Is Json.Net's TypeNameHandling.Auto Setting a Security Risk for External JSON Deserialization?

Is External JSON Vulnerable Due to Json.Net TypeNameHandling Auto?

In the realm of web applications, handling JSON requests is a common practice. However, concerns have been raised regarding the potential threats posed by automatic type deserialization using JSON frameworks like Json.Net.

Understanding the Problem

When a JSON payload is deserialized without adequate validation, particularly when dynamic or object-typed properties are present, it becomes possible for an attacker to supply a malicious payload containing a "$type" key. This key can specify an attack gadget, which when deserialized, can execute arbitrary code on the receiving system.

TypeNameHandling and Vulnerability

Json.Net provides a TypeNameHandling setting that determines how JSON payloads containing "$type" keys are handled:

• None: Disables deserialization of "$type" keys.
• Auto: Automatically resolves the type specified by the "$type" key.

By default, this setting is often left as "Auto," which raises concerns about potential vulnerabilities.

Safe Approach with TypeNameHandling.Auto

In the specific scenario where incoming JSON is only deserialized to a specific type (MyObject) and there are no object or dynamic typed members within MyObject or its subobjects, it is unlikely that a vulnerability exists.

However, it is important to note that this is not a guarantee of safety. Unexpected types or collections containing untyped items could still allow for the deserialization of an attack gadget.

Mitigations and Best Practices

To further mitigate the risk, consider the following best practices:

• Use a custom SerializationBinder to validate incoming types.
• Limit the use of object, dynamic, and IDynamicMetaObjectProvider types.
• Exercise caution when deserializing collections or values that share a base type with potential attack gadgets.
• Set DefaultContractResolver.IgnoreSerializableInterface = true to prevent deserialization of types implementing ISerializable.

Conclusion

While utilizing Json.Net's TypeNameHandling.Auto setting may reduce the risk of vulnerability, it is essential to thoroughly validate incoming JSON data and implement additional protective measures to mitigate potential threats.

The above is the detailed content of Is Json.Net's TypeNameHandling.Auto Setting a Security Risk for External JSON Deserialization?. For more information, please follow other related articles on the PHP Chinese website!