In 2015, Bitcoin suffered a garbage transaction attack, which had a far-reaching impact on the development of the network.

The Bitcoin network suffered a series of spam transaction attacks in the summer of 2015, and these events continue to have a profound impact on the Bitcoin community. At that time, the debate over block size was at a fever pitch, and attackers tried to prove that the 1MB block limit was too small and could easily be filled with spam transactions.

The attack is divided into four rounds. The first round was announced on June 20, with a plan to generate 1MB of transaction data every 5 minutes, aiming to create a backlog of 241MB of transactions. However, due to technical issues, this attack did not proceed as planned.

The second wave of attacks began on June 29, causing a certain impact on the network. Some users reported that transaction confirmation times were significantly extended, but some mining pools successfully filtered out junk transactions. This has sparked a debate about whether transaction filtering harms the interchangeability of Bitcoin.

The third round of attacks occurred on July 7, with a scale and impact exceeding the previous two rounds. The attackers employed various strategies to generate a large number of garbage transactions, including sending dust transactions to public wallets and utilizing known private key addresses. It was reported that this attack cost over $8,000 in fees. Some mining pools cleaned up these garbage outputs by creating large consolidated transactions.

The last round of attacks occurred in September, where the attackers changed their strategy and directly posted private keys with balances on the forum. This resulted in over 90,000 transactions, but due to many being conflicting transactions, the actual impact was not as severe as the third round.

These attacks had a significant impact on the Bitcoin network. They not only prompted some technical changes, such as raising the minimum relay fees and introducing memory pool limits, but also exacerbated the community's divisions over the block size issue. Although supporters of smaller blocks eventually gained the upper hand, the debate over how to define and handle spam transactions continues to this day.

This series of events indicates that even in the early stages, the Bitcoin network had already faced similar challenges. In contrast, some transactions on the current network, while regarded as "garbage", may not have intentions as clear or malicious as the attacks in 2015. Another significant difference is that the transaction fees for such transactions recently have reached hundreds of millions of dollars, far exceeding the approximately $10,000 level of 2015.