Key Highlights
- Prysm bug caused a 25% drop in Ethereum validator participation post-Fusaka upgrade, threatening network finality.
- Voting fell to 74.7% and sync participation to 75%, recovering later to nearly 99% and 97%, respectively.
- Lighthouse now dominates 52.55% of nodes, raising concerns over Ethereum’s client diversity.
Ethereum briefly faced one of its most serious stability scares of 2025 after a bug in the Prysm consensus client caused a sharp fall in validator participation, dropping by 25%. The decline pushed the network close to the threshold where it could lose finality.
Shortly after the Fusaka upgrade went live, Prysm’s latest release (v7.0.0) began generating unnecessary old states while processing outdated attestations.
According to Prysm developer Terence Tsao, this flaw caused the client to get stuck replaying heavy historical data, pushing many nodes into a “DoS-like” state. As a temporary fix, developers urged users to restart the client with the flag “–disable-last-epoch-targets.” No change was needed for validator clients.
Immediate network impact
The impact of the bug showed up immediately in Ethereum’s participation metrics. Data from Beaconcha.in network show that at epoch 411,448, voting participation dropped to 74.7% and sync participation fell to 75%, a 25% drop from normal levels of over 99%.
Because Ethereum requires at least two-thirds of staked ETH to maintain finality, the network came within roughly 9% of losing that threshold. By epoch 411,712, participation had mostly recovered, returning to nearly 99% voting and 97% sync, indicating that most Prysm nodes were back online.
Client distribution and risks
The scale of the outage closely reflected Prysm’s role in the ecosystem. Before the issue, Prysm made up 22.71% of consensus clients. After the incident, that number dropped to around 18%, indicating that the bug primarily impacted Prysm operators.
However, the event also highlighted a wider concern about Ethereum’s client diversity. Lighthouse, the leading client, now represents 52.55% of the network, an increase from under 48.5% earlier in the week.
Researchers have long warned that no single client should exceed 33%, so that a bug in one client cannot threaten the entire network. As Ethereum educator Anthony Sassano noted, if Lighthouse had been the client with this bug, Ethereum likely would have lost finality.
Risks of losing finality
Losing finality is serious. If fewer than two-thirds of validators participate, blocks still get produced, but the chain is no longer fully secure. In such moments, bridges freeze, rollups pause withdrawals, and exchanges require more confirmations due to the higher risk of chain reorganisations.
Ethereum has experienced this before. In May 2023, the network lost finality twice in one day because of bugs in Prysm and Teku, at a time when Prysm still controlled nearly 68% of nodes.
At its peak in 2021–2022, Prysm ran on more than two-thirds of all consensus nodes, a level of dominance that could have made any single bug far more damaging.
Although Ethereum recovered quickly from the latest incident, it highlights a long-standing issue. The network still relies too much on a small number of clients, and improving client diversity is crucial to avoid future near-misses.
