gh-160506: Fix ntpath.realpath() for broken links via directory links

Resolve the directory containing a broken relative symlink before
joining the link target to it, so that '..' in the target is applied
to the real directory.

ANT-2026-27II3A20

Author: gpshead

Lib/ntpath.py

@@ -594,7 +594,16 @@ def _readlink_deep(path, ignored_error=OSError):
                     if not islink(old_path):
                         path = old_path
                         break
-                    path = normpath(join(dirname(old_path), path))
+                    # Resolve the directory holding the link first, so that
+                    # '..' in the target is not applied to a directory link
+                    # in old_path.
+                    link_dir = dirname(old_path)
+                    try:
+                        link_dir = _getfinalpathname(link_dir)
+                    except ignored_error as ex:
+                        if ex.winerror not in allowed_winerror:
+                            raise
+                    path = normpath(join(link_dir, path))
             except ignored_error as ex:
                 if ex.winerror in allowed_winerror:
                     break

Lib/test/test_ntpath.py

@@ -701,9 +701,9 @@ def test_realpath_broken_symlinks(self):
             self.assertPathEqual(ntpath.realpath(r"broken1\baz"),
                                  ABSTFN + r"\broken\bar\baz")
             self.assertPathEqual(ntpath.realpath("broken2"),
-                                 ABSTFN + r"\self\self\missing")
+                                 ABSTFN + r"\missing")
             self.assertPathEqual(ntpath.realpath("broken3"),
-                                 ABSTFN + r"\subdir\parent\subdir\parent\missing")
+                                 ABSTFN + r"\missing")
             self.assertPathEqual(ntpath.realpath("broken4"),
                                  ABSTFN + r"\missing")
             self.assertPathEqual(ntpath.realpath("broken5"),
@@ -718,14 +718,39 @@ def test_realpath_broken_symlinks(self):
             self.assertPathEqual(ntpath.realpath(rb"broken1\baz"),
                                  os.fsencode(ABSTFN + r"\broken\bar\baz"))
             self.assertPathEqual(ntpath.realpath(b"broken2"),
-                                 os.fsencode(ABSTFN + r"\self\self\missing"))
+                                 os.fsencode(ABSTFN + r"\missing"))
             self.assertPathEqual(ntpath.realpath(rb"broken3"),
-                                 os.fsencode(ABSTFN + r"\subdir\parent\subdir\parent\missing"))
+                                 os.fsencode(ABSTFN + r"\missing"))
             self.assertPathEqual(ntpath.realpath(b"broken4"),
                                  os.fsencode(ABSTFN + r"\missing"))
             self.assertPathEqual(ntpath.realpath(b"broken5"),
                                  os.fsencode(ABSTFN + r"\missing"))
 
+    @os_helper.skip_unless_symlink
+    @unittest.skipUnless(HAVE_GETFINALPATHNAME, 'need _getfinalpathname')
+    @_parameterize({}, {'strict': ALL_BUT_LAST}, {'strict': ALLOW_MISSING})
+    def test_realpath_broken_symlink_in_linked_dir(self, kwargs):
+        # A relative target is resolved against the directory containing
+        # the link, not against the path by which the link was reached.
+        ABSTFN = ntpath.abspath(os_helper.TESTFN)
+        os.makedirs(ABSTFN + r"\sub")
+        self.addCleanup(os_helper.rmtree, ABSTFN)
+        os.symlink(".", ABSTFN + r"\sub\alias", target_is_directory=True)
+        os.symlink(r"..\missing", ABSTFN + r"\sub\broken")
+        os.symlink(r"alias\alias\broken", ABSTFN + r"\sub\broken2")
+
+        expected = ABSTFN + r"\missing"
+        for name in "broken", r"alias\broken", "broken2", r"alias\broken2":
+            path = ABSTFN + "\\sub\\" + name
+            with self.subTest(name=name):
+                self.assertPathEqual(ntpath.realpath(path, **kwargs), expected)
+                self.assertPathEqual(
+                    ntpath.realpath(os.fsencode(path), **kwargs),
+                    os.fsencode(expected))
+                self.assertPathEqual(
+                    ntpath.realpath("\\\\?\\" + path, **kwargs),
+                    "\\\\?\\" + expected)
+
     @os_helper.skip_unless_symlink
     @unittest.skipUnless(HAVE_GETFINALPATHNAME, 'need _getfinalpathname')
     def test_realpath_symlink_loops(self):

Lib/test/test_tarfile.py

@@ -4506,6 +4506,62 @@ def test_parent_symlink2(self):
                 self.expect_file('current/')
                 self.expect_file('parent/evil')
 
+    @symlink_test
+    def test_parent_symlink3(self):
+        # Test a dangling symlink reached through a symlink to a directory
+
+        with ArchiveMaker() as arc:
+
+            # `current` links to `.` which is both the destination directory
+            # and `current` itself
+            arc.add('current', symlink_to='.')
+
+            # `current/link` is also available as `./link`, and points
+            # to a file outside the destination which does not exist yet
+            arc.add('current/link', symlink_to='../evil')
+
+            # Writing through the link creates `../evil`
+            arc.add('current/link', content='evil')
+
+        with self.check_context(arc.open(), 'fully_trusted'):
+            if os_helper.can_symlink():
+                self.expect_file('current', symlink_to='.')
+                self.expect_file('link', symlink_to='../evil')
+                self.expect_file('../evil', content='evil')
+            else:
+                self.expect_file('current/link', content='evil')
+
+        with self.check_context(arc.open(), 'tar'):
+            if os_helper.can_symlink():
+                # Fail when extracting a file outside destination
+                self.expect_exception(
+                        tarfile.OutsideDestinationError,
+                        "'current/link' would be extracted to "
+                        + """['"].*evil['"], which is outside """
+                        + "the destination")
+            else:
+                self.expect_file('current/link', content='evil')
+
+        with self.check_context(arc.open(), 'data'):
+            if os_helper.can_symlink():
+                if self.dotdot_resolves_early:
+                    # 'current/../evil' normalises to 'evil', so the link is
+                    # created; fail when extracting a file through it
+                    self.expect_exception(
+                            tarfile.OutsideDestinationError,
+                            "'current/link' would be extracted to "
+                            + """['"].*evil['"], which is outside """
+                            + "the destination")
+                else:
+                    # Fail as soon as we have a symlink outside the destination
+                    self.expect_exception(
+                            tarfile.LinkOutsideDestinationError,
+                            "'current/link' would link to "
+                            + """['"].*evil['"], which is outside """
+                            + "the destination")
+            else:
+                self.expect_file('current/link', content='evil')
+
     @symlink_test
     def test_absolute_symlink(self):
         # Test symlink to an absolute path

Misc/NEWS.d/next/Security/2026-10-22-08-08-23.gh-issue-160506.rN4dLq.rst

@@ -0,0 +1,4 @@
+Fix :func:`os.path.realpath` on Windows returning the wrong path for a broken
+relative symbolic link reached through a symbolic link to a directory. This
+allowed :mod:`tarfile` extraction with the ``'data'`` or ``'tar'`` filter to
+create a file outside the destination directory.