From dceb2685d82bb18080312084aaacb9059d89f59e Mon Sep 17 00:00:00 2001
From: Cary Hawkins <hawkinscary23@gmail.com>
Date: Thu, 15 Oct 2026 14:03:28 -0700
Subject: [PATCH] [3.12] 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                              | 28 +++++++++++--------
 Lib/test/test_mimetypes.py                    |  7 +++++
 ...-10-15-03-47-08.gh-issue-159428.XHRKwY.rst |  2 ++
 3 files changed, 26 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 814b3f9c9f9ecc..a3d2f80d417e58 100644
--- a/Lib/mimetypes.py
+++ b/Lib/mimetypes.py
@@ -45,17 +45,23 @@
     "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 c366698b21b460..0dd4e97fff962a 100644
--- a/Lib/test/test_mimetypes.py
+++ b/Lib/test/test_mimetypes.py
@@ -50,6 +50,10 @@ def test_default_data(self):
         eq(self.db.guess_type("foo.tar.bz2"), ("application/x-tar", "bzip2"))
         eq(self.db.guess_type("foo.tar.xz"), ("application/x-tar", "xz"))
 
+    @unittest.skipIf(sys.platform == "win32", "non-Windows only")
+    def test_default_knownfiles(self):
+        self.assertIn("/etc/mime.types", knownfiles)
+
     def test_data_urls(self):
         eq = self.assertEqual
         guess_type = self.db.guess_type
@@ -296,6 +300,9 @@ 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 00000000000000..d481ed2b74a20e
--- /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.