The attacks leverage unprotected resources for a denial-of-service by filling the disk and exhausting the CPU with unnecessary header and block data. This forces the node to halt operation. The attack difficulty ranges from difficult to easy. There are currently limited guards for some of the attacks that require checkpoints to be enabled. This paper describes a solution that does not require enabling or maintaining checkpoints and provides improved security.
The complete paper: https://bcoin.io/papers/bitcoin-chain-expansion.pdf
Further discussion: https://www.mail-archive.com/bitcoin-dev@lists.linuxfoundation.org/msg08391.html
The branch: https://github.com/bcoin-org/bcoin/tree/chain-expansion