Blockchain Historical Review: The 2015 Bitcoin Network Garbage Transaction Attack

Recently, there has been a proposal to remove the limit on the size of OP_Return outputs in the Bitcoin Core software, which has sparked a new round of discussions about spam transactions on the Bitcoin blockchain. This article reviews a series of spam transaction attacks that the Bitcoin network suffered during the summer of 2015, compares the situation then and now, and summarizes the relevant lessons learned.

The attack in 2015 occurred against the backdrop of the block size debate. Supporters of large blocks argue that the 1MB limit is too small and can easily be filled with junk transactions, and that the block size limit should be increased. Supporters of small blocks argue that increasing the block size would lower fees, making junk transactions cheaper.

Between June and September 2015, an exchange called CoinWallet.eu launched four rounds of "stress tests" aimed at filling blocks with a large number of spam transactions to prove that it was necessary to increase the block size limit.

The first round of attacks failed due to technical reasons. The second round of attacks had some impact, with some users complaining about delayed transaction confirmations. The third round was the most intense, costing about $8,000 in fees and using various strategies to generate spam transactions. The fourth round involved giving away Bitcoin by disclosing private keys, resulting in a large number of transactions.

These attacks had a certain impact on the network. The number of unconfirmed transactions in the memory pool once reached 80,000, transaction fees rose, and confirmation times were extended. Some mining pools responded by filtering out spam transactions or merging outputs.

These attacks have prompted the Bitcoin network to make some adjustments, such as miners increasing block size limits, raising minimum relay fees, and introducing limits on memory pool size. It has also intensified the divide between supporters of larger blocks.

Overall, the attacks in 2015 indicated that spending around $10,000 could significantly impact the Bitcoin network. In contrast, some recent controversial transactions have cost hundreds of millions of dollars. This reminds us that defining and addressing spam transactions remains a topic that needs ongoing discussion.