How Secure is Your JSON Deserialization with Json.Net's TypeNameHandling?

External JSON Exposure: Understanding the Risks of TypeNameHandling with Json.Net

JSON deserialization with automatic type handling can pose security threats. This article aims to clarify the potential vulnerabilities when using TypeNameHandling with settings set to Auto in Json.Net.

Understanding TypeNameHandling in Json.Net

TypeNameHandling controls how JSON.Net deserializes types with "$type" properties, which specify the fully qualified name of the type to instantiate. When set to Auto, Json.Net attempts to resolve the specified type and construct an instance.

Potential Hazards

Without immediate object or dynamic members in your data model, you may assume protection from deserialization attacks. However, certain scenarios can still introduce risks:

• Untyped Collections: Deserializing untyped collections like ArrayList or List
• CollectionBase: Types inheriting from CollectionBase allow runtime item validation, creating a potential loophole for attack gadget construction.
• Shared Base Types: Polymorphic values with base types or interfaces shared by attack gadgets are susceptible to deserialization attacks.
• ISerializable Types: Types implementing ISerializable may deserialize untyped members, including the Exception.Data dictionary.
• Conditional Serialization: Members marked as non-serialized via ShouldSerialize methods can still be deserialized if they are present in JSON input.

Mitigation Measures

To enhance security, consider the following:

• Custom SerializationBinder: Implement a custom SerializationBinder to validate expected types and prevent deserialization of unexpected types.
• TypeNameHandling.None: Consider setting TypeNameHandling to None, which effectively disables type resolution during deserialization.
• Alertness to Unexpected/Hidden Typing: Stay vigilant for untyped members or hidden serialization behaviors in your data model.
• Disable Default Serialization Contract: Avoid setting DefaultContractResolver.IgnoreSerializableInterface or DefaultContractResolver.IgnoreSerializableAttribute to false.

Conclusion

While certain mechanisms in Json.Net help mitigate vulnerabilities, it is crucial to carefully consider the potential risks posed by TypeNameHandling in external JSON deserialization. By following the recommended precautions, such as implementing a custom SerializationBinder and verifying your data model's typing, you can increase the security of your application while utilizing Json.Net's features.

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