Poison Block Attacks and Shadow Mining

These may sound like terms from a video game or sci-fi film but really they’re attack types on blockchains. They were mentioned as possible scenarios during the BCH fork back in August 2018, however they can be employed outside of a hardfork situation in order to disrupt network activity and attack other miners.

Poison Block Attack

A poison block attack, or sometimes referred to as a ‘big block attack’, is where a malicious miner will create a very large block which takes smaller miners with less hash power a long time to validate. This effectively wastes their time and delays them in finding their own blocks, thus the malicious miner is able to mine a valid block during this time and hopefully claim the block reward for adding it to the chain.

This can be used against miners or mining pools with lower hash power and therefore makes their mining less profitable since they can mine less blocks in the time. In the non-blockchain world, it’s akin to wasting your competitors time so they can see less customers and therefore they make less sales.

Shadow Mining

When the BCH SV team fell behind by a significant number of blocks the day after the BCH fork, the BCH ABC team began to worry that a shadow mining attack may be on the cards. This is where a miner will secretly mine blocks but not broadcast them to the network. Once they are a number of blocks ahead of the other chain, they broadcast their chain and force a re-org of the blockchain, since, by Nakamoto consensus, the longest chain wins. As such, any transactions which were on the previous chain but not in the new chain will be discounted, double spending can occur and general blockchain chaos can ensue.

This is usually only an attack vector for recently forked chains which have more than one protocol vying to be the main chain, and is a reason why caution is sounded about moving coins around after a split.