CHANGE for webhook-mode deployments that never set
TELEGRAM_WEBHOOK_SECRET. The error message explains remediation:

  export TELEGRAM_WEBHOOK_SECRET="$(openssl rand -hex 32)"

and instructs registering it with Telegram via setWebhook's secret_token
parameter. Release notes must call this out.

Reported in GHSA-3vpc-7q5r-276h by @bupt-Yy-young. Hardening — not CVE
per SECURITY.md §3 "Public Exposure: Deploying the gateway to the
public internet without external authentication or network protection"
covers the historical default, but shipping a fail-open webhook as the
default was the wrong choice and the guard aligns us with the SECURITY.md
threat model.

tests/gateway/test_telegram_webhook_secret.py

@@ -0,0 +1,100 @@
+"""Tests for GHSA-3vpc-7q5r-276h — Telegram webhook secret required.
+
+Previously, when TELEGRAM_WEBHOOK_URL was set but TELEGRAM_WEBHOOK_SECRET
+was not, python-telegram-bot received secret_token=None and the webhook
+endpoint accepted any HTTP POST.
+
+The fix refuses to start the adapter in webhook mode without the secret.
+"""
+
+from __future__ import annotations
+
+import re
+import sys
+from pathlib import Path
+
+import pytest
+
+_repo = str(Path(__file__).resolve().parents[2])
+if _repo not in sys.path:
+    sys.path.insert(0, _repo)
+
+
+class TestTelegramWebhookSecretRequired:
+    """Direct source-level check of the webhook-secret guard.
+
+    The guard is embedded in TelegramAdapter.connect() and hard to isolate
+    via mocks (requires a full python-telegram-bot ApplicationBuilder
+    chain). These tests exercise it via source inspection — verifying the
+    check exists, raises RuntimeError with the advisory link, and only
+    fires in webhook mode. End-to-end validation is covered by CI +
+    manual deployment tests.
+    """
+
+    def _get_source(self) -> str:
+        path = Path(_repo) / "gateway" / "platforms" / "telegram.py"
+        return path.read_text(encoding="utf-8")
+
+    def test_webhook_branch_checks_secret(self):
+        """The webhook-mode branch of connect() must read
+        TELEGRAM_WEBHOOK_SECRET and refuse when empty."""
+        src = self._get_source()
+        # The guard must appear after TELEGRAM_WEBHOOK_URL is set
+        assert re.search(
+            r'TELEGRAM_WEBHOOK_SECRET.*?\.strip\(\)\s*\n\s*if not webhook_secret:',
+            src, re.DOTALL,
+        ), (
+            "TelegramAdapter.connect() must strip TELEGRAM_WEBHOOK_SECRET "
+            "and raise when the secret is empty — see GHSA-3vpc-7q5r-276h"
+        )
+
+    def test_guard_raises_runtime_error(self):
+        """The guard raises RuntimeError (not a silent log) so operators
+        see the failure at startup."""
+        src = self._get_source()
+        # Between the "if not webhook_secret:" line and the next blank
+        # line block, we should see a RuntimeError being raised
+        guard_match = re.search(
+            r'if not webhook_secret:\s*\n\s*raise\s+RuntimeError\(',
+            src,
+        )
+        assert guard_match, (
+            "Missing webhook secret must raise RuntimeError — silent "
+            "fall-through was the original GHSA-3vpc-7q5r-276h bypass"
+        )
+
+    def test_guard_message_includes_advisory_link(self):
+        """The RuntimeError message should reference the advisory so
+        operators can read the full context."""
+        src = self._get_source()
+        assert "GHSA-3vpc-7q5r-276h" in src, (
+            "Guard error message must cite the advisory for operator context"
+        )
+
+    def test_guard_message_explains_remediation(self):
+        """The error should tell the operator how to fix it."""
+        src = self._get_source()
+        # Should mention how to generate a secret
+        assert "openssl rand" in src or "TELEGRAM_WEBHOOK_SECRET=" in src, (
+            "Guard error message should show operators how to set "
+            "TELEGRAM_WEBHOOK_SECRET"
+        )
+
+    def test_polling_branch_has_no_secret_guard(self):
+        """Polling mode (else-branch) must NOT require the webhook secret —
+        polling authenticates via the bot token, not a webhook secret."""
+        src = self._get_source()
+        # The guard should appear inside the `if webhook_url:` branch,
+        # not the `else:` polling branch. Rough check: the raise is
+        # followed (within ~60 lines) by an `else:` that starts the
+        # polling branch, and there's no secret-check in that polling
+        # branch.
+        webhook_block = re.search(
+            r'if webhook_url:\s*\n(.*?)\n            else:\s*\n(.*?)\n',
+            src, re.DOTALL,
+        )
+        if webhook_block:
+            webhook_body = webhook_block.group(1)
+            polling_body = webhook_block.group(2)
+            assert "TELEGRAM_WEBHOOK_SECRET" in webhook_body
+            assert "TELEGRAM_WEBHOOK_SECRET" not in polling_body