Is Python\'s `eval()` Function Safe When Handling Untrusted Strings?

Eval() in Python: Security Risks with Untrusted Strings

Introduction

The Python eval() function allows the execution of code dynamically from a string. While versatile, it poses significant security risks when evaluating untrusted strings. This article examines these risks and provides potential mitigations.

Security Risks with Untrusted Strings

1. Accessibility of Class Methods from Foo Object (eval(string, {"f": Foo()}, {}))

Yes, this is unsafe. Accessing the Foo class from its instance via eval(string) grants the potential to access sensitive modules such as os or sys from within a Foo instance.

2. Reaching Built-ins via Eval (eval(string, {}, {}))

Yes, this is also unsafe. Eval can access built-in functions like len and list, which can be exploited to access unsafe modules like os or sys.

3. Removing Built-ins from Eval Context

There is no viable way to remove built-ins entirely from the eval context in Python.

Mitigations

1. Careful String Validation

Validate user-provided strings thoroughly to prevent the execution of malicious code.

2. Restricted Local Variables

Use the locals parameter of eval() to restrict the variables available within the evaluated string.

3. Custom Safe Evaluation Function

Implement a custom, sandboxed evaluation function that restricts access to sensitive modules and objects.

Alternatives to eval()

Consider alternatives to eval(), such as:

• exec() with additional security measures in place.
• ast.literal_eval() for evaluating simple expressions.
• serializer modules for transmitting data securely.

Conclusion

Eval() with untrusted strings poses significant security risks. Implement rigorous mitigations or consider alternative approaches when handling user-provided code. Remember that eval() should only be used when absolutely necessary.

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