Patched code sample

import json
from typing import Any, Dict, Set

# Define placeholder classes to make the example self-contained.
# In the actual library, these would be the real APScheduler classes.
class DateTrigger:
    def __setstate__(self, state): pass
class IntervalTrigger:
    def __setstate__(self, state): pass
class JobInfo:
    def __setstate__(self, state): pass

# Define a custom exception to make the example self-contained.
class DeserializationError(Exception):
    pass

class FixedBaseSerializer:
    """
    This class represents the fixed version of the serializer, which uses an
    allow-list for deserialization to prevent arbitrary class instantiation.
    """
    __slots__ = '_deserializable_classes'

    def __init__(self):
        # The allow-list is created on initialization.
        self._deserializable_classes = {
            f'{cls.__module__}.{cls.__name__}': cls
            for cls in self.get_deserializable_classes()
        }

    @property
    def deserializable_classes(self) -> Dict[str, type]:
        return self._deserializable_classes

    def get_deserializable_classes(self) -> Set[type]:
        """
        Defines the allow-list of classes that are safe to deserialize.
        The vulnerability was that any class could be dynamically imported.
        """
        return {
            DateTrigger,
            IntervalTrigger,
            JobInfo
        }

    def unmarshal_object(self, data: Dict[str, Any]) -> Any:
        """
        The fixed unmarshal_object function. It looks up the class from a
        pre-defined allow-list instead of dynamically importing it from the payload.
        """
        classpath = data.get('__class__')
        if not isinstance(classpath, str):
            raise DeserializationError('The value of "__class__" must be a string')

        # Secure: Look up the class in the pre-approved dictionary.
        cls = self.deserializable_classes.get(classpath)
        if cls is None:
            # This is the core of the fix: if the class is not in the allow-list,
            # deserialization is denied, preventing RCE.
            raise DeserializationError(
                f'Could not deserialize object of type {classpath}: '
                f'class not found in deserializable_classes')

        instance = cls.__new__(cls)
        if '__state__' in data:
            instance.__setstate__(data['__state__'])

        return instance

class JSONSerializer(FixedBaseSerializer):
    """A simplified JSON serializer demonstrating the fix."""
    def deserialize(self, payload: bytes) -> Any:
        def custom_decode(obj):
            if '__class__' in obj:
                return self.unmarshal_object(obj)
            return obj

        try:
            return json.loads(payload, object_hook=custom_decode)
        except (TypeError, ValueError) as e:
            raise DeserializationError from e