@@ -37,6 +37,10 @@ class NoSuchPathError(GitError, OSError):
     """Thrown if a path could not be access by the system."""
 
 
+class UnsafeOptionsUsedError(GitError):
+    """Thrown if unsafe protocols or options are passed without overridding."""
+
+
 class CommandError(GitError):
     """Base class for exceptions thrown at every stage of `Popen()` execution.
 
 
@@ -21,7 +21,12 @@
 )
 from git.config import GitConfigParser
 from git.db import GitCmdObjectDB
-from git.exc import InvalidGitRepositoryError, NoSuchPathError, GitCommandError
+from git.exc import (
+    GitCommandError,
+    InvalidGitRepositoryError,
+    NoSuchPathError,
+    UnsafeOptionsUsedError,
+)
 from git.index import IndexFile
 from git.objects import Submodule, RootModule, Commit
 from git.refs import HEAD, Head, Reference, TagReference
@@ -128,6 +133,7 @@ class Repo(object):
     re_envvars = re.compile(r"(\$(\{\s?)?[a-zA-Z_]\w*(\}\s?)?|%\s?[a-zA-Z_]\w*\s?%)")
     re_author_committer_start = re.compile(r"^(author|committer)")
     re_tab_full_line = re.compile(r"^\t(.*)$")
+    re_config_protocol_option = re.compile(r"-[-]?c(|onfig)\s+protocol\.", re.I)
 
     # invariants
     # represents the configuration level of a configuration file
@@ -1215,11 +1221,27 @@ def _clone(
         # END handle remote repo
         return repo
 
+    @classmethod
+    def unsafe_options(
+        cls,
+        url: str,
+        multi_options: Optional[List[str]] = None,
+    ) -> bool:
+        if "ext::" in url:
+            return True
+        if multi_options is not None:
+            if any(["--upload-pack" in m for m in multi_options]):
+                return True
+            if any([re.match(cls.re_config_protocol_option, m) for m in multi_options]):
+                return True
+        return False
+
     def clone(
         self,
         path: PathLike,
         progress: Optional[Callable] = None,
         multi_options: Optional[List[str]] = None,
+        unsafe_protocols: bool = False,
         **kwargs: Any,
     ) -> "Repo":
         """Create a clone from this repository.
@@ -1230,12 +1252,15 @@ def clone(
             option per list item which is passed exactly as specified to clone.
             For example ['--config core.filemode=false', '--config core.ignorecase',
             '--recurse-submodule=repo1_path', '--recurse-submodule=repo2_path']
+        :param unsafe_protocols: Allow unsafe protocols to be used, like ext
         :param kwargs:
             * odbt = ObjectDatabase Type, allowing to determine the object database
               implementation used by the returned Repo instance
             * All remaining keyword arguments are given to the git-clone command
 
         :return: ``git.Repo`` (the newly cloned repo)"""
+        if not unsafe_protocols and self.unsafe_options(path, multi_options):
+            raise UnsafeOptionsUsedError(f"{path} requires unsafe_protocols flag")
         return self._clone(
             self.git,
             self.common_dir,
@@ -1254,6 +1279,7 @@ def clone_from(
         progress: Optional[Callable] = None,
         env: Optional[Mapping[str, str]] = None,
         multi_options: Optional[List[str]] = None,
+        unsafe_protocols: bool = False,
         **kwargs: Any,
     ) -> "Repo":
         """Create a clone from the given URL
@@ -1268,11 +1294,14 @@ def clone_from(
             If you want to unset some variable, consider providing empty string
             as its value.
         :param multi_options: See ``clone`` method
+        :param unsafe_protocols: Allow unsafe protocols to be used, like ext
         :param kwargs: see the ``clone`` method
         :return: Repo instance pointing to the cloned directory"""
         git = cls.GitCommandWrapperType(os.getcwd())
         if env is not None:
             git.update_environment(**env)
+        if not unsafe_protocols and cls.unsafe_options(url, multi_options):
+            raise UnsafeOptionsUsedError(f"{url} requires unsafe_protocols flag")
         return cls._clone(git, url, to_path, GitCmdObjectDB, progress, multi_options, **kwargs)
 
     def archive(
 
@@ -13,6 +13,7 @@
 import pickle
 import sys
 import tempfile
+import uuid
 from unittest import mock, skipIf, SkipTest
 
 import pytest
@@ -37,6 +38,7 @@
 )
 from git.exc import (
     BadObject,
+    UnsafeOptionsUsedError,
 )
 from git.repo.fun import touch
 from test.lib import TestBase, with_rw_repo, fixture
@@ -263,6 +265,40 @@ def test_leaking_password_in_clone_logs(self, rw_dir):
             to_path=rw_dir,
         )
 
+    def test_unsafe_options(self):
+        self.assertFalse(Repo.unsafe_options("github.com/deploy/deploy"))
+
+    def test_unsafe_options_ext_url(self):
+        self.assertTrue(Repo.unsafe_options("ext::ssh"))
+
+    def test_unsafe_options_multi_options_upload_pack(self):
+        self.assertTrue(Repo.unsafe_options("", ["--upload-pack='touch foo'"]))
+
+    def test_unsafe_options_multi_options_config_user(self):
+        self.assertFalse(Repo.unsafe_options("", ["--config user"]))
+
+    def test_unsafe_options_multi_options_config_protocol(self):
+        self.assertTrue(Repo.unsafe_options("", ["--config protocol.foo"]))
+
+    def test_clone_from_forbids_helper_urls_by_default(self):
+        with self.assertRaises(UnsafeOptionsUsedError):
+            Repo.clone_from("ext::sh -c touch% /tmp/foo", "tmp")
+
+    @with_rw_repo("HEAD")
+    def test_clone_from_allow_unsafe(self, repo):
+        bad_filename = pathlib.Path(f'{tempfile.gettempdir()}/{uuid.uuid4()}')
+        bad_url = f'ext::sh -c touch% {bad_filename}'
+        try:
+            repo.clone_from(
+                bad_url, 'tmp',
+                multi_options=["-c protocol.ext.allow=always"],
+                unsafe_protocols=True
+            )
+        except GitCommandError:
+            pass
+        self.assertTrue(bad_filename.is_file())
+        bad_filename.unlink()
+
     @with_rw_repo("HEAD")
     def test_max_chunk_size(self, repo):
         class TestOutputStream(TestBase):