[3.11] gh-135034: Normalize link targets in tarfile, add os.path.realpath(strict='allow_missing') (GH-135037)

Addresses CVEs 2024-12718, 2025-4138, 2025-4330, and 2025-4517.
(cherry picked from commit 3612d8f)
(cherry picked from commit c358142)

Co-authored-by: Łukasz Langa <[email protected]>
Signed-off-by: Łukasz Langa <[email protected]>
Co-authored-by: Petr Viktorin <[email protected]>
Co-authored-by: Seth Michael Larson <[email protected]>
Co-authored-by: Adam Turner <[email protected]>
Co-authored-by: Serhiy Storchaka <[email protected]>

Author: 5 people

Doc/library/os.path.rst

@@ -352,10 +352,26 @@ the :mod:`glob` module.)
    links encountered in the path (if they are supported by the operating
    system).
 
-   If a path doesn't exist or a symlink loop is encountered, and *strict* is
-   ``True``, :exc:`OSError` is raised. If *strict* is ``False``, the path is
-   resolved as far as possible and any remainder is appended without checking
-   whether it exists.
+   By default, the path is evaluated up to the first component that does not
+   exist, is a symlink loop, or whose evaluation raises :exc:`OSError`.
+   All such components are appended unchanged to the existing part of the path.
+
+   Some errors that are handled this way include "access denied", "not a
+   directory", or "bad argument to internal function". Thus, the
+   resulting path may be missing or inaccessible, may still contain
+   links or loops, and may traverse non-directories.
+
+   This behavior can be modified by keyword arguments:
+
+   If *strict* is ``True``, the first error encountered when evaluating the path is
+   re-raised.
+   In particular, :exc:`FileNotFoundError` is raised if *path* does not exist,
+   or another :exc:`OSError` if it is otherwise inaccessible.
+
+   If *strict* is :py:data:`os.path.ALLOW_MISSING`, errors other than
+   :exc:`FileNotFoundError` are re-raised (as with ``strict=True``).
+   Thus, the returned path will not contain any symbolic links, but the named
+   file and some of its parent directories may be missing.
 
    .. note::
       This function emulates the operating system's procedure for making a path
@@ -374,6 +390,15 @@ the :mod:`glob` module.)
    .. versionchanged:: 3.10
       The *strict* parameter was added.
 
+   .. versionchanged:: next
+      The :py:data:`~os.path.ALLOW_MISSING` value for the *strict* parameter
+      was added.
+
+.. data:: ALLOW_MISSING
+
+   Special value used for the *strict* argument in :func:`realpath`.
+
+   .. versionadded:: next
 
 .. function:: relpath(path, start=os.curdir)
 

Doc/library/tarfile.rst

@@ -239,6 +239,15 @@ The :mod:`tarfile` module defines the following exceptions:
    Raised to refuse extracting a symbolic link pointing outside the destination
    directory.
 
+.. exception:: LinkFallbackError
+
+   Raised to refuse emulating a link (hard or symbolic) by extracting another
+   archive member, when that member would be rejected by the filter location.
+   The exception that was raised to reject the replacement member is available
+   as :attr:`!BaseException.__context__`.
+
+   .. versionadded:: next
+
 
 The following constants are available at the module level:
 
@@ -1037,6 +1046,12 @@ reused in custom filters:
   Implements the ``'data'`` filter.
   In addition to what ``tar_filter`` does:
 
+  - Normalize link targets (:attr:`TarInfo.linkname`) using
+    :func:`os.path.normpath`.
+    Note that this removes internal ``..`` components, which may change the
+    meaning of the link if the path in :attr:`!TarInfo.linkname` traverses
+    symbolic links.
+
   - :ref:`Refuse <tarfile-extraction-refuse>` to extract links (hard or soft)
     that link to absolute paths, or ones that link outside the destination.
 
@@ -1065,6 +1080,10 @@ reused in custom filters:
 
   Return the modified ``TarInfo`` member.
 
