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Minecraft exploits and how to fix them

Preface

This guide will assume you are running Purpur. Some settings required for this guide to work are not available in Paper or Spigot. Similar exploits might exist in vanilla, fabric or forge servers, and you will have to find mods that fix them on those loaders.

The most important thing

Keep up to date on your server software. There are exploits that are fixed in newer versions of the software, even if it's not announced. If you are running a server, you should be running the latest version possible to avoid critical vulnerabilities.

Exploits

Armor stand lag machines

Armor stands can be used to create lag machines. This is done by placing huge amounts of armor stands and forcing them to move via water, pistons or other means. This can be fixed by disabling armor stand tick and collision lookups. This will cause armor stands to not being able to be pushed or pulled (even by gravity).

paper-world-defaults.yml

entities:
  armor-stands:
    do-collision-entity-lookups: false
    tick: false

Book exploits

Books are a common target for exploitation techniques, and have been used for all sorts of nasty things in the past including duplication exploits, crash exploits, and forcing servers to run out of memory while producing massive amounts of chunk data.

Option 1

Moderately reducing page-max can mitigate most book exploits without affecting vanilla clients.

paper-global.yml

item-validation:
  book-size:
    page-max: 1024

Option 2

Disabling books completely prevents the most book exploits (as long as plugins don't create non-standard books). Pufferfish config option prevents players from writing to the book, therefore ruling out anything that requires player input to work.

pufferfish.yml

enable-books: false

Collision lag machines

This exploit is similar to the armor stand one, but instead of armor stands, it uses entities that can be pushed by other entities. This can be fixed by setting a smaller limit on how many entities can collide with a singular entity. You can set the max-entity-collisions to 2 to still have relatively natural behavior, or set it to 0 to completely disable collisions. The option to fix climbing bypassing cramming rule is also available, but it's disabled by default.

paper-world-defaults.yml

collisions:
  max-entity-collisions: 2
  fix-climbing-bypassing-cramming-rule: true

Command suggestion packet spam

Some commands on the server might have a lot of logic involved with their command suggestions. This can be exploited by sending a lot of packets that request the server to send suggestions for the command. This can be mitigated by setting packet limit for command suggestions that is just slightly above fast rate of typing commands, so normal players won't be affected.

paper-global.yml

packet-limiter:
  overrides:
    ServerboundCommandSuggestionPacket:
      action: DROP
      interval: 1.0
      max-packet-rate: 15.0

Command spam

While even spigot will protect you from this exploit, there's a slight oversight that will enable a single command to be usable to perform this one. To fix this, simply remove /skill command from the spam exclusions list in spigot.yml.

spigot.yml

commands:
  spam-exclusions: []

Join spam

Sometimes shear quantity of players joining the server can cause the server to lag out. This is especially true for bot attacks and moments after server restart. This can be fixed by setting max joins per tick. Players joining will be delayed in time so server can properly tick between handling them. If joins still overwhelm the server, you can also enable max-joins-per-second in purpur.yml that will make it so value from paper config will be applied per second instead of per tick.

paper-global.yml

misc:
  max-joins-per-tick: 3

purpur.yml

settings:
  network:
    max-joins-per-second: true

Neighbor update lag machines

Limiting the amount of consecutive neighbor updates before skipping additional ones. Moderately reducing this value has no game impact and helps alleviate various 'neighbor update lag machines'.

server.properties

max-chained-neighbor-updates=10000

Projectile suspension

Projectiles can be suspended in bubble columns indefinitely. They can also be transported into unloaded chunks in mass. If anyone loads the chunk with amassed projectiles, server can crash due to loading too many things at once. This can be stopped by limiting how much time projectiles can exist in the world and how many of them are saved and loaded with a chunk. If the timeout is applied to ender pearls, it will prevent usage of ender pearl suspension contraption.

paper-world-defaults.yml

chunks:
  entity-per-chunk-save-limit:
    arrow: 8
    ender_pearl: 8
    experience_orb: 8
    fireball: 8
    small_fireball: 8
    snowball: 8

pufferfish.yml

entity_timeouts:
  ARROW: 200
  EGG: 200
  ENDER_PEARL: 200
  SNOWBALL: 200

Recipe book spam

Malicious players can use auto clicker or a mod to switch between recipe book recipes extremely quickly. This is pretty expensive operation for the server and can cause huge slowdowns. This is actually fixed in default paper config, but older configs might not have it added, so make sure it's there.

paper-global.yml

packet-limiter:
  overrides:
    ServerboundPlaceRecipePacket:
      action: DROP
      interval: 4.0
      max-packet-rate: 5.0

Treasure search

When new treasure map is generated, usually via cartographer villager or opening a chest with treasure map in it, the server searches for the treasure that map should lead to. This search is done in a way that in most cases causes a lot of chunks loading and possibly generating them. This search can halt the server for long enough that watchdog process kills it. It can also be triggered by feeding dolphins fish.

Option 1

To fix this one you basically have to disable treasure maps and dolphins searching for treasure. This is the recommended and 100% effective solution.

paper-world-defaults.yml

environment:
  treasure-maps:
    enabled: false

purpur.yml

world-settings:
  default:
    mobs:
      dolphin:
        disable-treasure-searching: true

Option 2

Alternatively, you can keep the maps enabled, but set up vanilla world border and make sure your world is pregenerated within the border. Then you can just make sure that already discovered treasures are valid as the search result. This will prevent the treasure search from loading chunks that are not generated.

paper-world-defaults.yml

  treasure-maps:
    find-already-discovered:
      loot-tables: true
      villager-trade: true