CA2327: Do not use insecure JsonSerializerSettings

Property	Value
Rule ID	CA2327
Title	Do not use insecure JsonSerializerSettings
Category	Security
Fix is breaking or non-breaking	Non-breaking
Enabled by default in .NET 8	No

Cause

This rule fires when both of the following conditions are true for a Newtonsoft.Json.JsonSerializerSettings instance:

• The TypeNameHandling property is a value other than None.
• The SerializationBinder property is null.

The JsonSerializerSettings instance must be used in one of the following ways:

• Initialized as a class field or property.
• Returned by a method.
• Passed to JsonSerializer.Create or JsonSerializer.CreateDefault.
• Passed to a JsonConvert method that has a JsonSerializerSettings parameter.

By default, this rule analyzes the entire codebase, but this is configurable.

Rule description

Insecure deserializers are vulnerable when deserializing untrusted data. An attacker could modify the serialized data to include unexpected types to inject objects with malicious side effects. An attack against an insecure deserializer could, for example, execute commands on the underlying operating system, communicate over the network, or delete files.

This rule finds Newtonsoft.Json.JsonSerializerSettings instances that are configured to deserialize types specified from input, but not configured to restrict deserialized types with a Newtonsoft.Json.Serialization.ISerializationBinder. If you want to disallow deserialization of types specified from input completely, disable rules CA2327, CA2328, CA2329, and CA2330, and enable rule CA2326 instead.

How to fix violations

• Use TypeNameHandling's None value, if possible.
• Make the serialized data tamper-proof. After serialization, cryptographically sign the serialized data. Before deserialization, validate the cryptographic signature. Protect the cryptographic key from being disclosed and design for key rotations.
• Restrict deserialized types. Implement a custom Newtonsoft.Json.Serialization.ISerializationBinder. Before deserializing with Json.NET, ensure your custom ISerializationBinder is specified in the Newtonsoft.Json.JsonSerializerSettings.SerializationBinder property. In the overridden Newtonsoft.Json.Serialization.ISerializationBinder.BindToType method, if the type is unexpected, return null or throw an exception to stop deserialization.

When to suppress warnings

It's safe to suppress a warning from this rule if:

• You know the input is trusted. Consider that your application's trust boundary and data flows may change over time.
• You've taken one of the precautions in How to fix violations.

Suppress a warning

If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

#pragma warning disable CA2327
// The code that's violating the rule is on this line.
#pragma warning restore CA2327

To disable the rule for a file, folder, or project, set its severity to none in the configuration file.

[*.{cs,vb}]
dotnet_diagnostic.CA2327.severity = none

For more information, see How to suppress code analysis warnings.

Configure code to analyze

Use the following options to configure which parts of your codebase to run this rule on.

• Exclude specific symbols
• Exclude specific types and their derived types

You can configure these options for just this rule, for all rules it applies to, or for all rules in this category (Security) that it applies to. For more information, see Code quality rule configuration options.

Exclude specific symbols

You can exclude specific symbols, such as types and methods, from analysis. For example, to specify that the rule should not run on any code within types named MyType, add the following key-value pair to an .editorconfig file in your project:

dotnet_code_quality.CAXXXX.excluded_symbol_names = MyType

Allowed symbol name formats in the option value (separated by |):

• Symbol name only (includes all symbols with the name, regardless of the containing type or namespace).
• Fully qualified names in the symbol's documentation ID format. Each symbol name requires a symbol-kind prefix, such as M: for methods, T: for types, and N: for namespaces.
• .ctor for constructors and .cctor for static constructors.

Examples:

Option Value	Summary
dotnet_code_quality.CAXXXX.excluded_symbol_names = MyType	Matches all symbols named MyType.
dotnet_code_quality.CAXXXX.excluded_symbol_names = MyType1|MyType2	Matches all symbols named either MyType1 or MyType2.
dotnet_code_quality.CAXXXX.excluded_symbol_names = M:NS.MyType.MyMethod(ParamType)	Matches specific method MyMethod with the specified fully qualified signature.
dotnet_code_quality.CAXXXX.excluded_symbol_names = M:NS1.MyType1.MyMethod1(ParamType)|M:NS2.MyType2.MyMethod2(ParamType)	Matches specific methods MyMethod1 and MyMethod2 with the respective fully qualified signatures.

Exclude specific types and their derived types

You can exclude specific types and their derived types from analysis. For example, to specify that the rule should not run on any methods within types named MyType and their derived types, add the following key-value pair to an .editorconfig file in your project:

dotnet_code_quality.CAXXXX.excluded_type_names_with_derived_types = MyType

Allowed symbol name formats in the option value (separated by |):

• Type name only (includes all types with the name, regardless of the containing type or namespace).
• Fully qualified names in the symbol's documentation ID format, with an optional T: prefix.

Examples:

Option Value	Summary
dotnet_code_quality.CAXXXX.excluded_type_names_with_derived_types = MyType	Matches all types named MyType and all of their derived types.
dotnet_code_quality.CAXXXX.excluded_type_names_with_derived_types = MyType1|MyType2	Matches all types named either MyType1 or MyType2 and all of their derived types.
dotnet_code_quality.CAXXXX.excluded_type_names_with_derived_types = M:NS.MyType	Matches specific type MyType with given fully qualified name and all of its derived types.
dotnet_code_quality.CAXXXX.excluded_type_names_with_derived_types = M:NS1.MyType1|M:NS2.MyType2	Matches specific types MyType1 and MyType2 with the respective fully qualified names, and all of their derived types.

