Immediately after the Bitcoin Cash hard fork, the blocks on the Bitcoin ABC chain started filling up with nonsensically small transactions. At the time of writing shortly after the fork, three such blocks had been mined with a total of more than 50,000 transactions worth less than 0.5 BCH total. As seen in the video below, the attack was ongoing around lunchtime Chicago time on November 15.
The method of attack seemed to be repeatedly sending transactions between the same addresses. This address, for instance, received 500 of the earliest spam transactions. It then appeared to send transactions to itself.
Given the large block size limit of Bitcoin Cash in general, it would take a lot more spam transactions than this to actually have a truly negative impact on the network. Collectively, the three blocks above did not surpass a size of 10MB. Over an extended period of time, say 2 to 3 weeks or more, however, this spam attack could have a negative impact in that it would fill up the hard drives of Bitcoin ABC full nodes, or if the breadth of it were increased it could potentially impact the cost of BCH transactions as well as the availability of block space. As a rule, however, Bitcoin ABC is not opposed to future block size increases.
In other chains with smaller block sizes, such attacks can have a serious impact on the network. In Bitcoin, an ongoing spam attack gets expensive quickly, due to the higher cost of smaller units of coin and the way transaction fees work. The closer to full a block becomes, the more expensive a transaction on it becomes for guaranteed inclusion. A sustained effort for several months in 2015 and again in 2016 resulted in higher fees, but ultimately all transactions were eventually processed, some hanging for weeks on end.
At this time there are no suspects in the spam attack, which technically began before the fork but increased in intensity following it. Bitcoin ABC mined more blocks in the interim, so it provides more data regarding transactions. It is also apparent that there are no dedicated Bitcoin Cash SV block explorers just yet (aside from CoinDance, which shows SV processing a number of large blocks but does not provide data on the individual transactions), making it difficult to determine whether this attack is affecting both chains or just ABC.
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