From ad1aa26ec00f71dec149fee671cff2907c4dfa88 Mon Sep 17 00:00:00 2001
From: Cary Hawkins <hawkinscary23@gmail.com>
Date: Thu, 15 Oct 2026 14:03:28 -0700
Subject: [PATCH] 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 04a29bf8f9ff8526cd96cb7368400173303a390a)

Co-authored-by: Cary Hawkins <hawkinscary23@gmail.com>
---
 Lib/mimetypes.py                              | 30 ++++++++++++-------
 Lib/test/test_mimetypes.py                    |  7 +++++
 ...-10-15-03-47-08.gh-issue-159428.XHRKwY.rst |  2 ++
 3 files changed, 28 insertions(+), 11 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Security/2026-10-15-03-47-08.gh-issue-159428.XHRKwY.rst

diff --git a/Lib/mimetypes.py b/Lib/mimetypes.py
index e7581d2ae0ace41..ba942c6f3baf2c7 100644
--- a/Lib/mimetypes.py
+++ b/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
diff --git a/Lib/test/test_mimetypes.py b/Lib/test/test_mimetypes.py
index a7d1d724ff0e18b..8704a7353cfcbf1 100644
--- a/Lib/test/test_mimetypes.py
+++ b/Lib/test/test_mimetypes.py
@@ -265,6 +265,10 @@ 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('')
@@ -505,6 +509,9 @@ 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):
diff --git a/Misc/NEWS.d/next/Security/2026-10-15-03-47-08.gh-issue-159428.XHRKwY.rst b/Misc/NEWS.d/next/Security/2026-10-15-03-47-08.gh-issue-159428.XHRKwY.rst
new file mode 100644
index 000000000000000..d481ed2b74a20ea
--- /dev/null
+++ b/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.