diff --git a/etc/checkstyle/checkstyle-suppressions.xml b/etc/checkstyle/checkstyle-suppressions.xml
index 003a53d73..8f3d30181 100644
--- a/etc/checkstyle/checkstyle-suppressions.xml
+++ b/etc/checkstyle/checkstyle-suppressions.xml
@@ -5,4 +5,6 @@
 <suppressions>
 	<suppress files=".*" checks="JavadocStyle" />
 	<suppress files="SpringAuthorizationServerVersion\.java" checks="HideUtilityClassConstructor"/>
+	<suppress files="[\\/]src[\\/]test[\\/]" checks="RegexpSinglelineJava" id="toLowerCaseWithoutLocale"/>
+	<suppress files="[\\/]src[\\/]test[\\/]" checks="RegexpSinglelineJava" id="toUpperCaseWithoutLocale"/>
 </suppressions>
diff --git a/etc/checkstyle/checkstyle.xml b/etc/checkstyle/checkstyle.xml
index da8d0119f..fe0fec97c 100644
--- a/etc/checkstyle/checkstyle.xml
+++ b/etc/checkstyle/checkstyle.xml
@@ -15,4 +15,22 @@
 		<property name="excludes" value="io.spring.javaformat.checkstyle.check.SpringHeaderCheck" />
 		<property name="excludes" value="com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocPackageCheck" />
 	</module>
+	<module name="com.puppycrawl.tools.checkstyle.TreeWalker">
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="toLowerCaseWithoutLocale"/>
+			<property name="format" value="\.toLowerCase\(\)"/>
+			<property name="maximum" value="0"/>
+			<property name="message"
+					  value="String.toLowerCase() should be String.toLowerCase(Locale.ROOT) or String.toLowerCase(Locale.ENGLISH)"/>
+			<property name="ignoreComments" value="true"/>
+		</module>
+		<module name="com.puppycrawl.tools.checkstyle.checks.regexp.RegexpSinglelineJavaCheck">
+			<property name="id" value="toUpperCaseWithoutLocale"/>
+			<property name="format" value="\.toUpperCase\(\)"/>
+			<property name="maximum" value="0"/>
+			<property name="message"
+					  value="String.toUpperCase() should be String.toUpperCase(Locale.ROOT) or String.toUpperCase(Locale.ENGLISH)"/>
+			<property name="ignoreComments" value="true"/>
+		</module>
+	</module>
 </module>
diff --git a/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/JdbcOAuth2AuthorizationService.java b/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/JdbcOAuth2AuthorizationService.java
index 316a1b0e2..32f9bfe1f 100644
--- a/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/JdbcOAuth2AuthorizationService.java
+++ b/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/JdbcOAuth2AuthorizationService.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2023 the original author or authors.
+ * Copyright 2020-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,6 +27,7 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import java.util.function.Function;
@@ -416,7 +417,8 @@ private static ColumnMetadata getColumnMetadata(JdbcOperations jdbcOperations, S
 			// But if it is not enclosed in double quotes,
 			// the name is converted to uppercase and this uppercase version is stored in
 			// the database as the case-normal form.
-			rs = databaseMetaData.getColumns(null, null, TABLE_NAME.toUpperCase(), columnName.toUpperCase());
+			rs = databaseMetaData.getColumns(null, null, TABLE_NAME.toUpperCase(Locale.ENGLISH),
+					columnName.toUpperCase(Locale.ENGLISH));
 			if (rs.next()) {
 				return rs.getInt("DATA_TYPE");
 			}
diff --git a/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/web/authentication/OAuth2EndpointUtils.java b/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/web/authentication/OAuth2EndpointUtils.java
index f58f88d6d..390ef3116 100644
--- a/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/web/authentication/OAuth2EndpointUtils.java
+++ b/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/web/authentication/OAuth2EndpointUtils.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2020-2023 the original author or authors.
+ * Copyright 2020-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 
 import java.util.Collections;
 import java.util.HashMap;
+import java.util.Locale;
 import java.util.Map;
 
 import jakarta.servlet.http.HttpServletRequest;
@@ -110,14 +111,14 @@ static void throwError(String errorCode, String parameterName, String errorUri)
 
 	static String normalizeUserCode(String userCode) {
 		Assert.hasText(userCode, "userCode cannot be empty");
-		StringBuilder sb = new StringBuilder(userCode.toUpperCase().replaceAll("[^A-Z\\d]+", ""));
+		StringBuilder sb = new StringBuilder(userCode.toUpperCase(Locale.ENGLISH).replaceAll("[^A-Z\\d]+", ""));
 		Assert.isTrue(sb.length() == 8, "userCode must be exactly 8 alpha/numeric characters");
 		sb.insert(4, '-');
 		return sb.toString();
 	}
 
 	static boolean validateUserCode(String userCode) {
-		return (userCode != null && userCode.toUpperCase().replaceAll("[^A-Z\\d]+", "").length() == 8);
+		return (userCode != null && userCode.toUpperCase(Locale.ENGLISH).replaceAll("[^A-Z\\d]+", "").length() == 8);
 	}
 
 }