Is Newtonsoft JSON's TypeNameHandling.All a Security Risk?

Newtonsoft JSON's TypeNameHandling.All: Security Implications

Newtonsoft JSON's documentation strongly cautions against using TypeNameHandling.All for deserializing JSON from untrusted sources. This article explores the potential security vulnerabilities associated with this setting and outlines mitigation strategies.

Vulnerabilities of TypeNameHandling.All

The TypeNameHandling.All setting allows Newtonsoft JSON to instantiate types based on metadata within the JSON payload. While convenient, this creates a significant security risk. An attacker can craft malicious JSON that forces the deserialization of harmful types, leading to arbitrary code execution.

For instance, a benign JSON payload might look like this:

{
   "$type": "Car",
   "Maker": "Ford",
   "Model": "Explorer"
}

However, a malicious actor could construct a payload targeting a system-level type:

{
   "$type": "System.CodeDom.Compiler.TempFileCollection",
   "BasePath": "%SYSTEMDRIVE%",
   "KeepFiles": "false",
   "TempDir": "%SYSTEMROOT%"
}

This would cause Newtonsoft JSON to create a TempFileCollection instance, potentially deleting arbitrary files on the system by manipulating BasePath and TempDir.

Effective Mitigation: Custom SerializationBinder

The key to securing JSON deserialization with type information is using a custom SerializationBinder. This allows for strict control over which types are permitted during deserialization, effectively preventing the instantiation of malicious types.

Implementing a custom SerializationBinder involves these steps:

1. Create a class that implements the IBindingSerializer interface.
2. Override the BindToName method to enforce type validation. This typically involves checking the type against a whitelist or blacklist.
3. Register your custom SerializationBinder with the Newtonsoft JSON serializer.

By implementing these steps, you can safely deserialize JSON from external sources while preventing potentially harmful type instantiations. This proactive approach significantly reduces the risk of exploitation.

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