A Review of the Spam Transaction Attack on the Bitcoin Network in 2015

In the summer of 2015, the Bitcoin network experienced a series of spam transaction attacks. These attacks became part of the early block size debate, where attackers attempted to prove the need to increase the block size limit.

Supporters of large blocks believe that the 1MB limit is too small and that blocks can easily be filled with junk transactions. They hope to increase the limit to make it more costly to fill up blocks. On the other hand, supporters of small blocks argue that allowing junk transactions to be quickly added to the blockchain would actually encourage attackers.

Attack Process

First Round

On June 20, 2015, a wallet provider claiming to be based in London announced that it would conduct a "stress test." They planned to generate 1MB of transaction data every 5 minutes, aiming to create a backlog of 241MB of transactions.

On June 24, the attacker stated that the attack did not succeed as expected because the server crashed after the mempool reached 12MB. This failed attack cost about 434 euros in fees.

Round Two

On June 24, the attackers announced that the second round of attacks would take place on June 29. This attack appears to be more effective, with some users reporting that their transactions have been unconfirmed for a long time.

A certain mining pool successfully filtered out junk transactions, but this has sparked controversy over interchangeability. Some believe that filtering out valid transactions sets a bad precedent.

Round Three

On July 7th, the third round of attacks occurred. This was the most intense one, causing considerable chaos. It was reported that over $8000 was spent on transaction fees.

Attackers adopted diversified strategies, such as sending dust transactions to public wallets and sending small amounts of Bitcoin to known private key addresses. This resulted in a large number of junk transactions.

A certain mining pool helped alleviate the situation by creating large integrated transactions to clear these junk outputs.

Round Four

In September, attackers conducted the final round of "stress tests". This time they released thousands of private keys with balances, resulting in over 90,000 transactions.

Due to many conflicting transactions, the "first-seen-safe" principle can be used to discard them, so the impact is not as severe as in the third round.

Impact and Results

These attacks have had a significant impact on Bitcoin:

• Miners have increased the block size limit strategy to 1MB
• The minimum relay fee has increased by 5 times.
• Bitcoin Core has introduced a memory pool limit.
• Intensified the divisions in the debate over block size limits

Overall, these attacks became significant events in the history of Bitcoin, influencing people's perceptions of junk transactions and driving a series of technological changes. Although it only cost about $10,000 at the time, the impact was far greater than some "junk" transactions today that cost hundreds of millions.