Reject syntax-bearing git config option names

GHSA-jm78-9fvv-mhgr reports that config option names containing Git
syntax can be serialized as unintended directives. A regression test showed
that set, set_value, and add_value accepted delimiter, comment, bracket, and
whitespace characters in option names.

Restrict written option names to GitPython's established safe character set of
letters, digits, hyphens, underscores, and dots. This blocks characters that
can change config syntax while preserving option names historically supported
by the writer and SectionConstraint.

A broader audit confirmed that every public option-creating config API and
SectionConstraint delegate reaches this validator; no separate config writer
sink was found. The behavior was checked against Git cf5497b14, and the full
config test module plus dotted-option regression pass.

Author: codex

git/config.py

@@ -75,6 +75,9 @@
 UNSAFE_CONFIG_CHARS_RE = re.compile(r"[\r\n\x00]")
 """Characters that cannot be safely written in config names or values."""
 
+VALID_CONFIG_OPTION_NAME_RE = re.compile(r"^[A-Za-z0-9_.-]+$")
+"""Pattern for option names that can be written without changing config syntax."""
+
 
 class MetaParserBuilder(abc.ABCMeta):  # noqa: B024
     """Utility class wrapping base-class methods into decorators that assure read-only
@@ -897,6 +900,8 @@ def _value_to_string_safe(self, value: Union[str, bytes, int, float, bool]) -> s
     def _assure_config_name_safe(self, name: "cp._SectionName", label: str) -> None:
         if isinstance(name, str) and UNSAFE_CONFIG_CHARS_RE.search(name):
             raise ValueError("Git config %s names must not contain CR, LF, or NUL" % label)
+        if label == "option" and isinstance(name, str) and not VALID_CONFIG_OPTION_NAME_RE.fullmatch(name):
+            raise ValueError("Git config option names may contain only letters, digits, '-', '_', or '.'")
         if label == "section" and isinstance(name, str):
             in_quotes = False
             escaped = False

test/test_config.py

@@ -194,6 +194,43 @@ def test_set_value_rejects_unsafe_section_and_option_names(self, rw_dir):
             self.assertEqual(git_config.get_value("user", "name"), "safe")
             self.assertFalse(git_config.has_section("core"))
 
+    @with_rw_directory
+    def test_writer_rejects_invalid_option_names(self, rw_dir):
+        config_path = osp.join(rw_dir, "config")
+        bad_options = (
+            "name=value",
+            "name#comment",
+            "name;comment",
+            "name with space",
+            "name\twith-tab",
+            "name[section",
+            "name]section",
+            "name:colon",
+            'name"quote',
+            "name\\escape",
+        )
+
+        with GitConfigParser(config_path, read_only=False) as git_config:
+            git_config.add_section("user")
+            for bad_option in bad_options:
+                with pytest.raises(ValueError, match="option name"):
+                    git_config.set("user", bad_option, "unsafe")
+                with pytest.raises(ValueError, match="option name"):
+                    git_config.set_value("user", bad_option, "unsafe")
+                with pytest.raises(ValueError, match="option name"):
+                    git_config.add_value("user", bad_option, "unsafe")
+
+            git_config.set_value("user", "safe-option1", "safe")
+            git_config.set_value("user", "safe_option2", "safe")
+            git_config.set_value("user", "3safe_option", "safe")
+            git_config.set_value("user", "safe.option3", "safe")
+
+        with GitConfigParser(config_path, read_only=True) as git_config:
+            self.assertEqual(git_config.get_value("user", "safe-option1"), "safe")
+            self.assertEqual(git_config.get_value("user", "safe_option2"), "safe")
+            self.assertEqual(git_config.get_value("user", "3safe_option"), "safe")
+            self.assertEqual(git_config.get_value("user", "safe.option3"), "safe")
+
     @with_rw_directory
     def test_writer_rejects_unquoted_section_terminators(self, rw_dir):
         config_path = osp.join(rw_dir, "config")