Is Using `TypeNameHandling.All` in Newtonsoft.Json Safe for Deserializing Untrusted Data?

Newtonsoft.Json's TypeNameHandling.All: Security Risks When Deserializing Untrusted Data

The TypeNameHandling property in Newtonsoft.Json dictates how polymorphic objects are deserialized. Setting it to TypeNameHandling.All enables Newtonsoft.Json to instantiate types based on the $type property within the incoming JSON. However, this presents significant security vulnerabilities when handling untrusted data.

Security Implications

Malicious actors can exploit TypeNameHandling.All by injecting a $type property specifying a harmful type within the JSON. This allows them to execute arbitrary code or perform unwanted actions on the target system.

Consider a seemingly harmless class:

public class Vehicle
{
    public string Make { get; set; }
    public string Model { get; set; }
}

A malicious JSON payload could look like this:

{
  "$type": "System.Diagnostics.Process",
  "Make": "Attack",
  "Model": "DeleteC:\ImportantFiles"
}

While the Make and Model properties are benign, the $type property forces the creation of a System.Diagnostics.Process object, potentially initiating harmful processes on the system. This bypasses normal type checking and opens the door to various attacks.

Mitigating the Risk

To prevent such attacks, avoid using TypeNameHandling.All when deserializing JSON from external, untrusted sources. Instead, employ TypeNameHandling.None to disable type name handling. Alternatively, implement a custom SerializationBinder to meticulously control which types are allowed during deserialization, effectively whitelisting safe types. This provides a more granular and secure approach to handling polymorphic deserialization.

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