gh-154490: Only keep permission bits of the mode argument in ZipFile.mkdir()

The file type bits of a caller-provided mode were previously mixed into
the entry's external attributes, so a mode such as 0o107777 could mark
the created entry as a socket instead of a directory.  Sanitize the mode
with & 0o7777, matching what stat.S_IMODE does, before combining it with
S_IFDIR.  Also spell the default mode as 0o777 instead of 511 in the
signature and documentation, for consistency with os.mkdir().

Author: DevoidSloth

Doc/library/zipfile.rst

@@ -538,17 +538,23 @@ ZipFile objects
       If set, it uses this value as the modification timestamp for the file
       written into the ZIP archive, instead of using the current time.
 
-.. method:: ZipFile.mkdir(zinfo_or_directory, mode=511)
+.. method:: ZipFile.mkdir(zinfo_or_directory, mode=0o777)
 
    Create a directory inside the archive.  If *zinfo_or_directory* is a string,
    a directory is created inside the archive with the mode that is specified in
-   the *mode* argument. If, however, *zinfo_or_directory* is
-   a :class:`ZipInfo` instance then the *mode* argument is ignored.
+   the *mode* argument.  Only the permission bits of *mode* are used; other
+   bits are ignored.  If, however, *zinfo_or_directory* is a :class:`ZipInfo`
+   instance then the *mode* argument is ignored.
 
    The archive must be opened with mode ``'w'``, ``'x'`` or ``'a'``.
 
    .. versionadded:: 3.11
 
+   .. versionchanged:: next
+      Bits of *mode* other than the permission bits are now ignored.
+      Previously, file type bits in *mode* could mark the created entry as
+      something other than a directory.
+
 
 .. method:: ZipFile.remove(zinfo_or_arcname)
 

Lib/test/test_zipfile/test_core.py

@@ -5478,10 +5478,24 @@ def test_mkdir(self):
             self.assertEqual(old_zinfo.filename, "directory4/")
             self.assertEqual(old_zinfo.external_attr, new_zinfo.external_attr)
 
+            # gh-154490: file type bits in *mode* must not leak into the
+            # entry's external attributes; only the permission bits are kept.
+            zf.mkdir("directory5", mode=0o107777)  # S_IFREG | 0o7777
+            zinfo = zf.filelist[4]
+            self.assertEqual(zinfo.filename, "directory5/")
+            self.assertEqual(zinfo.external_attr, (0o47777 << 16) | 0x10)
+
+            zf.mkdir("directory6", mode=0o120555)  # S_IFLNK | 0o555
+            zinfo = zf.filelist[5]
+            self.assertEqual(zinfo.filename, "directory6/")
+            self.assertEqual(zinfo.external_attr, (0o40555 << 16) | 0x10)
+
             target = os.path.join(TESTFN2, "target")
             os.mkdir(target)
             zf.extractall(target)
-            self.assertEqual(set(os.listdir(target)), {"directory", "directory2", "directory3", "directory4"})
+            self.assertEqual(set(os.listdir(target)),
+                             {"directory", "directory2", "directory3",
+                              "directory4", "directory5", "directory6"})
 
     def test_create_directory_with_write(self):
         with zipfile.ZipFile(TESTFN, "w") as zf:

Lib/zipfile/__init__.py

@@ -2579,7 +2579,7 @@ def writestr(self, zinfo_or_arcname, data,
             with self.open(zinfo, mode='w') as dest:
                 dest.write(data)
 
-    def mkdir(self, zinfo_or_directory_name, mode=511):
+    def mkdir(self, zinfo_or_directory_name, mode=0o777):
         """Creates a directory inside the zip archive."""
         if isinstance(zinfo_or_directory_name, ZipInfo):
             zinfo = zinfo_or_directory_name
@@ -2592,7 +2592,7 @@ def mkdir(self, zinfo_or_directory_name, mode=511):
             zinfo = ZipInfo(directory_name)
             zinfo.compress_size = 0
             zinfo.CRC = 0
-            zinfo.external_attr = ((0o40000 | mode) & 0xFFFF) << 16
+            zinfo.external_attr = (0o40000 | (mode & 0o7777)) << 16
             zinfo.file_size = 0
             zinfo.external_attr |= 0x10
         else:

Misc/NEWS.d/next/Library/2026-07-22-17-18-57.gh-issue-154490.tLFMqx.rst

@@ -0,0 +1,5 @@
+Fix :meth:`zipfile.ZipFile.mkdir` so that only the permission bits of the
+*mode* argument are stored.  Previously, file type bits in *mode* could mark
+the created entry as something other than a directory.  Also change the
+default value of *mode* in the signature and documentation from ``511`` to
+the equivalent but clearer ``0o777``.