Adjust issuer validation to accept differing paths

src/client/Microsoft.Identity.Client/Instance/Oidc/OidcRetrieverWithCache.cs

@@ -87,13 +87,17 @@ private static void ValidateIssuer(Uri authority, string issuer)
             string normalizedAuthority = authority.AbsoluteUri.TrimEnd('/');
             string normalizedIssuer = issuer?.TrimEnd('/');
 
-            // Primary validation: check if normalized authority starts with normalized issuer (case-insensitive comparison)
-            if (normalizedAuthority.StartsWith(normalizedIssuer, StringComparison.OrdinalIgnoreCase))
+            // OIDC validation: if the issuer's scheme and host match the authority's, consider it valid
+            if (!string.IsNullOrEmpty(issuer) && Uri.TryCreate(issuer, UriKind.Absolute, out Uri issuerUri))
             {
-                return;
+                if (string.Equals(authority.Scheme, issuerUri.Scheme, StringComparison.OrdinalIgnoreCase) &&
+                    string.Equals(authority.Host, issuerUri.Host, StringComparison.OrdinalIgnoreCase))
+                {
+                    return;
+                }
             }
 
-            // Extract tenant for CIAM scenarios. In a CIAM scenario the issuer is expected to have "{tenant}.ciamlogin.com"
+            // CIAM-specific validation: In a CIAM scenario the issuer is expected to have "{tenant}.ciamlogin.com"
             // as the host, even when using a custom domain.
             string tenant = null;
             try

tests/Microsoft.Identity.Test.Unit/CoreTests/InstanceTests/GenericAuthorityTests.cs

@@ -333,6 +333,56 @@ public async Task BadOidcResponse_ThrowsException_Async(string badOidcResponseTy
             }
         }
 
+        [TestMethod]
+        public async Task OidcIssuerValidation_AcceptsDifferentPath_Async()
+        {
+            using (var httpManager = new MockHttpManager())
+            {
+                // This test was made to cover an issue that realistically would only happen with Microsoft authorities in multi-tenant scenarios,
+                // so it uses a Microsoft host instead of the custom domain used in other tests.
+                string microsoftHost = "login.microsoftonline.com";
+                string authority = $"https://{microsoftHost}/organizations/2.0/";
+                string issuerWithDifferentPath = $"https://{microsoftHost}/someTenant/2.0/";
+
+                IConfidentialClientApplication app = ConfidentialClientApplicationBuilder
+                    .Create(TestConstants.ClientId)
+                    .WithHttpManager(httpManager)
+                    .WithOidcAuthority(authority)
+                    .WithClientSecret(TestConstants.ClientSecret)
+                    .Build();
+
+                // Create OIDC document with a Microsoft host and an issuer that has matching host but different path
+                string oidcDocumentWithDifferentPath = TestConstants.GenericOidcResponse.Replace(
+                        $"\"issuer\":\"{TestConstants.GenericAuthority}\"",
+                        $"\"issuer\":\"{issuerWithDifferentPath}\"");
+                oidcDocumentWithDifferentPath = oidcDocumentWithDifferentPath.Replace(
+                        "demo.duendesoftware.com",
+                        microsoftHost);
+
+                // Mock OIDC endpoint response
+                httpManager.AddMockHandler(new MockHttpMessageHandler
+                {
+                    ExpectedMethod = HttpMethod.Get,
+                    ExpectedUrl = $"{authority}{Constants.WellKnownOpenIdConfigurationPath}",
+                    ResponseMessage = MockHelpers.CreateSuccessResponseMessage(oidcDocumentWithDifferentPath)
+                });
+
+                // Mock token endpoint response
+                httpManager.AddMockHandler(
+                    CreateTokenResponseHttpHandler(
+                        $"https://{microsoftHost}/connect/token",
+                        scopesInRequest: "api",
+                        scopesInResponse: "api", 
+                        grant: "client_credentials"));
+
+                // Should not throw an exception with our updated validation
+                var result = await app.AcquireTokenForClient(new[] { "api" }).ExecuteAsync().ConfigureAwait(false);
+
+                Assert.IsNotNull(result);
+                Assert.IsNotNull(result.AccessToken);
+            }
+        }
+
         [TestMethod]
         public async Task OidcIssuerValidation_ThrowsForNonMatchingIssuer_Async()
         {