dotnet · jamesmontemagno · Jul 9, 2025 · Jun 23, 2025 · Jul 3, 2025 · Jul 3, 2025
diff --git a/release-notes/10.0/preview/preview6/libraries.md b/release-notes/10.0/preview/preview6/libraries.md
@@ -2,12 +2,64 @@
 
 Here's a summary of what's new in .NET Libraries in this preview release:
 
-- [Feature](#feature)
+- [Option to disallow duplicate JSON properties](#option-to-disallow-duplicate-json-properties)
+- [Strict JSON serialization options](#strict-json-serialization-options)
+- [Post-Quantum Cryptography (PQC)](#post-quantum-cryptography-pqc)
 
 .NET Libraries updates in .NET 10:
 
 - [What's new in .NET 10](https://learn.microsoft.com/dotnet/core/whats-new/dotnet-10/overview) documentation
 
-## Feature
+## Option to disallow duplicate JSON properties
 
-Something about the feature
+The JSON specification does not specify how to handle duplicate properties when deserializing a JSON payload. This can lead to unexpected results and security vulnerabilities. For example, see https://bishopfox.com/blog/json-interoperability-vulnerabilities and https://nvd.nist.gov/vuln/detail/CVE-2017-12635. This release introduces the `AllowDuplicateProperties` option to disallow duplicate JSON properties:
+
+```csharp
+string json = """{ "Value": 1, "Value": -1 }""";
+Console.WriteLine(JsonSerializer.Deserialize<MyRecord>(json).Value); // -1
+
+JsonSerializerOptions options = new() { AllowDuplicateProperties = false };
+JsonSerializer.Deserialize<MyRecord>(json, options);                // throws JsonException
+JsonSerializer.Deserialize<JsonObject>(json, options);              // throws JsonException
+JsonSerializer.Deserialize<Dictionary<string, int>>(json, options); // throws JsonException
+
+JsonDocumentOptions docOptions = new() { AllowDuplicateProperties = false };
+JsonDocument.Parse(json, docOptions);   // throws JsonException
+
+record MyRecord(int Value);
+```
+
+Duplicate detection works by checking if a value is assigned multiple times during deserialization, so it will work with as expected with other options like case-sensitivity and naming policy.
+
+## Strict JSON serialization options
+
+The JSON serializer accepts many options to customize serialization and deserialization, but the defaults may be too relaxed for some applications. This release adds a new `JsonSerializationOptions.Strict` preset which follows best practices by including the following options:
+
+- Applies the `JsonUnmappedMemberHandling.Disallow` policy
+- Disables `AllowDuplicateProperties`
+- Preserves case sensitive property binding
+- Enables both `RespectNullableAnnotations` and `RespectRequiredConstructorParameters` settings
+
+These options are read-compatible with `JsonSerializationOptions.Default` - an object serialized with `JsonSerializationOptions.Default` can be deserialized with `JsonSerializationOptions.Strict`.
+
+## Post-Quantum Cryptography (PQC)
+
+Windows announced Post-Quantum Cryptography support recently in a [blog post](https://techcommunity.microsoft.com/blog/microsoft-security-blog/post-quantum-cryptography-comes-to-windows-insiders-and-linux/4413803) and in this release we have started adding Windows CNG support to .NET. The following code from the previous release notes now also works on Windows versions with PQC:
+
+```csharp
+using System;
+using System.IO;
+using System.Security.Cryptography;
+
+private static bool ValidateMLDsaSignature(ReadOnlySpan<byte> data, ReadOnlySpan<byte> signature, string publicKeyPath)
+{
+    string publicKeyPem = File.ReadAllText(publicKeyPath);
+
+    using (MLDsa key = MLDsa.ImportFromPem(publicKeyPem))
+    {
+        return key.VerifyData(data, signature);
+    }
+}
+```
+
+We are also working on adding downlevel support in Microsoft.Bcl.Cryptography to allow use in .NET Framework. Try out the feature now by installing a Windows Insider build (only available in the Canary Channel at this time).