Crashing Minecraft with a Broken Clay Pot

There is a specific, nasty crash that haunted Minecraft Java Edition for a few months. It involved decorated pots and something called an identifier. The bug was in the game from version 1.19.4 until it was finally fixed in 1.20.2. It never made it to older servers, but for a while, it was a perfect way to ruin someone's game with a single item.

Here is the core of it. In Minecraft, many things are referenced by identifiers. Think of them like formal addresses. They follow a strict format: a namespace, a colon, and a path, like minecraft:diamond_sword or minecraft:entity/creeper.

The rules for these addresses are strict. The namespace and path can only contain certain characters: lowercase letters, numbers, underscores, dashes, periods, and forward slashes. If the game tries to create an identifier with illegal characters like a space or an exclamation mark, it throws a specific error called an InvalidIdentifierException.

Normally, when the game takes input from a player or a data file to make an identifier, it uses safe methods. These methods, often called tryParse or of, catch that error and just return null instead of crashing. It is a basic safety check.

The bug was in a place where that safety check was missing.

The Broken Render Pipeline

When you hold a decorated pot in your hand, the game needs to draw it. Before it can draw, it loads the pot's data. A pot can have up to four sherds (like minecraft:angler_pottery_sherd). These sherd types are stored in the item's NBT data as a list of identifier strings.

The crash happens in the function that prepares the pot for rendering. It reads the NBT list and, for each string in it, does this:

Registries.ITEM.get(new Identifier(nbtElement.asString()));

See the problem? It uses new Identifier(...) directly. That is the unsafe constructor. There is no tryParse here, no safety net.

If the string loaded from the NBT is "minecraft:angler_pottery_sherd", everything is fine. The constructor succeeds, the game finds the item, and the pot renders.

If the string is "POPBOB REAL", the constructor tries to make an identifier out of it. It immediately sees the spaces and capital letters, throws the InvalidIdentifierException, and because this happens on the main rendering thread with no one to catch it, the whole game client crashes.

The Exploit in Practice

This made decorated pots a potent crash vector. All an attacker needed to do was use a command or a modified client to create a pot item with an invalid sherd string in its NBT and hold the pot. When the item was dropped on the ground, or placed in a chest they opened, or literally rendered in any way, the rendering logic would call that unsafe constructor, and the player's screen would freeze, then close with a crash report.

The fix was exactly what you would expect. Mojang patched it by replacing the dangerous new Identifier() call with a safe method like Identifier.tryParse(). If the string is invalid, it returns null, and the game defaults to using a brick texture for that sherd spot. No crash, just a slightly weird looking pot.

The Bigger Picture

This is more than a quirky bug. It is a textbook example of a failure in input validation and sandboxing. The game's rendering engine, a core system, was exposed to raw, unsanitized data from the game world. In a sandboxed environment, these systems should be isolated. The rendering engine should receive only pre-validated, safe data objects, never raw strings it has to parse itself.

While the sherd crash is now fixed, the principle remains. In Minecraft, and in any complex software, even something that looks harmless can do a lil trolling to the entire software.