+  .. versionchanged:: next
+
+     Link targets are now normalized.
+
 
 .. _tarfile-extraction-refuse:
 
@@ -1091,6 +1110,7 @@ Here is an incomplete list of things to consider:
 * Extract to a :func:`new temporary directory <tempfile.mkdtemp>`
   to prevent e.g. exploiting pre-existing links, and to make it easier to
   clean up after a failed extraction.
+* Disallow symbolic links if you do not need the functionality.
 * When working with untrusted data, use external (e.g. OS-level) limits on
   disk, memory and CPU usage.
 * Check filenames against an allow-list of characters

Doc/whatsnew/3.11.rst

@@ -2786,3 +2786,37 @@ email
   check if the *strict* paramater is available.
   (Contributed by Thomas Dwyer and Victor Stinner for :gh:`102988` to improve
   the CVE-2023-27043 fix.)
+
+
+Notable changes in 3.11.13
+==========================
+
+os.path
+-------
+
+* The *strict* parameter to :func:`os.path.realpath` accepts a new value,
+  :data:`os.path.ALLOW_MISSING`.
+  If used, errors other than :exc:`FileNotFoundError` will be re-raised;
+  the resulting path can be missing but it will be free of symlinks.
+  (Contributed by Petr Viktorin for :cve:`2025-4517`.)
+
+tarfile
+-------
+
+* :func:`~tarfile.data_filter` now normalizes symbolic link targets in order to
+  avoid path traversal attacks.
+  (Contributed by Petr Viktorin in :gh:`127987` and :cve:`2025-4138`.)
+* :func:`~tarfile.TarFile.extractall` now skips fixing up directory attributes
+  when a directory was removed or replaced by another kind of file.
+  (Contributed by Petr Viktorin in :gh:`127987` and :cve:`2024-12718`.)
+* :func:`~tarfile.TarFile.extract` and :func:`~tarfile.TarFile.extractall`
+  now (re-)apply the extraction filter when substituting a link (hard or
+  symbolic) with a copy of another archive member, and when fixing up
+  directory attributes.
+  The former raises a new exception, :exc:`~tarfile.LinkFallbackError`.
+  (Contributed by Petr Viktorin for :cve:`2025-4330` and :cve:`2024-12718`.)
+* :func:`~tarfile.TarFile.extract` and :func:`~tarfile.TarFile.extractall`
+  no longer extract rejected members when
+  :func:`~tarfile.TarFile.errorlevel` is zero.
+  (Contributed by Matt Prodani and Petr Viktorin in :gh:`112887`
+  and :cve:`2025-4435`.)

Lib/genericpath.py

@@ -8,7 +8,7 @@
 
 __all__ = ['commonprefix', 'exists', 'getatime', 'getctime', 'getmtime',
            'getsize', 'isdir', 'isfile', 'samefile', 'sameopenfile',
-           'samestat']
+           'samestat', 'ALLOW_MISSING']
 
 
 # Does a path exist?
@@ -153,3 +153,12 @@ def _check_arg_types(funcname, *args):
                             f'os.PathLike object, not {s.__class__.__name__!r}') from None
     if hasstr and hasbytes:
         raise TypeError("Can't mix strings and bytes in path components") from None
+
+# A singleton with a true boolean value.
+@object.__new__
+class ALLOW_MISSING:
+    """Special value for use in realpath()."""
+    def __repr__(self):
+        return 'os.path.ALLOW_MISSING'
+    def __reduce__(self):
+        return self.__class__.__name__

Lib/ntpath.py

@@ -30,7 +30,8 @@
            "ismount", "expanduser","expandvars","normpath","abspath",
            "curdir","pardir","sep","pathsep","defpath","altsep",
            "extsep","devnull","realpath","supports_unicode_filenames","relpath",
-           "samefile", "sameopenfile", "samestat", "commonpath"]
+           "samefile", "sameopenfile", "samestat", "commonpath",
+           "ALLOW_MISSING"]
 
