Patched code sample

import ast

class SecurityError(Exception):
    """Custom exception for security violations in safe_eval."""
    pass

# Allowlist of safe built-in functions
_ALLOWED_BUILTINS = {
    'str', 'int', 'float', 'len', 'list', 'dict', 'set', 'tuple',
    'max', 'min', 'sum', 'abs', 'round', 'any', 'all', 'sorted',
    'isinstance', 'range'
}

# Allowlist of safe attributes, which is the core of the fix.
# It prevents access to dangerous attributes like 'gi_frame', 'f_back', etc.
_ALLOWED_ATTRIBUTES = {
    # Common and safe string attributes
    'strip', 'split', 'replace', 'lower', 'upper', 'title', 'capitalize',
    # Common and safe list/dict attributes
    'append', 'extend', 'insert', 'remove', 'pop', 'clear', 'index', 'count', 'sort', 'reverse', 'copy',
    'keys', 'values', 'items', 'get',
}


class _SafeExpressionValidator(ast.NodeVisitor):
    """
    AST visitor to validate expressions for safe evaluation.
    This fixed version uses an allowlist for attribute access.
    """
    def visit_Call(self, node: ast.Call) -> None:
        if isinstance(node.func, ast.Name) and node.func.id not in _ALLOWED_BUILTINS:
            raise SecurityError(f"Calling the function '{node.func.id}' is not allowed.")
        self.generic_visit(node)

    def visit_Attribute(self, node: ast.Attribute) -> None:
        # Still blocks attributes starting with underscore as a first-level defense.
        if node.attr.startswith('_'):
            raise SecurityError(f"Access to private attribute '{node.attr}' is not allowed.")
        
        # ** THE FIX **
        # Checks the attribute against a safe allowlist, preventing access to
        # dangerous attributes like 'gi_frame', 'f_globals', 'f_builtins', etc.
        if node.attr not in _ALLOWED_ATTRIBUTES:
            raise SecurityError(f"Access to attribute '{node.attr}' is not allowed for security reasons.")
        
        self.generic_visit(node)

# Example of how this validator would be used in the safe evaluation function.
# The original vulnerable function did not have the `if node.attr not in _ALLOWED_ATTRIBUTES:` check.
def _safe_eval_expression(expression: str, context: dict):
    """
    Safely evaluates a Python expression string.
    """
    try:
        tree = ast.parse(expression, mode='eval')
        validator = _SafeExpressionValidator()
        validator.visit(tree)
        
        # The 'builtins' dictionary is carefully controlled.
        safe_builtins = {b: __builtins__[b] for b in _ALLOWED_BUILTINS if b in __builtins__}
        
        # Evaluation is done with a restricted global and local context.
        return eval(compile(tree, filename='<string>', mode='eval'), {"__builtins__": safe_builtins}, context)
    except (SecurityError, SyntaxError, NameError, TypeError, Exception) as e:
        raise SecurityError(f"Failed to safely evaluate expression: {e}") from e