Many common libraries that are often used for reading configuration files and deserializing objects are very dangerous because they can allow execution of arbitrary code. By default, libraries such as PyYAML and pickle do not provide strong separation of data and code, and thus allow code to be embedded inside the input.
Often the input to these libraries is untrusted or only partially trusted. These unsafe inputs can come from configuration files or be provided via REST APIs. For example, we often use YAML for configuration files but YAML files can also contain embedded Python code. This may provide an attacker with a method to execute code.
Many, but not all, of these libraries, offer safe interfaces that disable features that enable code execution. You always want to use the safe functions to load input. Often the obvious function to use is not the safe one and we should check the documentation for libraries not covered here.
We often use YAML, pickle, or eval to load data into our Python programs, but this is dangerous. PyYAML has a safe way to load code, but pickle and eval do not.
|PyYAML||Allows creating arbitrary Python objects.||yaml.safe_load||yaml.load|
|pickle||Allows creating arbitrary Python objects.||Do not use||pickle.load, pickle.loads|
|cPickle||Allows creating arbitrary Python objects.||Do not use||cPickle.load, cPickle.loads|
|eval||Runs all input as Python code||Do not use||eval|
|exec||Runs all input as Python code (Python 3.x)||Do not use||exec|
yaml.load is the obvious function to use but it is dangerous:
import yaml import pickle conf_str = ''' !!python/object:__main__.AttackerObj key: 'value' ''' conf = yaml.load(conf_str)
Using pickle or cPickle with untrusted input can result in arbitrary code execution.
import pickle import cPickle user_input = "cos\nsystem\n(S'cat /etc/passwd'\ntR.'\ntR." cPickle.loads(user_input) # results in code execution pickle.loads(user_input) # results in code execution
Similarly eval and exec are difficult to use safely with input that comes from an untrusted source.
user_input = "os.system('cat /etc/passwd')" eval(user_input) # execute python expressions user_input = "import os; os.system('cat /etc/passwd')" exec(user_input) # execute _any_ python code
Here we use PyYAMLs safe YAML loading function:
import yaml conf_str = ''' - key: 'value' - key: 'value' ''' conf = yaml.safe_load(conf_str)
There is no safe alternative for pickle.load. However in most cases using pickle for serialization of data objects is something that can be avoided altogether.