Ethereum’s development community has officially confirmed the dates for the upcoming Glamsterdam event, a gathering that brings together researchers, developers, and enthusiasts to explore the future of the blockchain ecosystem. While the announcement has generated excitement across the ecosystem, the Ethereum Foundation also issued a stark warning: the presence of so‑called “fake” builders could pose a serious risk to the smooth operation of the network during this critical period. In the context of Ethereum’s evolving consensus mechanism, a “builder” refers to an entity that assembles blocks for the network, selecting transactions and ordering them in a way that maximizes the rewards for validators. Historically, most builders have been legitimate participants seeking to optimize block profitability while adhering to the protocol’s rules.
However, recent research and testing have uncovered a new class of actors—dubbed “fake” builders—who exploit the system in ways that can undermine both efficiency and security. One of the primary tools these malicious builders wield is free test ether, a resource typically distributed to developers for experimentation on testnets such as Sepolia and Goerli.
By providing test ether without cost, the Ethereum community encourages innovation and rapid prototyping. Unfortunately, this generosity can be turned against the network. Fake builders can use the free test ether to place artificially high bids for block space, outcompeting honest builders and effectively crowding them out of the market. Because these bids are backed by test funds rather than real economic value, they can be inflated to extreme levels without any real cost to the malicious actor.
Beyond merely outbidding honest participants, fake builders can also withhold transaction payloads. In a typical block‑building process, the builder assembles a payload—a collection of transactions—that is then proposed to validators for inclusion in the blockchain. When a builder decides to withhold this payload, it creates a situation where validators receive a block header that signals a certain amount of transaction data, but the actual data is delayed or never delivered.
This discrepancy can cause validators to waste time and resources attempting to retrieve missing information, potentially leading to delays in finalizing blocks. The combination of inflated bids and payload withholding can have a cascading effect on the network’s performance. Validators may experience longer latency when attempting to verify blocks, and the overall throughput of the chain could be reduced. Moreover, the uncertainty introduced by these tactics can erode trust among participants, making it harder for the ecosystem to maintain a stable and predictable environment for users and developers alike.
To mitigate these risks, the Ethereum client teams have taken proactive steps. One notable measure is the reduction of the review window for client updates before the Sepolia testnet launch. Historically, client teams have been allotted a generous period—often several weeks—to conduct thorough code reviews, run extensive test suites, and coordinate with other teams.
In the lead‑up to Sepolia, this review window has been cut to roughly half of its usual length. While this acceleration places additional pressure on developers, it also forces a more rapid identification and resolution of potential vulnerabilities that could be exploited by fake builders. The shortened timeline compels client teams to adopt more rigorous automated testing pipelines, leverage formal verification tools, and increase collaboration across the ecosystem.
By tightening the review process, the Ethereum community aims to close any gaps that could be taken advantage of by malicious actors before they become entrenched in the network’s operational fabric. In addition to procedural changes, the Ethereum Foundation is also exploring technical safeguards. One proposal under discussion involves implementing stricter verification of transaction payloads at the protocol level.
By requiring validators to perform additional checks on the integrity and completeness of payloads before accepting a block, the network could reduce the effectiveness of payload withholding attacks. Another avenue being examined is the introduction of a reputation system for builders, where historical behavior influences the priority given to their bids. Such a system could penalize builders that have previously engaged in disruptive practices, thereby discouraging malicious conduct. Community education also plays a vital role.
Developers and validators are being encouraged to stay informed about the tactics employed by fake builders and to adopt best practices for monitoring bid patterns and payload delivery. By fostering a culture of vigilance, the Ethereum ecosystem can collectively identify anomalies early and respond swiftly.
The Glamsterdam event itself will feature a series of workshops, panel discussions, and hands‑on labs focused on these very challenges. Experts from the Ethereum Foundation, leading client teams, and independent researchers will share insights into how the network can defend against sophisticated attacks while continuing to innovate.
Attendees will have the opportunity to experiment with new tools designed to detect and mitigate fake builder activity, as well as to contribute to the development of future protocol upgrades that enhance resilience. In summary, while the confirmation of Glamsterdam dates marks an exciting milestone for Ethereum’s roadmap, it also serves as a reminder that the network’s security and performance are continually tested by evolving threats. The emergence of fake builders—empowered by free test ether and capable of manipulating bids and payloads—highlights the need for both procedural diligence and technical innovation. By shortening client review periods, exploring protocol‑level safeguards, and promoting community awareness, Ethereum aims to stay ahead of these challenges and ensure that the chain remains robust, efficient, and trustworthy for all participants.