[3.13] 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)

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

@@ -408,9 +408,26 @@ the :mod:`glob` module.)
    system). On Windows, this function will also resolve MS-DOS (also called 8.3)
    style names such as ``C:\\PROGRA~1`` to ``C:\\Program Files``.
 
-   If a path doesn't exist or a symlink loop is encountered, and *strict* is
-   ``True``, :exc:`OSError` is raised. If *strict* is ``False`` these errors
-   are ignored, and so the result might be missing or otherwise inaccessible.
+   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
@@ -429,6 +446,15 @@ 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

@@ -249,6 +249,15 @@ 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:
 
@@ -1052,6 +1061,12 @@ 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.
 
@@ -1080,6 +1095,10 @@ reused in custom filters:
 
   Return the modified ``TarInfo`` member.
 
+  .. versionchanged:: next
+
+     Link targets are now normalized.
+
 
 .. _tarfile-extraction-refuse:
 
@@ -1106,6 +1125,7 @@ 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/3.13.rst

@@ -2829,3 +2829,36 @@ sys
 * The previously undocumented special function :func:`sys.getobjects`,
   which only exists in specialized builds of Python, may now return objects
   from other interpreters than the one it's called in.
+
+Notable changes in 3.13.4
+=========================
+
+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.Add commentMore actions
+  (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', 'isdevdrive', 'isdir', 'isfile', 'isjunction', 'islink',
-           'lexists', 'samefile', 'sameopenfile', 'samestat']
+           'lexists', 'samefile', 'sameopenfile', 'samestat', 'ALLOW_MISSING']
 
 
 # Does a path exist?
@@ -189,3 +189,12 @@ 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

@@ -29,7 +29,7 @@
            "abspath","curdir","pardir","sep","pathsep","defpath","altsep",
            "extsep","devnull","realpath","supports_unicode_filenames","relpath",
            "samefile", "sameopenfile", "samestat", "commonpath", "isjunction",
-           "isdevdrive"]
+           "isdevdrive", "ALLOW_MISSING"]
 
 def _get_bothseps(path):
     if isinstance(path, bytes):
@@ -601,9 +601,10 @@ def abspath(path):
     from nt import _findfirstfile, _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
@@ -636,7 +637,7 @@ 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
@@ -645,7 +646,7 @@ 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
@@ -673,25 +674,26 @@ 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
                 # If we get these errors, try to get the real name of the file without accessing it.
                 if ex.winerror in (1, 5, 32, 50, 87, 1920, 1921):
                     try:
                         name = _findfirstfile(path)
                         path, _ = split(path)
-                    except OSError:
+                    except ignored_error:
                         path, name = split(path)
                 else:
                     path, name = split(path)
@@ -721,24 +723,32 @@ def realpath(path, *, strict=False):
             if normcase(path) == 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

@@ -36,7 +36,7 @@
            "samefile","sameopenfile","samestat",
            "curdir","pardir","sep","pathsep","defpath","altsep","extsep",
            "devnull","realpath","supports_unicode_filenames","relpath",
-           "commonpath", "isjunction","isdevdrive"]
+           "commonpath", "isjunction","isdevdrive","ALLOW_MISSING"]
 
 
 def _get_sep(path):
@@ -402,6 +402,15 @@ def realpath(filename, *, strict=False):
         curdir = '.'
         pardir = '..'
         getcwd = os.getcwd
+    if strict is ALLOW_MISSING:
+        ignored_error = FileNotFoundError
+        strict = True
+    elif strict:
+        ignored_error = ()
+    else:
+        ignored_error = OSError
+
+    maxlinks = None
 
     # The stack of unresolved path parts. When popped, a special value of None
     # indicates that a symlink target has been resolved, and that the original
@@ -462,25 +471,28 @@ def realpath(filename, *, strict=False):
                 path = newpath
                 continue
             target = os.readlink(newpath)
-        except OSError:
-            if strict:
-                raise
-            path = newpath
+        except ignored_error:
+            pass
+        else:
+            # Resolve the symbolic link
+            if target.startswith(sep):
+                # Symlink target is absolute; reset resolved path.
+                path = sep
+            if maxlinks is None:
+                # Mark this symlink as seen but not fully resolved.
+                seen[newpath] = None
+                # Push the symlink path onto the stack, and signal its specialness
+                # by also pushing None. When these entries are popped, we'll
+                # record the fully-resolved symlink target in the 'seen' mapping.
+                rest.append(newpath)
+                rest.append(None)
+            # Push the unresolved symlink target parts onto the stack.
+            target_parts = target.split(sep)[::-1]
+            rest.extend(target_parts)
+            part_count += len(target_parts)
             continue
-        # Resolve the symbolic link
-        seen[newpath] = None # not resolved symlink
-        if target.startswith(sep):
-            # Symlink target is absolute; reset resolved path.
-            path = sep
-        # Push the symlink path onto the stack, and signal its specialness by
-        # also pushing None. When these entries are popped, we'll record the
-        # fully-resolved symlink target in the 'seen' mapping.
-        rest.append(newpath)
-        rest.append(None)
-        # Push the unresolved symlink target parts onto the stack.
-        target_parts = target.split(sep)[::-1]
-        rest.extend(target_parts)
-        part_count += len(target_parts)
+        # An error occurred and was ignored.
+        path = newpath
 
     return path