From 82ccfed3fd8ddae328a82d9b47f492ddc66cf7ed Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Sun, 27 Sep 2026 09:25:10 +0000
Subject: [PATCH] [patch] Return null, not throw, for stored JSON with no $type
 discriminator

System.Text.Json's polymorphic deserializer throws NotSupportedException for
well-formed JSON that lacks a $type property, such as {} or {"Token":"x"}.
Deserialize and DeserializeFromString caught only JsonException, so a
corrupt or foreign entry in a native store made CredentialCache.TryGet throw
instead of returning false. Both now treat that payload as unknown.

Fixes ktsu-dev/CredentialCache#164

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014jCUYfoRMwhybsUaSa1NFm
---
 CredentialCache.Test/UnknownPayloadTests.cs   | 73 +++++++++++++++++++
 .../Storage/CredentialSerialization.cs        | 10 +++
 2 files changed, 83 insertions(+)
 create mode 100644 CredentialCache.Test/UnknownPayloadTests.cs

diff --git a/CredentialCache.Test/UnknownPayloadTests.cs b/CredentialCache.Test/UnknownPayloadTests.cs
new file mode 100644
index 0000000..4616647
--- /dev/null
+++ b/CredentialCache.Test/UnknownPayloadTests.cs
@@ -0,0 +1,73 @@
+// Copyright (c) 2023-2026 ktsu-dev contributors
+
+namespace ktsu.CredentialCache.Test;
+
+using System.Collections.Concurrent;
+using System.Text;
+using ktsu.CredentialCache.Storage;
+
+/// <summary>
+/// Tests that a stored payload which is not a known credential reads back as "not found"
+/// rather than throwing, whatever shape the JSON takes.
+/// </summary>
+[TestClass]
+public class UnknownPayloadTests
+{
+	[TestMethod]
+	[DataRow("{}")]
+	[DataRow("{\"Token\":\"x\"}")]
+	[DataRow("{\"$type\":\"Bogus\"}")]
+	[DataRow("[]")]
+	[DataRow("123")]
+	public void DeserializeReturnsNullForJsonThatIsNotAKnownCredential(string json) =>
+		Assert.IsNull(CredentialSerialization.Deserialize(Encoding.UTF8.GetBytes(json)));
+
+	[TestMethod]
+	[DataRow("{}")]
+	[DataRow("{\"Token\":\"x\"}")]
+	[DataRow("{\"$type\":\"Bogus\"}")]
+	[DataRow("[]")]
+	[DataRow("123")]
+	public void DeserializeFromStringReturnsNullForJsonThatIsNotAKnownCredential(string json) =>
+		Assert.IsNull(CredentialSerialization.DeserializeFromString(json));
+
+	[TestMethod]
+	[DataRow("{}")]
+	[DataRow("{\"Token\":\"x\"}")]
+	public void TryGetReturnsFalseForAStoredEntryWithoutATypeDiscriminator(string json)
+	{
+		RawBlobCredentialStore store = new();
+		using CredentialCache cache = new(store);
+		PersonaGUID persona = CredentialCache.CreatePersonaGUID();
+		store.Blobs[persona] = Encoding.UTF8.GetBytes(json);
+
+		bool found = cache.TryGet(persona, out Credential? credential);
+
+		Assert.IsFalse(found);
+		Assert.IsNull(credential);
+	}
+}
+
+/// <summary>
+/// A store that holds raw bytes and reads them back the way the native stores do, so an entry
+/// written by another tool, or damaged, can be planted directly.
+/// </summary>
+public sealed class RawBlobCredentialStore : ICredentialStore
+{
+	public ConcurrentDictionary<PersonaGUID, byte[]> Blobs { get; } = new();
+
+	public string Name => "RawBlob";
+
+	public bool TryLoad(PersonaGUID persona, out Credential? credential)
+	{
+		credential = Blobs.TryGetValue(persona, out byte[]? blob)
+			? CredentialSerialization.DeserializeAndScrub([.. blob])
+			: null;
+		return credential is not null;
+	}
+
+	public void Save(PersonaGUID persona, Credential credential) =>
+		Blobs[persona] = CredentialSerialization.Serialize(credential);
+
+	public bool Remove(PersonaGUID persona) => Blobs.TryRemove(persona, out _);
+}
diff --git a/CredentialCache/Storage/CredentialSerialization.cs b/CredentialCache/Storage/CredentialSerialization.cs
index 1b2aba4..1f462b3 100644
--- a/CredentialCache/Storage/CredentialSerialization.cs
+++ b/CredentialCache/Storage/CredentialSerialization.cs
@@ -59,6 +59,11 @@ public static string SerializeToString(Credential credential) =>
 		{
 			return null;
 		}
+		catch (NotSupportedException)
+		{
+			// Well-formed JSON with no $type discriminator is not a known credential either
+			return null;
+		}
 	}
 
 	/// <summary>
@@ -123,5 +128,10 @@ internal static void Zero(byte[] buffer)
 		{
 			return null;
 		}
+		catch (NotSupportedException)
+		{
+			// Well-formed JSON with no $type discriminator is not a known credential either
+			return null;
+		}
 	}
 }