Pseudo-code examples

Violation

using Newtonsoft.Json;

public class BookRecord
{
    public string Title { get; set; }
    public object Location { get; set; }
}

public abstract class Location
{
    public string StoreId { get; set; }
}

public class AisleLocation : Location
{
    public char Aisle { get; set; }
    public byte Shelf { get; set; }
}

public class WarehouseLocation : Location
{
    public string Bay { get; set; }
    public byte Shelf { get; set; }
}

public class ExampleClass
{
    public BookRecord DeserializeBookRecord(string s)
    {
        JsonSerializerSettings settings = new JsonSerializerSettings();
        settings.TypeNameHandling = TypeNameHandling.Auto;
        return JsonConvert.DeserializeObject<BookRecord>(s, settings);    // CA2327 violation
    }
}

Imports Newtonsoft.Json

Public Class BookRecord
    Public Property Title As String
    Public Property Location As Location
End Class

Public MustInherit Class Location
    Public Property StoreId As String
End Class

Public Class AisleLocation
    Inherits Location

    Public Property Aisle As Char
    Public Property Shelf As Byte
End Class

Public Class WarehouseLocation
    Inherits Location

    Public Property Bay As String
    Public Property Shelf As Byte
End Class

Public Class ExampleClass
    Public Function DeserializeBookRecord(s As String) As BookRecord
        Dim settings As JsonSerializerSettings = New JsonSerializerSettings()
        settings.TypeNameHandling = TypeNameHandling.Auto
        Return JsonConvert.DeserializeObject(Of BookRecord)(s, settings)    ' CA2327 violation
    End Function
End Class

Solution

using System;
using Newtonsoft.Json;
using Newtonsoft.Json.Serialization;

public class BookRecordSerializationBinder : ISerializationBinder
{
    // To maintain backwards compatibility with serialized data before using an ISerializationBinder.
    private static readonly DefaultSerializationBinder Binder = new DefaultSerializationBinder();

    public void BindToName(Type serializedType, out string assemblyName, out string typeName)
    {
        Binder.BindToName(serializedType, out assemblyName, out typeName);
    }

    public Type BindToType(string assemblyName, string typeName)
    {
        // If the type isn't expected, then stop deserialization.
        if (typeName != "BookRecord" && typeName != "AisleLocation" && typeName != "WarehouseLocation")
        {
            return null;
        }

        return Binder.BindToType(assemblyName, typeName);
    }
}

public class BookRecord
{
    public string Title { get; set; }
    public object Location { get; set; }
}

public abstract class Location
{
    public string StoreId { get; set; }
}

public class AisleLocation : Location
{
    public char Aisle { get; set; }
    public byte Shelf { get; set; }
}

public class WarehouseLocation : Location
{
    public string Bay { get; set; }
    public byte Shelf { get; set; }
}

public class ExampleClass
{
    public BookRecord DeserializeBookRecord(string s)
    {
        JsonSerializerSettings settings = new JsonSerializerSettings();
        settings.TypeNameHandling = TypeNameHandling.Auto;
        settings.SerializationBinder = new BookRecordSerializationBinder();
        return JsonConvert.DeserializeObject<BookRecord>(s, settings);
    }
}

Imports System
Imports Newtonsoft.Json
Imports Newtonsoft.Json.Serialization

Public Class BookRecordSerializationBinder
    Implements ISerializationBinder

    ' To maintain backwards compatibility with serialized data before using an ISerializationBinder.
    Private Shared ReadOnly Property Binder As New DefaultSerializationBinder()

    Public Sub BindToName(serializedType As Type, ByRef assemblyName As String, ByRef typeName As String) Implements ISerializationBinder.BindToName
        Binder.BindToName(serializedType, assemblyName, typeName)
    End Sub

    Public Function BindToType(assemblyName As String, typeName As String) As Type Implements ISerializationBinder.BindToType
        ' If the type isn't expected, then stop deserialization.
        If typeName <> "BookRecord" AndAlso typeName <> "AisleLocation" AndAlso typeName <> "WarehouseLocation" Then
            Return Nothing
        End If

        Return Binder.BindToType(assemblyName, typeName)
    End Function
End Class

Public Class BookRecord
    Public Property Title As String
    Public Property Location As Location
End Class

Public MustInherit Class Location
    Public Property StoreId As String
End Class

Public Class AisleLocation
    Inherits Location

    Public Property Aisle As Char
    Public Property Shelf As Byte
End Class

Public Class WarehouseLocation
    Inherits Location

    Public Property Bay As String
    Public Property Shelf As Byte
End Class

Public Class ExampleClass
    Public Function DeserializeBookRecord(s As String) As BookRecord
        Dim settings As JsonSerializerSettings = New JsonSerializerSettings()
        settings.TypeNameHandling = TypeNameHandling.Auto
        settings.SerializationBinder = New BookRecordSerializationBinder()
        Return JsonConvert.DeserializeObject(Of BookRecord)(s, settings)
    End Function
End Class

Related rules

CA2326: Do not use TypeNameHandling values other than None

CA2328: Ensure that JsonSerializerSettings are secure

CA2329: Do not deserialize with JsonSerializer using an insecure configuration

CA2330: Ensure that JsonSerializer has a secure configuration when deserializing