Congesting Ethereum after EIP-1559

We provide two novel block congestion attacks on Ethereum that are applicable even in the presence of the EIP-1559 base fee mechanism, which aimed to make such attacks impossible or highly costly. Unlike traditional block congestion methods, our approaches allow the attacker to avoid paying large transaction fees in case the attack is unsuccessful. Moreover, our second attack avoids an explosion in the block base fee and can thus be used for prolonged congestion of an interval of blocks. Finally, we provide real-world examples of contracts currently deployed on the Ethereum blockchain which are vulnerable to such attacks. Thus, block congestion is both possible and profitable, even after EIP-1559^*.^*A longer version of this article, including a list of vulnerable contracts, is available at [1]. The research was partially supported by the Hong Kong Research Grants Council ECS Project 26208122.