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title: Jinja 2.8.1 Security Release

author: Armin Ronacher


We just pushed out a new release for Jinja (2.8.1) which includes a security related fix. If you are using the Jinja2 sandbox you are encouraged to upgrade or alternatively manually further lock down the sandbox.

The core of the issue is that Python's string format method that was added to strings can be used to discover potentially dangerous values including configuration values:

>>> config = {'SECRET_KEY': '12345'}
>>> class User(object):
...  def __init__(self, name):
... = name
>>> user = User('joe')
>>> '{0.__class__.__init__.__globals__[config]}'.format(user)
"{'SECRET_KEY': '12345'}"

For this reason you must never let user supply format strings in raw Python as its too easy to escape them. However specifically for the Jinja2 sandbox we changed the behavior now that we're using the same sandboxing functionality that Jinja2 uses for its own runtime also for Python string formatting.

This means that with 2.8.1 and higher templates from sandboxed environments will intercept format strings the same way as with normal cases:

>>> from jinja2.sandbox import SandboxedEnvironment
>>> env = SandboxedEnvironment()
>>> class User(object):
...  def __init__(self, name):
... = name
>>> t = env.from_string(
...  '{{ "{0.__class__.__init__.__globals__}".format(user) }}')
>>> t.render(user=User('joe'))
Traceback (most recent call last):
jinja2.exceptions.SecurityError: ...

If you don't want or you cannot upgrade Jinja2, you can override the is_safe_attribute method on the sandbox and explicitly disallow all format attributes on strings.

Thank you to Olivier Dony for reporting the issue.

categories: releases, security

pub_date: 2016-12-29

twitter_handle: mitsuhiko