This past week Bitcoin Cash (BCH) and Bitcoin SV (BSV) both had their first block halving in history, with their block rewards dropping from 12.5 coins to 6.25 coins. It was expected that the hashrate of Bitcoin Cash (BCH) and Bitcoin SV (BSV) would drop following their respective block halvings, and that is exactly what happened according to data from Coin Dance. What was perhaps unexpected is that the hash rates of Bitcoin Cash (BCH) and Bitcoin SV (BSV) are now dropping so low on a regular basis that they are highly susceptible to 51% attacks.
Before the Bitcoin Cash (BCH) halving happened on April 8, the hash rate was fluctuating between 2.0-5.5 EH/s. Within a day of the halving the hash rate dropped as low as 0.4 EH/s, and now is in a steady daily cycle with the hash rate fluctuating between 0.3 EH/s and 4 EH/s.
Likewise, before the Bitcoin SV (BSV) halving on April 9 the hash rate was fluctuating between 2.0-3.5 EH/s, and since the halving the hash rate is fluctuating between 0.5-2.5 EH/s.
Theoretically, if the Bitcoin Cash (BCH) and Bitcoin SV (BSV) prices doubled when the block halving happened, then the hash rates could have stayed the same. However, in reality Bitcoin Cash (BCH) saw a brief speculative rally from $230 to $275 and has already dropped back below $230, while Bitcoin SV (BSV) saw a rally from $170 to $220 but has already dropped to $180.
Since the price of Bitcoin SV (BSV) and Bitcoin Cash (BCH) before the block halving is roughly the same as it is now, miners have seen a 50% reduction in their revenues. A 50% hit to mining revenue is enough to wipe out the entire profit margin for most mining operations, and therefore it is not surprising that the hash rates for Bitcoin Cash (BCH) and Bitcoin SV (BSV) are declining as low as 0.3-0.5 EH/s on a daily basis.
Based on data from Crypto51, a site which calculates how much money it would take to 51% attack various cryptocurrencies, during the times of the day when the hash rate drops to 0.3-0.5 EH/s it would cost approximately $2,000-$3,000 to rent hash rate from a cloud mining service and 51% attack Bitcoin Cash (BCH) or Bitcoin SV (BSV) for an hour.
This cost is lowered even further by the fact that a 51% attacker would receive half of the mining rewards or even more. Assuming a 51% attacker receives half of the block rewards, they would receive over $4,000 an hour for Bitcoin Cash (BCH) and $3,500 an hour for Bitcoin SV (BSV).
Essentially, a 51% attack on Bitcoin Cash (BCH) or Bitcoin SV (BSV) would practically pay for itself due to the block reward income.
All of this being said, although renting cloud mining hash rate to attack Bitcoin Cash (BCH) or Bitcoin SV (BSV) could be a threat according to the data from Crypto51, it is not the cheapest way of conducting such an attack.
Perhaps the biggest threat is if a medium sized Bitcoin (BTC) mining farm with 1 EH/s of hash rate, which is approximately 1% of the Bitcoin (BTC) hash rate, decides to go rogue and attack Bitcoin Cash (BCH) and/or Bitcoin SV (BSV).
Based on the data from Coin Dance, the hash rate for Bitcoin Cash (BCH) is below 1 EH/s for more than 6 hours every day since the halving, and this pattern is quite predictable, so a rogue Bitcoin (BTC) mining farm with 1 EH/s of hash rate could easily attack.
Some factors which will help to make Bitcoin Cash (BCH) and Bitcoin SV (BSV) safer over the next month are that their mining difficulty will adjust lower at some point, making it more profitable to mine, and therefore their hash rates will rise significantly when the difficulty adjustment happens. Also, when the Bitcoin (BTC) halving comes in the middle of May it could help drive some miners back to Bitcoin Cash (BCH) and Bitcoin SV (BSV).
However, until the difficulty adjustment and/or the Bitcoin (BTC) block halving happens, Bitcoin Cash (BCH) and Bitcoin SV (BSV) are sitting ducks.
Ultimately, it is unacceptable that Bitcoin Cash (BCH) and Bitcoin SV (BSV), which have market caps of $4.2 billion and $3.4 billion respectively, have such low network security, and investors should probably avoid holding these coins until their hash rates rise back to safe levels, since it just takes a single rogue mining farm at this point to conduct a serious attack on either cryptocurrency.