additional check for safer_getattr (#285)

* add an additional check for potential breakout capability via Inspection Attributes Names in the provided safer_getattr method.

---------

Co-authored-by: Dieter Maurer <d-maurer@users.noreply.github.com>
Co-authored-by: Michael Howitz <icemac@gmx.net>

CHANGES.rst

@@ -6,6 +6,10 @@ Changes
 
 - Allow to use the package with Python 3.13 -- Caution: No security
   audit has been done so far.
+- Increase the safety level of ``safer_getattr`` allowing applications to use
+  it as ``getattr`` implementation. Such use should now follow the same policy
+  and give the same level of protection as direct attribute access in an
+  environment based on ``RestrictedPython``'s ``safe_builtints``.
 
 
 7.2 (2024-08-02)

src/RestrictedPython/Guards.py

@@ -18,6 +18,7 @@
 import builtins
 
 from RestrictedPython._compat import IS_PY311_OR_GREATER
+from RestrictedPython.transformer import INSPECT_ATTRIBUTES
 
 
 safe_builtins = {}
@@ -253,6 +254,10 @@ def safer_getattr(object, name, default=None, getattr=getattr):
             (isinstance(object, type) and issubclass(object, str))):
         raise NotImplementedError(
             'Using the format*() methods of `str` is not safe')
+    if name in INSPECT_ATTRIBUTES:
+        raise AttributeError(
+            f'"{name}" is a restricted name,'
+            ' that is forbidden to access in RestrictedPython.')
     if name.startswith('_'):
         raise AttributeError(
             '"{name}" is an invalid attribute name because it '

tests/test_Guards.py

@@ -295,6 +295,40 @@ def test_Guards__safer_getattr__4():
     assert 'type(name) must be str' == str(err.value)
 
 
+SAFER_GETATTR_BREAKOUT2 = """\
+g = None
+leak = None
+def test():
+    global g, leak
+    leak = getattr(getattr(getattr(g, "gi_frame"), "f_back"), "f_back")
+    yield leak
+g = test()
+g.send(None)
+os = getattr(leak, "f_builtins").get('__import__')('os')
+result = os.getgid()
+"""
+
+
+def test_Guards__safer_getattr__5():
+    restricted_globals = dict(
+        __builtins__=safe_builtins,
+        __name__=None,
+        __metaclass__=type,
+        # _write_=_write_,
+        getattr=safer_getattr,
+        result=None,
+    )
+
+    # restricted_exec(SAFER_GETATTR_BREAKOUT2, restricted_globals)
+    # assert restricted_globals['result'] == 20
+    with pytest.raises(AttributeError) as err:
+        restricted_exec(SAFER_GETATTR_BREAKOUT2, restricted_globals)
+    assert (
+        '"gi_frame" is a restricted name, '
+        'that is forbidden to access in RestrictedPython.'
+    ) == str(err.value)
+
+
 def test_call_py3_builtins():
     """It should not be allowed to access global builtins in Python3."""
     result = compile_restricted_exec('builtins["getattr"]')