From bc4d0134ebd00f7e4062ee4f5e73ca01e52b3beb Mon Sep 17 00:00:00 2001
From: tobias-tengler <45513122+tobias-tengler@users.noreply.github.com>
Date: Tue, 27 Oct 2026 09:28:28 +0000
Subject: [PATCH] Reject selection sets on __typename for abstract parent types

---
 .../Core/src/Validation/ErrorHelper.cs        |  2 +-
 .../Rules/LeafFieldSelectionsRule.cs          | 16 +++++++
 .../LeafFieldSelectionsRuleTests.cs           | 42 +++++++++++++++++++
 3 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/src/HotChocolate/Core/src/Validation/ErrorHelper.cs b/src/HotChocolate/Core/src/Validation/ErrorHelper.cs
index c92e1fe4917..bd44f36e3c6 100644
--- a/src/HotChocolate/Core/src/Validation/ErrorHelper.cs
+++ b/src/HotChocolate/Core/src/Validation/ErrorHelper.cs
@@ -170,7 +170,7 @@ public IError FieldDoesNotExist(FieldNode node, IComplexTypeDefinition outputTyp
 
         public IError LeafFieldsCannotHaveSelections(
             FieldNode node,
-            IComplexTypeDefinition declaringType,
+            ITypeDefinition declaringType,
             IType fieldType)
         {
             return ErrorBuilder.New()
diff --git a/src/HotChocolate/Core/src/Validation/Rules/LeafFieldSelectionsRule.cs b/src/HotChocolate/Core/src/Validation/Rules/LeafFieldSelectionsRule.cs
index 781003f7e18..c2561146543 100644
--- a/src/HotChocolate/Core/src/Validation/Rules/LeafFieldSelectionsRule.cs
+++ b/src/HotChocolate/Core/src/Validation/Rules/LeafFieldSelectionsRule.cs
@@ -102,6 +102,22 @@ private void ValidateField(
     {
         var namedParentType = parentType.NamedType();
 
+        // __typename can be selected on unions and interfaces, which don't have this field,
+        // so we need this special case for it.
+        if (IntrospectionFieldNames.TypeName.Equals(field.Name.Value, StringComparison.Ordinal))
+        {
+            if (field.SelectionSet is not null)
+            {
+                context.ReportError(
+                    context.LeafFieldsCannotHaveSelections(
+                        field,
+                        namedParentType,
+                        new NonNullType(context.Schema.Types["String"])));
+            }
+
+            return;
+        }
+
         if (namedParentType is not IComplexTypeDefinition complex
             || !complex.Fields.TryGetField(field.Name.Value, out var fieldDef))
         {
diff --git a/src/HotChocolate/Core/test/Validation.Tests/LeafFieldSelectionsRuleTests.cs b/src/HotChocolate/Core/test/Validation.Tests/LeafFieldSelectionsRuleTests.cs
index 9b68e76b97c..b7343672adf 100644
--- a/src/HotChocolate/Core/test/Validation.Tests/LeafFieldSelectionsRuleTests.cs
+++ b/src/HotChocolate/Core/test/Validation.Tests/LeafFieldSelectionsRuleTests.cs
@@ -44,6 +44,48 @@ public void Validate_Should_RejectInvalidSelections_WhenEnabled(
         Assert.Equal(expectedMessage, Assert.Single(context.Errors).Message);
     }
 
+    [Theory]
+    [InlineData("{ dog { __typename { name } } }")]
+    [InlineData("{ human { pets { __typename { name } } } }")]
+    [InlineData("{ catOrDog { __typename { name } } }")]
+    [InlineData("{ catOrDog { ...f } } fragment f on CatOrDog { __typename { name } }")]
+    public void Validate_Should_RejectSelectionsOnTypeName_When_ParentIsAnyCompositeType(string sourceText)
+    {
+        // arrange
+        var rule = new LeafFieldSelectionsRule();
+        var document = Utf8GraphQLParser.Parse(sourceText);
+        var context = ValidationUtils.CreateContext(document);
+
+        // act
+        rule.Validate(context, document);
+
+        // assert
+        Assert.Equal(
+            "Field \"__typename\" must not have a selection since type \"String!\" has no subfields.",
+            Assert.Single(context.Errors).Message);
+    }
+
+    [Fact]
+    public void TypeNameOnAbstractTypes()
+    {
+        ExpectValid(
+            """
+            {
+              dog {
+                __typename
+              }
+              human {
+                pets {
+                  __typename
+                }
+              }
+              catOrDog {
+                __typename
+              }
+            }
+            """);
+    }
+
     [Fact]
     public void ScalarSelection()
     {