Ethereum sees 25% validator drop after Prysm bug post-Fusaka
Ethereum experienced a 25% decline in validator voting participation shortly after the Fusaka network upgrade, following a bug in the Prysm consensus client that took a portion of validators offline. The drop left the network less than 9 percentage points from losing the two-thirds supermajority required for finality.
In an announcement on Thursday, Prysm said version v7.0.0 of its client was unnecessarily generating old states while processing outdated attestations, which prevented affected nodes from operating properly. Prysm core developer Terence Tsao confirmed the issue and recommended running the client with the “–disable-last-epoch-targets” flag as a temporary workaround.
Beaconcha.in data shows that at epoch 411,448 the network recorded 75% sync participation and 74.7% voting participation. The 25% drop in voting participation brought the network to within under 9% of the two-thirds threshold needed to maintain finality and normal operations.
As of the current epoch (411,712), voting participation has rebounded to nearly 99%, with sync participation at 97%, indicating a recovery. Before the incident, epochs commonly posted well over 99% voting participation.
The magnitude of the decline aligns with the estimated share of validators running Prysm, which was about 22.71% on Wednesday and fell to roughly 18% after the event, suggesting the attestation failure was concentrated among Prysm validators.
Client diversity chart. Source: MigaLabs
The Ethereum Foundation and Offchain Labs, the organization behind Prysm development, had not responded to requests for comment by the time of publication.
Brush with finality loss
If voting participation drops below two-thirds of total staked Ether (ETH), the network loses finality. In such a scenario, blocks may still be produced, but the chain is not considered finalized.
Potential consequences include freezes on layer-2 bridges, paused withdrawals on rollups, and higher confirmation requirements on exchanges due to elevated chain reorganization risk.
A comparable disruption occurred in early May 2023, when the Ethereum mainnet experienced two separate finality losses within 24 hours due to issues in handling old-target attestations in the Prysm and Teku consensus clients.
The impact could have been more severe previously, as Prysm was estimated by its developers to be running on more than two-thirds of consensus nodes in September 2021. Data published in January 2022 by Michael Sproul, a developer on Lighthouse, indicated Prysm then accounted for 68.1% of nodes.
Client diversity chart. Source: Michael Sproul
Client diversity remains limited
While client diversity has improved since 2022, it remains short of the target where no single client exceeds 33%, a threshold that would prevent a solitary client bug from endangering finality. Current MigaLabs data shows Lighthouse at 52.55% of consensus nodes and Prysm at 18%.
Client diversity chart. Source: MigaLabs
These figures reflect a shift from before the incident, when Lighthouse was below 48.5% and Prysm around 22.71%, per MigaLabs. Ethereum educator Anthony Sassano noted on X that if Lighthouse had encountered the bug instead, the network would have lost finalization.
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