Lioba Heimbach

2025

1. Sec 25

   Deanonymizing Ethereum Validators: The P2P Network Has a Privacy Issue

   Lioba Heimbach, Yann Vonlanthen, Juan Villacis, and 2 more authors

   In 34th USENIX Security Symposium, Seattle, WA, USA, Aug 2025

   Awarded a bug bounty by the Ethereum Foundation.

   Non-archival: TUM Blockchain Conference (TBC), Munich, Germany, September 2024

   Non-archival: Ethereumzuri.ch Conference, Zuich, Switzerland, January 2025

   Abstract Bibtex PDF Blog

   Many blockchain networks aim to preserve the anonymity of validators in the peer-to-peer (P2P) network, ensuring that no adversary can link a validator’s identifier to the IP address of a peer due to associated privacy and security concerns.
   This work demonstrates that the Ethereum P2P network does not offer this anonymity. We present a methodology that enables any node in the network to identify validators hosted on connected peers and empirically verify the feasibility of our proposed method. Using data collected from four nodes over three days, we located more than 15% of Ethereum validators in the P2P network. The insights gained from our deanonymization technique provide valuable infoarmation on the distribution of validators across peers, their geographic locations, and hosting organizations. We further discuss the implications and risks associated with the lack of anonymity in the P2P network and propose methods to help validators protect their privacy.
   The Ethereum Foundation awarded us a bug bounty, acknowledging the impact of our results.

   @inproceedings{heimbach2025deanonymizing,
     author = {Heimbach, Lioba and Vonlanthen, Yann and Villacis, Juan and Kiffer, Lucianna and Wattenhofer, Roger},
     title = {Deanonymizing Ethereum Validators: The P2P Network Has a Privacy Issue},
     booktitle = {34th USENIX Security Symposium, Seattle, WA, USA},
     year = {2025},
     month = aug,
   }

2024

1. S&P 24

   Non-Atomic Arbitrage in Decentralized Finance

   Lioba Heimbach, Vabuk Pahari, and Eric Schertenleib

   In IEEE Symposium on Security and Privacy (SP), San Francisco, USA, May 2024

   Non-archival: 4th Workshop on Decentralized Finance (DeFi), March 2024

   Abstract Bibtex External PDF Code Slides Talk

   The prevalence of maximal extractable value (MEV) in the Ethereum ecosystem has led to a characterization of the latter as a dark forest. Studies of MEV have thus far largely been restricted to purely on-chain MEV, i.e., sandwich attacks, cyclic arbitrage, and liquidations. In this work, we shed light on the prevalence of non-atomic arbitrage on decentralized exchanges (DEXes) on the Ethereum blockchain. Importantly, non-atomic arbitrage exploits price differences between DEXes on the Ethereum blockchain as well as exchanges outside the Ethereum blockchain (i.e., centralized exchanges or DEXes on other blockchains). Thus, non-atomic arbitrage is a type of MEV that involves actions on and off the Ethereum blockchain.
   In our study of non-atomic arbitrage, we uncover that more than a fourth of the volume on Ethereum’s biggest five DEXes from the merge until 31 October 2023 can likely be attributed to this type of MEV. We further highlight that only eleven searchers are responsible for more than 80% of the identified non-atomic arbitrage volume sitting at a staggering 137 billion US$ and draw a connection between the centralization of the block construction market and non-atomic arbitrage. Finally, we discuss the security implications of these high-value transactions that account for more than 10% of Ethereum’s total block value and outline possible mitigations.

   @inproceedings{heimbach2024nonatomic,
     author = {Heimbach, Lioba and Pahari, Vabuk and Schertenleib, Eric},
     title = {Non-Atomic Arbitrage in Decentralized Finance},
     booktitle = {IEEE Symposium on Security and Privacy (SP), San Francisco, USA},
     year = {2024},
     month = may,
   }

2023

1. IMC 23

   Ethereum’s Proposer-Builder Separation: Promises and Realities

   Lioba Heimbach, Lucianna Kiffer, Christof Ferreira Torres, and 1 more author

   In 2023 ACM Internet Measurement Conference (IMC), Montreal, QC, Canada, Oct 2023

