diff --git a/src/SignhostAPIClient.Tests/IVerificationDeserializationTests.cs b/src/SignhostAPIClient.Tests/IVerificationDeserializationTests.cs
new file mode 100644
index 00000000..7983a6d1
--- /dev/null
+++ b/src/SignhostAPIClient.Tests/IVerificationDeserializationTests.cs
@@ -0,0 +1,44 @@
+using System.Text.Json;
+using FluentAssertions;
+using Signhost.APIClient.Rest.DataObjects;
+using Xunit;
+
+namespace Signhost.APIClient.Rest.Tests;
+
+public class IVerificationDeserializationTests
+{
+	[Fact]
+	public void Given_unknown_verification_type_When_deserializing_to_IVerification_Then_returns_unknown_verification()
+	{
+		// Arrange
+		const string json = "{\"Type\":\"SomethingUnknown\"}";
+
+		// Act
+		var verification = JsonSerializer.Deserialize<IVerification>(json, SignhostJsonSerializerOptions.Default);
+
+		// Assert
+		var unknownVerification = verification.Should().BeOfType<UnknownVerification>().Subject;
+		unknownVerification.Type.Should().Be("SomethingUnknown");
+	}
+
+	[Fact]
+	public void Given_unknown_verification_with_additional_fields_When_deserializing_to_IVerification_Then_additional_data_is_available()
+	{
+		// Arrange
+		const string json = "{\"Type\":\"SomethingUnknown\",\"DisplayName\":\"External\",\"Requires2FA\":true,\"Meta\":{\"Source\":\"partner\"}}";
+
+		// Act
+		var verification = JsonSerializer.Deserialize<IVerification>(json, SignhostJsonSerializerOptions.Default);
+
+		// Assert
+		var unknownVerification = verification.Should().BeOfType<UnknownVerification>().Subject;
+		unknownVerification.Type.Should().Be("SomethingUnknown");
+		unknownVerification.AdditionalData.Should().NotBeNull();
+		unknownVerification.AdditionalData!.Should().ContainKey("DisplayName");
+		unknownVerification.AdditionalData["DisplayName"].GetString().Should().Be("External");
+		unknownVerification.AdditionalData.Should().ContainKey("Requires2FA");
+		unknownVerification.AdditionalData["Requires2FA"].GetBoolean().Should().BeTrue();
+		unknownVerification.AdditionalData.Should().ContainKey("Meta");
+		unknownVerification.AdditionalData["Meta"].GetProperty("Source").GetString().Should().Be("partner");
+	}
+}
diff --git a/src/SignhostAPIClient/Rest/DataObjects/Verifications/IVerification.cs b/src/SignhostAPIClient/Rest/DataObjects/Verifications/IVerification.cs
index 9c586474..918a4bed 100644
--- a/src/SignhostAPIClient/Rest/DataObjects/Verifications/IVerification.cs
+++ b/src/SignhostAPIClient/Rest/DataObjects/Verifications/IVerification.cs
@@ -1,22 +1,9 @@
 using System.Text.Json.Serialization;
+using Signhost.APIClient.Rest.JsonConverters;
 
 namespace Signhost.APIClient.Rest.DataObjects;
 
-[JsonPolymorphic(TypeDiscriminatorPropertyName = "Type")]
-[JsonDerivedType(typeof(ConsentVerification), "Consent")]
-[JsonDerivedType(typeof(DigidVerification), "DigiD")]
-[JsonDerivedType(typeof(EidasLoginVerification), "eIDAS Login")]
-[JsonDerivedType(typeof(IdealVerification), "iDeal")]
-[JsonDerivedType(typeof(IdinVerification), "iDIN")]
-[JsonDerivedType(typeof(IPAddressVerification), "IPAddress")]
-[JsonDerivedType(typeof(ItsmeIdentificationVerification), "itsme Identification")]
-[JsonDerivedType(typeof(PhoneNumberVerification), "PhoneNumber")]
-[JsonDerivedType(typeof(ScribbleVerification), "Scribble")]
-[JsonDerivedType(typeof(SurfnetVerification), "SURFnet")]
-[JsonDerivedType(typeof(CscVerification), "CSC Qualified")]
-[JsonDerivedType(typeof(EherkenningVerification), "eHerkenning")]
-[JsonDerivedType(typeof(OidcVerification), "OpenID Providers")]
-[JsonDerivedType(typeof(OnfidoVerification), "Onfido")]
+[JsonConverter(typeof(VerificationConverter))]
 public interface IVerification
 {
 }
diff --git a/src/SignhostAPIClient/Rest/DataObjects/Verifications/UnknownVerification.cs b/src/SignhostAPIClient/Rest/DataObjects/Verifications/UnknownVerification.cs
new file mode 100644
index 00000000..c02cb238
--- /dev/null
+++ b/src/SignhostAPIClient/Rest/DataObjects/Verifications/UnknownVerification.cs
@@ -0,0 +1,14 @@
