diff --git a/src/jdk.jlink/share/classes/jdk/tools/jimage/JImageTask.java b/src/jdk.jlink/share/classes/jdk/tools/jimage/JImageTask.java
index 3f9936e5112..71566933de5 100644
--- a/src/jdk.jlink/share/classes/jdk/tools/jimage/JImageTask.java
+++ b/src/jdk.jlink/share/classes/jdk/tools/jimage/JImageTask.java
@@ -180,7 +180,7 @@ class JImageTask {
             String[] remaining = args;
             try {
                 command = args[0];
-                options.task = Enum.valueOf(Task.class, args[0].toUpperCase(Locale.ENGLISH));
+                options.task = Enum.valueOf(Task.class, args[0].toUpperCase(Locale.ROOT));
                 remaining = args.length > 1 ? Arrays.copyOfRange(args, 1, args.length)
                                             : new String[0];
             } catch (IllegalArgumentException ex) {
@@ -212,7 +212,7 @@ class JImageTask {
                 } else {
                     try {
                         log.println(TASK_HELPER.getMessage("main.usage." +
-                                options.task.toString().toLowerCase()));
+                                options.task.toString().toLowerCase(Locale.ROOT)));
                     } catch (MissingResourceException ex) {
                         throw TASK_HELPER.newBadArgs("err.not.a.task", command);
                     }
diff --git a/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/JlinkTask.java b/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/JlinkTask.java
index a833d64af23..b108ffa52e5 100644
--- a/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/JlinkTask.java
+++ b/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/JlinkTask.java
@@ -810,7 +810,7 @@ public class JlinkTask {
 
                 try (Stream<Archive.Entry> entries = modularJarArchive.entries()) {
                     boolean hasSignatures = entries.anyMatch((entry) -> {
-                        String name = entry.name().toUpperCase(Locale.ENGLISH);
+                        String name = entry.name().toUpperCase(Locale.ROOT);
 
                         return name.startsWith("META-INF/") && name.indexOf('/', 9) == -1 && (
                                 name.endsWith(".SF") ||
diff --git a/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/plugins/VersionPropsPlugin.java b/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/plugins/VersionPropsPlugin.java
index 455cbf71869..c917b954731 100644
--- a/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/plugins/VersionPropsPlugin.java
+++ b/src/jdk.jlink/share/classes/jdk/tools/jlink/internal/plugins/VersionPropsPlugin.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019, 2020, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2019, 2023, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -25,6 +25,7 @@
 
 package jdk.tools.jlink.internal.plugins;
 
+import java.util.Locale;
 import java.util.Map;
 import jdk.internal.classfile.Classfile;
 import jdk.internal.classfile.ClassTransform;
@@ -70,7 +71,7 @@ abstract class VersionPropsPlugin extends AbstractPlugin {
      * @param field The name of the java.lang.VersionProps field to be redefined
      */
     protected VersionPropsPlugin(String field) {
-        this(field, field.toLowerCase().replace('_', '-'));
+        this(field, field.toLowerCase(Locale.ROOT).replace('_', '-'));
     }
 
     @Override
diff --git a/src/jdk.jlink/share/classes/jdk/tools/jmod/JmodTask.java b/src/jdk.jlink/share/classes/jdk/tools/jmod/JmodTask.java
index 334dbf79992..ed1620ec0f0 100644
--- a/src/jdk.jlink/share/classes/jdk/tools/jmod/JmodTask.java
+++ b/src/jdk.jlink/share/classes/jdk/tools/jmod/JmodTask.java
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2015, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2015, 2023, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -1471,7 +1471,7 @@ public class JmodTask {
                 throw new CommandException("err.missing.mode").showUsage(true);
             String verb = words.get(0);
             try {
-                options.mode = Enum.valueOf(Mode.class, verb.toUpperCase());
+                options.mode = Enum.valueOf(Mode.class, verb.toUpperCase(Locale.ROOT));
             } catch (IllegalArgumentException e) {
                 throw new CommandException("err.invalid.mode", verb).showUsage(true);
             }