diff --git a/src/NetSparkle/Downloaders/WebRequestAppCastDataDownloader.cs b/src/NetSparkle/Downloaders/WebRequestAppCastDataDownloader.cs
index 250836d1..c1c6c8cf 100644
--- a/src/NetSparkle/Downloaders/WebRequestAppCastDataDownloader.cs
+++ b/src/NetSparkle/Downloaders/WebRequestAppCastDataDownloader.cs
@@ -79,8 +79,12 @@ public async Task<string> DownloadAndGetAppCastDataAsync(string url)
         {
             _appcastUrl = url;
             // configure ssl cert link
+            // .NET 9 -> ServicePointManager is deprecated
+            // we have the lambda below for trusting all connections anyway, so this
+            // is sort of redundant at the moment.
+#if NETSTANDARD || NET6 || NET7 || NET8 
             ServicePointManager.ServerCertificateValidationCallback += ValidateRemoteCertificate;
-
+#endif
             var handler = new HttpClientHandler();
             if (RedirectHandler != null)
             {
@@ -111,7 +115,10 @@ public async Task<string> DownloadAndGetAppCastDataAsync(string url)
                     if (response.IsSuccessStatusCode)
                     {
                         Stream responseStream = await response.Content.ReadAsStreamAsync().ConfigureAwait(false);
+                        // .NET 9 -> ServicePointManager is deprecated
+#if NETSTANDARD || NET6 || NET7 || NET8
                         ServicePointManager.ServerCertificateValidationCallback -= ValidateRemoteCertificate;
+#endif
                         using (StreamReader reader = new StreamReader(responseStream, GetAppCastEncoding()))
                         {
                             return await reader.ReadToEndAsync().ConfigureAwait(false);
@@ -143,8 +150,10 @@ public async Task<string> DownloadAndGetAppCastDataAsync(string url)
                     }
                     else
                     {
-                        Stream responseStream = await httpClient.GetStreamAsync(url).ConfigureAwait(false);
+                        Stream responseStream = await httpClient.GetStreamAsync(url).ConfigureAwait(false);// .NET 9 -> ServicePointManager is deprecated
+#if NETSTANDARD || NET6 || NET7 || NET8
                         ServicePointManager.ServerCertificateValidationCallback -= ValidateRemoteCertificate;
+#endif
                         using (StreamReader reader = new StreamReader(responseStream, GetAppCastEncoding()))
                         {
                             return await reader.ReadToEndAsync().ConfigureAwait(false);
diff --git a/src/NetSparkle/SparkleUpdater.cs b/src/NetSparkle/SparkleUpdater.cs
index dda36c66..b94c08e1 100644
--- a/src/NetSparkle/SparkleUpdater.cs
+++ b/src/NetSparkle/SparkleUpdater.cs
@@ -150,11 +150,19 @@ public SparkleUpdater(string appcastUrl, ISignatureVerifier signatureVerifier, s
 
         #region Properties
 
+#if NETSTANDARD || NET6 || NET7 || NET8
         /// <summary>
         /// The security protocol used by NetSparkle. Setting this property will also set this 
-        /// for the current AppDomain of the caller. Needs to be set to 
-        /// SecurityProtocolType.Tls12 for some cases (such as when downloading from GitHub).
-        /// </summary>
+        /// for the current AppDomain of the caller. May need to be set to 
+        /// SecurityProtocolType.Tls12 for some cases (such as when downloading from GitHub);
+        /// however, if your app is using a more recent version of .NET, you should be OK.
+        /// See also: https://stackoverflow.com/a/59398678/3938401
+        /// .NET 9 -> ServicePointManager is deprecated
+        /// </summary>
+        [Obsolete("Deprecated in .NET 9; this property may be removed at any time " + 
+            "(including minor/patch updates to this library); override pertinent " + 
+            "functions (e.g. to HttpClient) in WebRequestAppCastDataDownloader or " + 
+            "similar to make equivalent changes to your application")]
         public SecurityProtocolType SecurityProtocolType
         {
             get
@@ -166,6 +174,7 @@ public SecurityProtocolType SecurityProtocolType
                 ServicePointManager.SecurityProtocol = value;
             }
         }
+#endif
 
         /// <summary>
         /// Set the user interaction mode for Sparkle to use when there is a valid update for the software