fix: check loopback before cert creation

stapler/cert_manager.py

@@ -5,6 +5,8 @@ import ssl
 import subprocess
 import typing
 
+import requests
+
 from stapler.strings import valid_host
 
 if typing.TYPE_CHECKING:
@@ -49,6 +51,18 @@ class CertManager:
     def exists(self, host: str) -> bool:
         return self.__exists_certbot(host) or self.__exists_self_signed(host)
 
+    def valid_host(self, host: str) -> bool:
+        try:
+            response = requests.head(
+                url=f"http://{host}/.well-known/stapler",
+                allow_redirects=True,
+                timeout=5,
+                stream=False,
+            )
+            return type(response.status_code) is int and response.status_code < 400
+        except Exception:
+            return False
+
     def init_cert(self, host: str) -> bool:
         if not self.exists(host):
             return self.__create_self_signed(host)
@@ -196,7 +210,9 @@ class CertManager:
         if host is None or not valid_host(host):
             return None
         self.logger.debug("servername callback: %s", host)
-        if not self.exists(host) and not self.create_or_update(host):
+        if not self.exists(host) and (
+            not self.valid_host(host) or not self.create_or_update(host)
+        ):
             return None
         cert_file = self.get_cert(host)
         key_file = self.get_key(host)

tests/test_cert_manager.py

@@ -4,6 +4,8 @@ import subprocess
 import typing
 import unittest.mock
 
+import requests
+
 from stapler.cert_manager import CertManager, CertManagerError
 from stapler.params import Parameters
 
@@ -170,10 +172,24 @@ class TestRegistry(BaseTestCase):
                 self.socket_mock, None, self.context_mock
             )
 
-    def test_servername_callback_fail(self) -> None:
+    def test_servername_callback_fail_no_valid_host(self) -> None:
         self._make_self_signed("example.com")
+        with (
+            self.patch("requests.head") as request_mock,
+            self.patch("ssl.create_default_context", count=0),
+        ):
+            request_mock.side_effect = Exception()
+            self.cert_manager.servername_callback(
+                self.socket_mock, "example.fr", self.context_mock
+            )
+
+    def test_servername_callback_fail_no_binaries(self) -> None:
+        self._make_self_signed("example.com")
+        response = requests.Response()
+        response.status_code = 200
         with (
             self.patch("shutil.which", count=3),
+            self.patch("requests.head", response),
             self.patch("ssl.create_default_context", count=0),
         ):
             self.cert_manager.servername_callback(