 def _get_bothseps(path):
     if isinstance(path, bytes):
@@ -578,9 +579,10 @@ def abspath(path):
     from nt import _getfinalpathname, readlink as _nt_readlink
 except ImportError:
     # realpath is a no-op on systems without _getfinalpathname support.
-    realpath = abspath
+    def realpath(path, *, strict=False):
+        return abspath(path)
 else:
-    def _readlink_deep(path):
+    def _readlink_deep(path, ignored_error=OSError):
         # These error codes indicate that we should stop reading links and
         # return the path we currently have.
         # 1: ERROR_INVALID_FUNCTION
@@ -613,7 +615,7 @@ def _readlink_deep(path):
                         path = old_path
                         break
                     path = normpath(join(dirname(old_path), path))
-            except OSError as ex:
+            except ignored_error as ex:
                 if ex.winerror in allowed_winerror:
                     break
                 raise
@@ -622,7 +624,7 @@ def _readlink_deep(path):
                 break
         return path
 
-    def _getfinalpathname_nonstrict(path):
+    def _getfinalpathname_nonstrict(path, ignored_error=OSError):
         # These error codes indicate that we should stop resolving the path
         # and return the value we currently have.
         # 1: ERROR_INVALID_FUNCTION
@@ -649,17 +651,18 @@ def _getfinalpathname_nonstrict(path):
             try:
                 path = _getfinalpathname(path)
                 return join(path, tail) if tail else path
-            except OSError as ex:
+            except ignored_error as ex:
                 if ex.winerror not in allowed_winerror:
                     raise
                 try:
                     # The OS could not resolve this path fully, so we attempt
                     # to follow the link ourselves. If we succeed, join the tail
                     # and return.
-                    new_path = _readlink_deep(path)
+                    new_path = _readlink_deep(path,
+                                              ignored_error=ignored_error)
                     if new_path != path:
                         return join(new_path, tail) if tail else new_path
-                except OSError:
+                except ignored_error:
                     # If we fail to readlink(), let's keep traversing
                     pass
                 path, name = split(path)
@@ -690,24 +693,32 @@ def realpath(path, *, strict=False):
             if normcase(path) == normcase(devnull):
                 return '\\\\.\\NUL'
         had_prefix = path.startswith(prefix)
+
+        if strict is ALLOW_MISSING:
+            ignored_error = FileNotFoundError
+            strict = True
+        elif strict:
+            ignored_error = ()
+        else:
+            ignored_error = OSError
+
         if not had_prefix and not isabs(path):
             path = join(cwd, path)
         try:
             path = _getfinalpathname(path)
             initial_winerror = 0
         except ValueError as ex:
             # gh-106242: Raised for embedded null characters
-            # In strict mode, we convert into an OSError.
+            # In strict modes, we convert into an OSError.
             # Non-strict mode returns the path as-is, since we've already
             # made it absolute.
             if strict:
                 raise OSError(str(ex)) from None
             path = normpath(path)
-        except OSError as ex:
-            if strict:
-                raise
+        except ignored_error as ex:
             initial_winerror = ex.winerror
-            path = _getfinalpathname_nonstrict(path)
+            path = _getfinalpathname_nonstrict(path,
+                                               ignored_error=ignored_error)
         # The path returned by _getfinalpathname will always start with \\?\ -
         # strip off that prefix unless it was already provided on the original
         # path.

Lib/posixpath.py

@@ -35,7 +35,7 @@
            "samefile","sameopenfile","samestat",
            "curdir","pardir","sep","pathsep","defpath","altsep","extsep",
            "devnull","realpath","supports_unicode_filenames","relpath",
-           "commonpath"]
+           "commonpath", "ALLOW_MISSING"]
 
 
 def _get_sep(path):
@@ -427,6 +427,15 @@ def _joinrealpath(path, rest, strict, seen):
         sep = '/'
         curdir = '.'
         pardir = '..'
+        getcwd = os.getcwd
+    if strict is ALLOW_MISSING:
+        ignored_error = FileNotFoundError
+    elif strict:
+        ignored_error = ()
+    else:
+        ignored_error = OSError
+
+    maxlinks = None
 
     if isabs(rest):
         rest = rest[1:]
@@ -449,7 +458,7 @@ def _joinrealpath(path, rest, strict, seen):
         newpath = join(path, name)
         try:
             st = os.lstat(newpath)
-        except OSError:
+        except ignored_error:
             if strict:
                 raise
             is_link = False