The worst problem with the Java sandbox was not the bytecode (though it did have its share of bytecode validation bugs). The worst problem was the vast API surface with its "blacklist" security model: all methods which should not be called from the sandbox have to explicitly call a AccessController.checkPermission() or similar. Miss one, and you have a hole in the sandbox. WASM instead follows a "whitelist" approach, which is much more robust.

How is Java bytecode fundamentally insecure? (Example?)