fix: adds explicit error message for invalid json pointers

src/Microsoft.OpenApi/Reader/JsonNodeHelper.cs

@@ -167,6 +167,18 @@ public static Dictionary<string, HashSet<T>> CreateArrayMap<T>(this JsonNode? no
             return jsonObject.TryGetPropertyValue("$ref", out var refNode) ? refNode?.GetScalarValue() : null;
         }
 
+        public static void ValidateReferencePointerFormat(string pointer)
+        {
+            var hashIndex = pointer.IndexOf('#');
+            if (hashIndex < 0) return;
+            var slashIndex = pointer.IndexOf('/', hashIndex);
+            if (slashIndex >= 0 &&
+                slashIndex != hashIndex + 1)
+            {
+                throw new OpenApiException(string.Format(SRResource.ReferenceHasInvalidFormat, pointer));
+            }
+        }
+
         /// <summary>
         /// Returns the value of $dynamicRef if $ref is absent. Used to create a schema reference
         /// for bare $dynamicRef schemas (no $ref) so they participate in reference resolution.

src/Microsoft.OpenApi/Reader/V2/OpenApiV2Deserializer.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license.
 
 using System;
@@ -88,7 +88,7 @@ private static IOpenApiExtension LoadExtension(string name, JsonNode node, Parsi
         private static (string, string?) GetReferenceIdAndExternalResource(string pointer)
         {
             var refSegments = pointer.Split('/');
-            var refId = refSegments[refSegments.Count() -1];
+            var refId = refSegments[refSegments.Length - 1];
             var isExternalResource = !refSegments[0].StartsWith("#", StringComparison.OrdinalIgnoreCase);
 
             string? externalResource = isExternalResource ? $"{refSegments[0]}/{refSegments[1].TrimEnd('#')}" : null;

src/Microsoft.OpenApi/Reader/V3/OpenApiV3Deserializer.cs

@@ -152,8 +152,10 @@ private static IOpenApiExtension LoadExtension(string name, JsonNode node, Parsi
 
         private static (string, string?) GetReferenceIdAndExternalResource(string pointer)
         {
+            JsonNodeHelper.ValidateReferencePointerFormat(pointer);
+
             var refSegments = pointer.Split('/');
-            var refId = refSegments[refSegments.Count() -1];
+            var refId = refSegments[refSegments.Length - 1];
             var isExternalResource = !refSegments[0].StartsWith("#", StringComparison.OrdinalIgnoreCase);
 
             string? externalResource = null;

src/Microsoft.OpenApi/Reader/V31/OpenApiV31Deserializer.cs

@@ -1,4 +1,4 @@
-// Copyright (c) Microsoft Corporation. All rights reserved.
+// Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT license. 
 
 using System;
@@ -152,14 +152,16 @@ private static IOpenApiExtension LoadExtension(string name, JsonNode node, Parsi
 
         private static (string, string?) GetReferenceIdAndExternalResource(string pointer)
         {
+            JsonNodeHelper.ValidateReferencePointerFormat(pointer);
+
             /* Check whether the reference pointer is a URL
              * (id keyword allows you to supply a URL for the schema as a target for referencing)
              * E.g. $ref: 'https://example.com/schemas/resource.json' 
              * or its a normal json pointer fragment syntax
              * E.g. $ref: '#/components/schemas/pet'
              */
             var refSegments = pointer.Split('/');
-            string refId = !pointer.Contains('#') ? pointer : refSegments[refSegments.Count()-1];
+            string refId = !pointer.Contains('#') ? pointer : refSegments[refSegments.Length - 1];
 
             var isExternalResource = !refSegments[0].StartsWith("#", StringComparison.OrdinalIgnoreCase);
             string? externalResource = null;

test/Microsoft.OpenApi.Readers.Tests/V31Tests/OpenApiDocumentTests.cs

@@ -16,6 +16,40 @@ public class OpenApiDocumentTests
     {
         private const string SampleFolderPath = "V31Tests/Samples/OpenApiDocument/";
 
+        [Fact]
+        public void ParseDocumentWithMalformedReferenceShouldYieldExpectedDiagnostic()
+        {
+            var result = OpenApiDocument.Parse(
+                """
+                openapi: 3.1.1
+                info:
+                  title: test
+                  version: 1.0.0
+                paths:
+                  /test:
+                    get:
+                      responses:
+                        '200':
+                          description: successful operation
+                          content:
+                            application/json:
+                              schema:
+                                $ref: '#components/schemas/Item'
+                components:
+                  schemas:
+                    Item:
+                      type: object
+                      properties:
+                        id:
+                          type: integer
+                """,
+                OpenApiConstants.Yaml,
+                SettingsFixture.ReaderSettings);
+
+            var error = Assert.Single(result.Diagnostic.Errors);
+            Assert.Equal("The reference string '#components/schemas/Item' has invalid format.", error.Message);
+        }
+
         [Fact]
         public async Task ParseDocumentWithWebhooksShouldSucceed()
         {

test/Microsoft.OpenApi.Readers.Tests/V3Tests/OpenApiDocumentTests.cs

@@ -153,6 +153,40 @@ public async Task ParseBrokenMinimalDocumentShouldYieldExpectedDiagnostic()
                 }, result.Diagnostic);
         }
 
+        [Fact]
+        public void ParseDocumentWithMalformedReferenceShouldYieldExpectedDiagnostic()
+        {
+            var result = OpenApiDocument.Parse(
+                """
+                openapi: 3.0.3
+                info:
+                  title: test
+                  version: 1.0.0
+                paths:
+                  /test:
+                    get:
+                      responses:
+                        '200':
+                          description: successful operation
+                          content:
+                            application/json:
+                              schema:
+                                $ref: '#components/schemas/Item'
+                components:
+                  schemas:
+                    Item:
+                      type: object
+                      properties:
+                        id:
+                          type: integer
+                """,
+                OpenApiConstants.Yaml,
+                SettingsFixture.ReaderSettings);
+
+            var error = Assert.Single(result.Diagnostic.Errors);
+            Assert.Equal("The reference string '#components/schemas/Item' has invalid format.", error.Message);
+        }
+
         [Fact]
         public async Task ParseMinimalDocumentShouldSucceed()
         {