Ethereum Prysm bug drops participation to 75%; patch issued
An Ethereum client bug in Prysm led to a significant validation disruption on Dec. 4, reducing network participation to 75% and resulting in an estimated 382 Ether (ETH) in missed attestation rewards. The issue, detailed in a post-mortem published Sunday by Ethereum developer Terence Tsao, traced the root cause to a flaw introduced a month earlier during testnet preparations for the Fusaka upgrade.
According to the post-mortem, Prysm nodes encountered “resource exhaustion” while handling attestations from out-of-sync peers. This behavior caused Prysm to replay prior epoch blocks and recompute costly state transitions, imposing a heavy computational load and degrading performance across affected nodes.
The report stated the bug originated in Prysm PR 15965 and had been deployed to testnets for a month before the event without being triggered. While testnets aim to surface such issues, they do not guarantee detection of every failure mode. In a separate episode in May 2023 — shortly after the Shanghai hard fork — the Ethereum network temporarily lost transaction finality for roughly 25 minutes, and again for more than an hour the following day, before self-recovering.
Prysm patch issued
During the incident, Prysm regenerated earlier states from scratch rather than using the current head state, contributing to the resource strain. Over more than 42 epochs, the network recorded an 18.5% missed slot rate, with overall participation falling to 75%. Node operators were advised to apply an interim workaround while developers prepared an update for Prysm clients.
Client diversity limited impact
Developers noted the outcome could have been more severe had the bug affected Ethereum’s leading consensus client, Lighthouse. Offchain Labs’ Prysm is the second-largest Ethereum consensus client with a 17.6% share, according to ClientDiversity. The post-mortem added that if a single client controlled more than one-third of the network, a similar bug could have temporarily halted finality and increased missed blocks.
The incident also underscored concentration risks. Lighthouse’s share currently stands at 52.6%, down from about 56% at the time of the disruption, placing it near the two-thirds threshold at which a single-client bug could potentially finalize an invalid chain. Ethereum developers continue to advocate for broader client diversity.
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