Try a way of determining required rights for the installer

java/net/i2p/router/Elevator.java

@@ -10,6 +10,10 @@ public class Elevator {
     }
 
     public static void executeAsAdministrator(String command, String args) {
+        if (command == "" || command == null) {
+            System.out.println("No command specified");
+            return;
+        }
         Shell32X.SHELLEXECUTEINFO execInfo = new Shell32X.SHELLEXECUTEINFO();
         execInfo.lpFile = new WString(command);
         if (args != null)

java/net/i2p/router/WinUpdateProcess.java

@@ -42,7 +42,10 @@ class WinUpdateProcess implements Runnable {
         var workingDir = workDir();
         var logFile = new File(workingDir, "log-" + version + ".txt");
 
-        ProcessBuilder pb = new ProcessBuilder(file.getAbsolutePath());
+        if (logFile.canWrite()) {
+            // check if we can write to the log file. If we can, use the ProcessBuilder to
+            // run the installer.
+            ProcessBuilder pb = new ProcessBuilder(file.getAbsolutePath(), "/S", "/D=" + workingDir.getAbsolutePath());
             var env = pb.environment();
             env.put("OLD_I2P_VERSION", version);
             env.remove("RESTART_I2P");
@@ -56,6 +59,12 @@ class WinUpdateProcess implements Runnable {
             } catch (IOException ex) {
                 System.out.println("Unable to run update-program in background. Update will fail.");
             }
+        } else {
+            // If we cant write to the log file and we're on Windows, use the elevator to
+            // execute the installer instead of the ProcessBuilder.
+            Elevator.executeAsAdministrator(file.getAbsolutePath(), " /S /D=" + workingDir.getAbsolutePath());
+        }
+
     }
 
     @Override

java/net/i2p/router/WindowsUpdatePostProcessor.java

@@ -43,6 +43,7 @@ public class WindowsUpdatePostProcessor implements UpdatePostProcessor {
     public void updateDownloadedandVerified(UpdateType type, int fileType, String version, File file)
             throws IOException {
         _log.info("Got an update to post-process");
+        if (SystemVersion.isWindows()) {
 
             if (type != UpdateType.ROUTER_SIGNED_SU3 && type != UpdateType.ROUTER_DEV_SU3) {
                 _log.warn("Unsupported update type " + type);
@@ -63,8 +64,9 @@ public class WindowsUpdatePostProcessor implements UpdatePostProcessor {
             }
 
             _log.info("adding shutdown hook");
-        ctx.addFinalShutdownTask(new WinUpdateProcess(ctx, this::getVersion, this::getFile));
 
+            ctx.addFinalShutdownTask(new WinUpdateProcess(ctx, this::getVersion, this::getFile));
+        }
     }
 
     private File moveUpdateInstaller(File file) throws IOException {