Python 보안: 새로운 문자열 형식 취약점 분석 및 솔루션

>>> 'class of {0} is {0.__class__}'.format(42)
"class of 42 is "

CONFIG = {
    'SECRET_KEY': 'super secret key'
}
 
class Event(object):
    def __init__(self, id, level, message):
        self.id = id
        self.level = level
        self.message = message
 
def format_event(format_string, event):
    return format_string.format(event=event)

{event.__init__.__globals__[CONFIG][SECRET_KEY]}

from string import Formatter
from collections import Mapping
 
class MagicFormatMapping(Mapping):
    """This class implements a dummy wrapper to fix a bug in the Python
    standard library for string formatting.
 
    See http://bugs.python.org/issue13598 for information about why
    this is necessary.
    """
 
    def __init__(self, args, kwargs):
        self._args = args
        self._kwargs = kwargs
        self._last_index = 0
 
    def __getitem__(self, key):
        if key == '':
            idx = self._last_index
            self._last_index += 1
            try:
                return self._args[idx]
            except LookupError:
                pass
            key = str(idx)
        return self._kwargs[key]
 
    def __iter__(self):
        return iter(self._kwargs)
 
    def __len__(self):
        return len(self._kwargs)
 
# This is a necessary API but it's undocumented and moved around
# between Python releases
try:
    from _string import formatter_field_name_split
except ImportError:
    formatter_field_name_split = lambda \
        x: x._formatter_field_name_split()
{C} 
class SafeFormatter(Formatter):
 
    def get_field(self, field_name, args, kwargs):
        first, rest = formatter_field_name_split(field_name)
        obj = self.get_value(first, args, kwargs)
        for is_attr, i in rest:
            if is_attr:
                obj = safe_getattr(obj, i)
            else:
                obj = obj[i]
        return obj, first
 
def safe_getattr(obj, attr):
    # Expand the logic here.  For instance on 2.x you will also need
    # to disallow func_globals, on 3.x you will also need to hide
    # things like cr_frame and others.  So ideally have a list of
    # objects that are entirely unsafe to access.
    if attr[:1] == '_':
        raise AttributeError(attr)
    return getattr(obj, attr)
 
def safe_format(_string, *args, **kwargs):
    formatter = SafeFormatter()
    kwargs = MagicFormatMapping(args, kwargs)
    return formatter.vformat(_string, args, kwargs)

>>> '{0.__class__}'.format(42)
""
>>> safe_format('{0.__class__}', 42)
Traceback (most recent call last):
  File "", line 1, in
AttributeError: __class__