fix: block format_map just like format

src/RestrictedPython/Guards.py

@@ -244,11 +244,12 @@ def safer_getattr(object, name, default=None, getattr=getattr):
 
     format() is considered harmful:
     http://lucumr.pocoo.org/2016/12/29/careful-with-str-format/
+    format_map() is equivalent to format with dict as argument.
 
     """
-    if isinstance(object, str) and name == 'format':
+    if isinstance(object, str) and name in ('format', 'format_map'):
         raise NotImplementedError(
-            'Using format() on a %s is not safe.' % object.__class__.__name__)
+            'Using %s() on a %s is not safe.' % (name, object.__class__.__name__))
     if name.startswith('_'):
         raise AttributeError(
             '"{name}" is an invalid attribute name because it '

tests/test_Guards.py

@@ -219,6 +219,26 @@ def test_Guards__safer_getattr__3():
     assert restricted_globals['result'] == 2
 
 
+STRING_DOT_FORMAT_MAP_DENIED = """\
+a = 'Hello {name}'
+b = a.format_map({name: 'world'})
+"""
+
+
+def test_Guards__safer_getattr__4():
+    """It prevents using the format_map method of a string.
+
+    format_map() is similar to format()
+    http://lucumr.pocoo.org/2016/12/29/careful-with-str-format/
+    """
+    glb = {
+        '__builtins__': safe_builtins,
+    }
+    with pytest.raises(NotImplementedError) as err:
+        restricted_exec(STRING_DOT_FORMAT_DENIED, glb)
+    assert 'Using format() on a str is not safe.' == 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"]')