Paper 2025/1070

Zeus: Defending against Fee Stealing and Griefing Attacks in Multi-Hop Payments

Abstract

Payment Channel Networks (PCNs) are the most scalable and trust-minimized solution to Bitcoin's scalability challenges. Within PCNs, connected payer and payee can make arbitrary off-chain transactions through multi-hop payments (MHPs), while intermediate relays charge relay fees by providing liquidity. However, current MHPs face critical security threats including fee-stealing attacks and griefing attacks. In this paper, we first identify new fee-stealing attacks targeting most existing MHP protocols. Then we demonstrate the impossibility of griefing resistance by reducing the griefing resistance MHP to a fair secret exchange problem. Finally, we introduce Zeus, the first Bitcoin-compatible MHP that is secure against fee-stealing attacks and offers bounded griefing protection against $k$-cost-sensitive adversaries—those who only launch griefing attacks when the damage exceeds a $k$ fraction of their cost. These guarantees are established through rigorous proofs in the Global Universal Composability (GUC) framework. Our comprehensive evaluation demonstrates that Zeus reduces worst-case griefing damage to 28% and 75% compared to MHP schemes such as AMHL~(NDSS'19) and Blitz~(USENIX SEC'21), respectively. Our results further show that, even under the most adverse configurations in the Lightning Network, Zeus ensures attackers incur costs at least 10x higher than potential damage.