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Race condition easily crashes servers using block entities #3717
Description
Server Implementation
Paper
Server Version
26.3
Describe the bug
A race condition with how FastAsyncWorldEdit edits block entities is able to cause the Minecraft server to crash entirely with a ConcurrentModificationException. This can be used to easily crash any server which gives players WorldEdit access.
To Reproduce
- Recreate this exact structure containing observer clocks, pistons, and block entities in a single chunk
-
Select the shulker boxes (or whatever block entities you used) with your wand.
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Set your selection to air. If nothing happens, undo it and try it again. Keep doing it.
Expected behaviour
I expect the server to not crash while messing with block entities
Screenshots / Videos
Error log (if applicable)
---- Minecraft Crash Report ----
// Oops.
Time: 2026-11-02 02:11:22
Description: Exception ticking world
java.util.ConcurrentModificationException
at java.base/java.util.HashMap.compute(HashMap.java:1326)
at net.minecraft.world.level.chunk.LevelChunk.updateBlockEntityTicker(LevelChunk.java:913)
at net.minecraft.world.level.chunk.LevelChunk.addAndRegisterBlockEntity(LevelChunk.java:513)
at net.minecraft.world.level.Level.setBlockEntity(Level.java:1707)
at net.minecraft.world.level.block.piston.PistonBaseBlock.moveBlocks(PistonBaseBlock.java:408)
at net.minecraft.world.level.block.piston.PistonBaseBlock.triggerEvent(PistonBaseBlock.java:163)
at net.minecraft.world.level.block.state.BlockBehaviour$BlockStateBase.triggerEvent(BlockBehaviour.java:912)
at net.minecraft.server.level.ServerLevel.doBlockEvent(ServerLevel.java:2082)
at net.minecraft.server.level.ServerLevel.runBlockEvents(ServerLevel.java:2059)
at net.minecraft.server.level.ServerLevel.tick(ServerLevel.java:869)
at net.minecraft.server.MinecraftServer.tickChildren(MinecraftServer.java:1807)
at net.minecraft.server.MinecraftServer.tickServer(MinecraftServer.java:1604)
at net.minecraft.server.dedicated.DedicatedServer.tickServer(DedicatedServer.java:404)
at net.minecraft.server.MinecraftServer.processPacketsAndTick(MinecraftServer.java:1662)
at net.minecraft.server.MinecraftServer.runServer(MinecraftServer.java:1332)
at net.minecraft.server.MinecraftServer.lambda$spin$0(MinecraftServer.java:299)
at java.base/java.lang.Thread.run(Thread.java:1474)
A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------
-- Head --
Thread: Server thread
Stacktrace:
at java.base/java.util.HashMap.compute(HashMap.java:1326)
at net.minecraft.world.level.chunk.LevelChunk.updateBlockEntityTicker(LevelChunk.java:913)
at net.minecraft.world.level.chunk.LevelChunk.addAndRegisterBlockEntity(LevelChunk.java:513)
at net.minecraft.world.level.Level.setBlockEntity(Level.java:1707)
at net.minecraft.world.level.block.piston.PistonBaseBlock.moveBlocks(PistonBaseBlock.java:408)
at net.minecraft.world.level.block.piston.PistonBaseBlock.triggerEvent(PistonBaseBlock.java:163)
at net.minecraft.world.level.block.state.BlockBehaviour$BlockStateBase.triggerEvent(BlockBehaviour.java:912)
at net.minecraft.server.level.ServerLevel.doBlockEvent(ServerLevel.java:2082)
at net.minecraft.server.level.ServerLevel.runBlockEvents(ServerLevel.java:2059)
Fawe Debugpaste
https://athion.net/ISPaster/paste/view/f71b01dd0a174189ade4a2c4ac876095
Are you interested in contributing a fix?
No
Activity
[-]Easily abusable race condition easily crashes servers using block entities[/-][+]Race condition easily crashes servers using block entities[/+]RedstoneFuture commentedon Nov 10, 2026
Note: Last change about FAWE processing and block entities: #3714
RedstoneFuture commentedon Nov 10, 2026
With the FAWE
v.2.16.3(dev version) the server doesn’t crash for me on a PaperMC 26.2 test-server. Instead, the observer-clocks gradually "freeze" one by one. Which is, of course, better than a crash.