+using System.Collections.Generic;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace Signhost.APIClient.Rest.DataObjects;
+
+public class UnknownVerification
+	: IVerification
+{
+	public string Type { get; set; } = default!;
+
+	[JsonExtensionData]
+	public Dictionary<string, JsonElement>? AdditionalData { get; set; }
+}
diff --git a/src/SignhostAPIClient/Rest/JsonConverters/VerificationConverter.cs b/src/SignhostAPIClient/Rest/JsonConverters/VerificationConverter.cs
new file mode 100644
index 00000000..477839d9
--- /dev/null
+++ b/src/SignhostAPIClient/Rest/JsonConverters/VerificationConverter.cs
@@ -0,0 +1,96 @@
+using System;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using Signhost.APIClient.Rest.DataObjects;
+
+namespace Signhost.APIClient.Rest.JsonConverters;
+
+public class VerificationConverter
+	: JsonConverter<IVerification>
+{
+	public override IVerification Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
+	{
+		var jsonElement = JsonSerializer.Deserialize<JsonElement>(ref reader, options);
+		if (jsonElement.ValueKind is not JsonValueKind.Object) {
+			throw new JsonException("Verification must be a JSON object.");
+		}
+
+		if (
+			!jsonElement.TryGetProperty("Type", out var typeElement) ||
+			typeElement.ValueKind is not JsonValueKind.String ||
+			string.IsNullOrWhiteSpace(typeElement.GetString())
+		) {
+			throw new JsonException("Verification must contain a non-empty Type property.");
+		}
+
+		string discriminator = typeElement.GetString()!;
+		string json = jsonElement.GetRawText();
+		var verificationType = ResolveType(discriminator);
+
+		if (verificationType is not null) {
+			return (IVerification)JsonSerializer.Deserialize(json, verificationType, options)!;
+		}
+
+		return JsonSerializer.Deserialize<UnknownVerification>(json, options)!;
+	}
+
+	public override void Write(Utf8JsonWriter writer, IVerification value, JsonSerializerOptions options)
+	{
+		if (value is UnknownVerification unknownVerification) {
+			JsonSerializer.Serialize(writer, unknownVerification, options);
+			return;
+		}
+
+		string discriminator = ResolveDiscriminator(value)
+			?? throw new JsonException($"Verification type '{value.GetType().Name}' is not supported.");
+
+		string json = JsonSerializer.Serialize(value, value.GetType(), options);
+		using var jsonDocument = JsonDocument.Parse(json);
+
+		writer.WriteStartObject();
+		writer.WriteString("Type", discriminator);
+
+		foreach (var property in jsonDocument.RootElement.EnumerateObject()) {
+			writer.WritePropertyName(property.Name);
+			property.Value.WriteTo(writer);
+		}
+
+		writer.WriteEndObject();
+	}
+
+	private static Type? ResolveType(string discriminator) => discriminator switch {
+		"Consent" => typeof(ConsentVerification),
+		"DigiD" => typeof(DigidVerification),
+		"eIDAS Login" => typeof(EidasLoginVerification),
+		"iDeal" => typeof(IdealVerification),
+		"iDIN" => typeof(IdinVerification),
+		"IPAddress" => typeof(IPAddressVerification),
+		"itsme Identification" => typeof(ItsmeIdentificationVerification),
+		"PhoneNumber" => typeof(PhoneNumberVerification),
+		"Scribble" => typeof(ScribbleVerification),
+		"SURFnet" => typeof(SurfnetVerification),
+		"CSC Qualified" => typeof(CscVerification),
+		"eHerkenning" => typeof(EherkenningVerification),
+		"OpenID Providers" => typeof(OidcVerification),
+		"Onfido" => typeof(OnfidoVerification),
+		_ => null,
+	};
+
+	private static string? ResolveDiscriminator(IVerification value) => value switch {
+		ConsentVerification => "Consent",
+		DigidVerification => "DigiD",
+		EidasLoginVerification => "eIDAS Login",
+		IdealVerification => "iDeal",
+		IdinVerification => "iDIN",
+		IPAddressVerification => "IPAddress",
+		ItsmeIdentificationVerification => "itsme Identification",
+		PhoneNumberVerification => "PhoneNumber",
+		ScribbleVerification => "Scribble",
+		SurfnetVerification => "SURFnet",
+		CscVerification => "CSC Qualified",
+		EherkenningVerification => "eHerkenning",
+		OidcVerification => "OpenID Providers",
+		OnfidoVerification => "Onfido",
+		_ => null,
+	};
+}