[3.15] gh-159428: Don't read mimetypes.knownfiles by default on Windows (GH-159445)

Make knownfiles empty on Windows so that only the registry is used by default. Files passed to init() or assigned to knownfiles explicitly are still read.
(cherry picked from commit 04a29bf)

Co-authored-by: Cary Hawkins <hawkinscary23@gmail.com>

Author: miss-islington

Lib/mimetypes.py

@@ -23,6 +23,8 @@
 read_mime_types(file) -- parse one file, return a dictionary or None
 """
 
+import sys
+
 try:
     from _winapi import _mimetypes_read_windows_registry
 except ImportError:
@@ -40,17 +42,23 @@
     "suffix_map", "encodings_map", "types_map", "common_types"
 ]
 
-knownfiles = [
-    "/etc/mime.types",
-    "/etc/httpd/mime.types",                    # Mac OS X
-    "/etc/httpd/conf/mime.types",               # Apache
-    "/etc/apache/mime.types",                   # Apache 1
-    "/etc/apache2/mime.types",                  # Apache 2
-    "/usr/local/etc/httpd/conf/mime.types",
-    "/usr/local/lib/netscape/mime.types",
-    "/usr/local/etc/httpd/conf/mime.types",     # Apache 1.2
-    "/usr/local/etc/mime.types",                # Apache 1.3
-    ]
+if sys.platform == "win32":
+    # The default paths would resolve to the root of the current drive on Windows
+    # (e.g. C:\etc\mime.types), which unprivileged users can usually create.
+    # Only the registry is used by default.
+    knownfiles = []
+else:
+    knownfiles = [
+        "/etc/mime.types",
+        "/etc/httpd/mime.types",                    # Mac OS X
+        "/etc/httpd/conf/mime.types",               # Apache
+        "/etc/apache/mime.types",                   # Apache 1
+        "/etc/apache2/mime.types",                  # Apache 2
+        "/usr/local/etc/httpd/conf/mime.types",
+        "/usr/local/lib/netscape/mime.types",
+        "/usr/local/etc/httpd/conf/mime.types",     # Apache 1.2
+        "/usr/local/etc/mime.types",                # Apache 1.3
+        ]
 
 inited = False
 _db = None

Lib/test/test_mimetypes.py

@@ -271,6 +271,10 @@ def test_init_knownfiles(self):
         mimetypes.init()
         self.assertEqual(guess_file_type("file.test2")[0], "testing/test2")
 
+    @unittest.skipIf(sys.platform == "win32", "non-Windows only")
+    def test_default_knownfiles(self):
+        self.assertIn("/etc/mime.types", knownfiles)
+
     def test_added_types_are_used(self):
         mimetypes.add_type('testing/default-type', '')
         mime_type, _ = mimetypes.guess_type('')
@@ -508,6 +512,9 @@ def test_registry_parsing(self):
         eq(self.db.guess_type("image.jpg"), ("image/jpeg", None))
         eq(self.db.guess_type("image.png"), ("image/png", None))
 
+    def test_default_knownfiles(self):
+        self.assertEqual(knownfiles, [])
+
     @unittest.skipIf(not hasattr(_winapi, "_mimetypes_read_windows_registry"),
                      "read_windows_registry accelerator unavailable")
     def test_registry_accelerator(self):

Misc/NEWS.d/next/Security/2026-10-15-03-47-08.gh-issue-159428.XHRKwY.rst

@@ -0,0 +1,2 @@
+:mod:`mimetypes` no longer reads the files listed in :data:`mimetypes.knownfiles`
+by default on Windows, and uses only the registry.