diff --git a/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.Unix.cs b/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.Unix.cs
index e432395e3e922a..fa79e2845b4dad 100644
--- a/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.Unix.cs
+++ b/src/libraries/Common/tests/TestUtilities/System/PlatformDetection.Unix.cs
@@ -18,6 +18,7 @@ public static partial class PlatformDetection
         public static bool IsLinux => RuntimeInformation.IsOSPlatform(OSPlatform.Linux);
         public static bool IsOpenSUSE => IsDistroAndVersion("opensuse");
         public static bool IsUbuntu => IsDistroAndVersion("ubuntu");
+        public static bool IsUbuntu26 => IsDistroAndVersion("ubuntu", 26);
         public static bool IsUbuntu24 => IsDistroAndVersion("ubuntu", 24);
         public static bool IsUbuntu24OrHigher => IsDistroAndVersionOrHigher("ubuntu", 24);
         public static bool IsDebian => IsDistroAndVersion("debian");
diff --git a/src/libraries/System.Net.Security/tests/UnitTests/NegotiateAuthenticationTests.cs b/src/libraries/System.Net.Security/tests/UnitTests/NegotiateAuthenticationTests.cs
index fbb75382a44782..433c3dfe269ae6 100644
--- a/src/libraries/System.Net.Security/tests/UnitTests/NegotiateAuthenticationTests.cs
+++ b/src/libraries/System.Net.Security/tests/UnitTests/NegotiateAuthenticationTests.cs
@@ -17,13 +17,23 @@ namespace System.Net.Security.Tests
 {
     public class NegotiateAuthenticationTests
     {
-        private static bool IsNtlmAvailable => Capability.IsNtlmInstalled() || OperatingSystem.IsAndroid() || OperatingSystem.IsTvOS();
+        // Ubuntu 24 and 26 ship with broekn gss-ntlmssp 1.2
+        private static bool UseManagedNtlm => PlatformDetection.IsUbuntu24 || PlatformDetection.IsUbuntu26;
+        private static bool IsNtlmAvailable => UseManagedNtlm || Capability.IsNtlmInstalled() || OperatingSystem.IsAndroid() || OperatingSystem.IsTvOS();
         private static bool IsNtlmUnavailable => !IsNtlmAvailable;
 
         private static NetworkCredential s_testCredentialRight = new NetworkCredential("rightusername", "rightpassword");
         private static NetworkCredential s_testCredentialWrong = new NetworkCredential("rightusername", "wrongpassword");
         private static readonly byte[] s_Hello = "Hello"u8.ToArray();
 
+        static NegotiateAuthenticationTests()
+        {
+            if (UseManagedNtlm)
+            {
+                AppContext.SetSwitch("System.Net.Security.UseManagedNtlm", true);
+            }
+        }
+
         [Fact]
         public void Constructor_Overloads_Validation()
         {
@@ -85,7 +95,6 @@ public void Package_Supported_NTLM()
         }
 
         [ConditionalFact(nameof(IsNtlmUnavailable))]
-        [ActiveIssue("https://github.com/dotnet/runtime/issues/111639", typeof(PlatformDetection), nameof(PlatformDetection.IsUbuntu24OrHigher))]
         public void Package_Unsupported_NTLM()
         {
             NegotiateAuthenticationClientOptions clientOptions = new NegotiateAuthenticationClientOptions { Package = "NTLM", Credential = s_testCredentialRight, TargetName = "HTTP/foo" };