use ConcurrentHashMap

worldedit-bukkit/src/main/java/com/sk89q/worldedit/bukkit/BukkitPermissionAttachmentManager.java

@@ -1,15 +1,15 @@
 package com.sk89q.worldedit.bukkit;
 
 import javax.annotation.Nullable;
-import java.util.HashMap;
 import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
 import org.bukkit.entity.Player;
 import org.bukkit.permissions.PermissionAttachment;
 
 public class BukkitPermissionAttachmentManager {
 
     private final WorldEditPlugin plugin;
-    private final Map<Player, PermissionAttachment> attachments = new HashMap<>();
+    private final Map<Player, PermissionAttachment> attachments = new ConcurrentHashMap<>();
 
     public BukkitPermissionAttachmentManager(WorldEditPlugin plugin) {
         this.plugin = plugin;