   Abstract Bibtex External PDF Code Slides Talk

   With Ethereum’s transition from Proof-of-Work to Proof-of-Stake in September 2022 came another paradigm shift, the Proposer-Builder Separation (PBS) scheme. PBS was introduced to decouple the roles of selecting and ordering transactions in a block (i.e., the builder), from those validating its contents and proposing the block to the network as the new head of the blockchain (i.e., the proposer). In this landscape, proposers are the validators in the Proof-of-Stake consensus protocol, while now relying on specialized block builders for creating blocks with the highest value for the proposer. Additionally, relays act as mediators between builders and proposers. We study PBS adoption and show that the current landscape exhibits significant centralization amongst the builders and relays. Further, we explore whether PBS effectively achieves its intended objectives of enabling hobbyist validators to maximize block profitability and preventing censorship. Our findings reveal that although PBS grants validators the opportunity to access optimized and competitive blocks, it tends to stimulate censorship rather than reduce it. Additionally, we demonstrate that relays do not consistently uphold their commitments and may prove unreliable. Specifically, proposers do not always receive the complete promised value, and the censorship or filtering capabilities pledged by relays exhibit significant gaps.

   @inproceedings{heimbach2023ethereum,
     title = {Ethereum's Proposer-Builder Separation: Promises and Realities},
     author = {Heimbach, Lioba and Kiffer, Lucianna and Ferreira Torres, Christof and Wattenhofer, Roger},
     booktitle = {2023 ACM Internet Measurement Conference (IMC), Montreal, QC, Canada},
     month = oct,
     year = {2023},
   }

2. DISC 23

   Base Fee Manipulation In Ethereum’s EIP-1559 Transaction Fee Mechanism

   Sarah Azouvi, Guy Goren, Lioba Heimbach, and 1 more author

   In 34th International Symposium on Distributed Computing (DISC), L’Aquila, Italy, Oct 2023

   Abstract Bibtex External PDF Code Slides

   In 2021 Ethereum adjusted the transaction pricing mechanism by implementing EIP-1559, which introduces the base fee - a fixed network fee per block that is burned and adjusted dynamically in accordance with network demand. The authors of the Ethereum Improvement Proposal (EIP) noted that a miner with more than 50% of the mining power might have an incentive to deviate from the honest mining strategy. Instead, such a miner could propose a series of empty blocks to increase its future rewards.
   In this paper, we generalize this attack and show that under rational player behavior, deviating from the honest strategy can be profitable for a miner with less than 50% of the mining power. Further, even when miners do not collaborate, it is rational for smaller mining power miners to join the attack.

   @inproceedings{azouvi2023base,
     title = {Base Fee Manipulation In Ethereum's EIP-1559 Transaction Fee Mechanism},
     author = {Azouvi, Sarah and Goren, Guy and Heimbach, Lioba and Hicks, Alexander},
     booktitle = {34th International Symposium on Distributed Computing (DISC), L'Aquila, Italy},
     month = oct,
     year = {2023},
   }

2022

1. AFT 22

   SoK: Preventing Transaction Reordering Manipulations in Decentralized Finance

   Lioba Heimbach, and Roger Wattenhofer

   In 4th ACM Conference on Advances in Financial Technologies (AFT), Cambridge, MA, USA, Sep 2022

   Non-archival: SNB-CIF Conference on Cryptoassets and Financial Innovation, May 2023

   Abstract Bibtex External PDF Slides

   User transactions on Ethereum’s peer-to-peer network are at risk of being attacked. The smart contracts building decentralized finance (DeFi) have introduced a new transaction ordering dependency to the Ethereum blockchain. As a result, attackers can profit from front- and back-running transactions. Multiple approaches to mitigate transaction reordering manipulations have surfaced recently. However, the success of individual approaches in mitigating such attacks and their impact on the entire blockchain remains largely unstudied.
   In this systematization of knowledge (SoK), we categorize and analyze state-of-the-art transaction reordering manipulation mitigation schemes. Instead of restricting our analysis to a scheme’s success at preventing transaction reordering attacks, we evaluate its full impact on the blockchain. Therefore, we are able to provide a complete picture of the strengths and weaknesses of current mitigation schemes. We find that currently no scheme fully meets all the demands of the blockchain ecosystem. In fact, all approaches demonstrate unsatisfactory performance in at least one area relevant to the blockchain ecosystem.

   @inproceedings{Heimbach2022sok,
     author = {Heimbach, Lioba and Wattenhofer, Roger},
     title = {SoK: Preventing Transaction Reordering Manipulations in Decentralized Finance},
     booktitle = {4th ACM Conference on Advances in Financial Technologies (AFT), Cambridge, MA, USA},
     month = sep,
     year = {2022},
   }