PaperMC/paper-server/nms-patches/net/minecraft/world/item/ItemLeash.patch

--- a/net/minecraft/world/item/ItemLeash.java
+++ b/net/minecraft/world/item/ItemLeash.java
@@ -13,6 +13,8 @@
 import net.minecraft.world.level.block.state.IBlockData;
 import net.minecraft.world.phys.AxisAlignedBB;
 
+import org.bukkit.event.hanging.HangingPlaceEvent; // CraftBukkit
+
 public class ItemLeash extends Item {
 
     public ItemLeash(Item.Info item_info) {
@@ -54,9 +56,25 @@
             if (entityinsentient.getLeashHolder() == entityhuman) {
                 if (entityleash == null) {
                     entityleash = EntityLeash.getOrCreateKnot(world, blockposition);
+
+                    // CraftBukkit start - fire HangingPlaceEvent
+                    HangingPlaceEvent event = new HangingPlaceEvent((org.bukkit.entity.Hanging) entityleash.getBukkitEntity(), entityhuman != null ? (org.bukkit.entity.Player) entityhuman.getBukkitEntity() : null, world.getWorld().getBlockAt(i, j, k), org.bukkit.block.BlockFace.SELF);
+                    world.getCraftServer().getPluginManager().callEvent(event);
+
+                    if (event.isCancelled()) {
+                        entityleash.discard();
+                        return EnumInteractionResult.PASS;
+                    }
+                    // CraftBukkit end
                     entityleash.playPlacementSound();
                 }
 
+                // CraftBukkit start
+                if (org.bukkit.craftbukkit.event.CraftEventFactory.callPlayerLeashEntityEvent(entityinsentient, entityleash, entityhuman).isCancelled()) {
+                    continue;
+                }
+                // CraftBukkit end
+
                 entityinsentient.setLeashedTo(entityleash, true);
                 flag = true;
             }