What really went wrong with Ethereum’s Fusaka upgrade: inside Prysm’s near-miss with finality loss
Ethereum’s December 4 Fusaka mainnet upgrade was supposed to be a landmark step for scalability. Instead, for several tense hours, the network came uncomfortably close to losing finality — a worst-case scenario that could have frozen key infrastructure across the ecosystem.
The root cause was not Fusaka itself, but a serious bug in Prysm, one of Ethereum’s consensus clients. In a post-mortem, Prysm’s developers detailed how a flaw tied to historical state handling led to resource exhaustion, validator failures, and a sharp drop in network participation immediately after the upgrade activated.
How the incident unfolded
The issue appeared right as Fusaka went live at epoch 411392 on December 4, 2025, at 21:49 UTC. Almost instantly, a subset of validators running Prysm began to experience severe performance issues.
Instead of smoothly processing attestations — the validator votes needed to confirm blocks — affected Prysm nodes were overwhelmed by expensive state recomputations. These computations were triggered when processing particular kinds of attestations referencing older states, causing machines to burn through CPU and memory.
As the problem escalated, Prysm validators started timing out, failing to attest, or dropping offline entirely. Network-wide validator participation, which normally sits comfortably above 95%, plunged to about 75%. Ethereum missed 41 consecutive epochs, and the ecosystem collectively lost an estimated 382 ETH in missed proof rewards.
The technical core of the bug: historical state replay
At the heart of the failure was the way Prysm handled historical states — snapshots of the chain at previous points in time.
Prysm core developer Terence Tsao highlighted that “historical state is compute memory heavy,” meaning replaying or recomputing it can become extremely expensive. If a node is forced to process many such replays at once, it can effectively be denied service by its own workload.
In this incident, specific attestations caused Prysm to recompute obsolete historical states repeatedly and in parallel. Instead of rejecting or optimizing these expensive operations, the client attempted to process them all, overloading resources. This turned historical state processing into a de facto denial-of-service vector for nodes running Prysm.
While the bug did not stem from the Fusaka upgrade logic itself, the timing of the upgrade changed network conditions in a way that made the issue visible immediately after activation.
How close Ethereum came to losing finality
The consequences went far beyond a few missed blocks. Ethereum’s proof-of-stake mechanism relies on a large majority of validators to finalize blocks — that is, to make them cryptographically and economically irreversible.
As Prysm validators struggled, overall participation crashed to around 75%. With 15% to 22.71% of all validators using Prysm at the time, the hit was large enough to bring the network dangerously close to the finality threshold. Any deeper drop could have prevented finality from being reached at all.
Importantly, this near-miss was partly a matter of which client was affected. If a more widely used consensus client such as Lighthouse had suffered the same bug under similar conditions, finality might have been lost entirely. That could have triggered cascading issues across the ecosystem:
– Layer 2 rollups might have been forced to pause or severely limit operations, since they depend on Ethereum finality to secure their state.
– Validator withdrawals could have been frozen until the root cause was identified and fixed.
– User confidence in Ethereum’s liveness and safety guarantees would have taken a serious hit.
In other words, this was not just a localized hiccup in one client. It was a live-fire test of Ethereum’s resilience design.
Why Layer 2s are so sensitive to finality
The incident also underscores how deeply Layer 2 systems are intertwined with Ethereum’s consensus. Rollups and other scaling solutions rely on the assumption that Ethereum will finalize blocks within a predictable timeframe.
If finality stalls:
– Rollups may delay posting state roots or proofs, slowing withdrawals and cross-chain activity.
– Bridges and interoperability solutions could halt transfers to avoid inconsistent states.
– Risk models for exchanges and institutional participants — many of which assume a certain number of finalized blocks before considering transactions “safe” — would break down.
The Prysm bug didn’t fully test that scenario, but it came close enough to highlight the systemic stakes involved.
Emergency response: runtime flags and rapid patches
Once the malfunction was identified, Prysm’s team moved quickly. They first introduced emergency runtime flags — configuration options that allowed operators to temporarily sidestep the worst performance traps without upgrading the full client immediately.
These stopgap measures gave validators enough breathing room to restore some stability while the developers worked on permanent fixes.
Within a short window, Prysm released patched versions, v7.0.1 and v7.1.0. These updates reworked how the client handled historical state computation and eliminated the pathological behavior that led to resource exhaustion.
At the ecosystem level, the Ethereum Foundation issued urgent guidance to node operators running Prysm, instructing them on how to apply the temporary flags and then upgrade to the corrected versions. The fast coordination between client developers, foundations, and operators helped contain the incident to roughly a one-day disturbance.
By December 5, network participation had already climbed back to nearly 99%, and block finality returned to normal.
Fusaka itself worked as designed
Ironically, the core functionality introduced by Fusaka — including PeerDAS (Peer Data Availability Sampling) — did not malfunction at all.
The Fusaka upgrade aimed to significantly expand blob capacity on Ethereum, effectively multiplying by eight the data throughput available for rollups and other scaling mechanisms. From an execution and protocol standpoint, the upgrade proceeded smoothly and without downtime.
The problem emerged not from Fusaka’s PeerDAS enhancements, but from how one particular consensus client implementation reacted under new post-upgrade conditions. That nuance matters: the protocol upgrade was technically successful; the client ecosystem exposed an implementation weakness.
Client diversity: Ethereum’s safety net
The episode is a concrete demonstration of why Ethereum’s multi-client philosophy is more than just an ideological stance — it is a critical safety feature.
While Prysm validators were struggling, ten other consensus clients, including Lighthouse, Nimbus, and Teku, continued to function normally. Because the validator set is distributed across these different implementations, around 75% to 85% of the network’s validators stayed fully operational throughout the incident.
This diversity:
– Prevented the bug in Prysm from turning into a protocol-wide failure.
– Maintained sufficient participation to preserve finality.
– Ensured that transactions continued to be processed, even if at reduced participation levels.
From a system design perspective, this is exactly the outcome client diversity was meant to deliver: isolation of implementation failures and graceful degradation instead of total collapse.
What if Prysm had a larger market share?
The incident also raises a sobering question: what if Prysm had controlled a much larger share of validators?
If a single client accounts for a majority of the stake, a critical bug like this could:
– Instantly drag overall participation below the finality threshold.
– Cause large validator slashing events if misbehavior becomes correlated.
– Effectively centralize power in the hands of one client team, whose reliability becomes synonymous with the network’s liveness.
This is why Ethereum governance and technical leadership repeatedly urge operators to spread their validators across multiple clients. Even modest shifts in client distribution can dramatically reduce systemic risk.
Lessons for validators and infrastructure providers
For individual validators and staking providers, the Fusaka-Prysm incident offers several concrete takeaways:
1. Avoid single-client dependence. Running all validators on one client — no matter how battle-tested — creates a single point of failure. Splitting across at least two consensus clients meaningfully reduces correlated risk.
2. Monitor resource usage aggressively. Tools that track CPU, memory, and network utilization can reveal abnormal spikes early, giving operators time to react before machines fail.
3. Stay prepared for emergency guidance. Upgrading promptly, using temporary runtime flags, and following best practices shared by client teams can be the difference between a minor revenue loss and prolonged downtime.
4. Test upgrades in staging. Running shadow or testnet infrastructure mirrors can uncover edge cases in advance, especially for large operators handling thousands of validators.
Implications for protocol and client development
On the development side, the incident is a reminder that performance and resource safety must be treated as first-class concerns, not afterthoughts. Historical state handling, in particular, has emerged as a recurring pain point across multiple clients and ecosystems.
Future work is likely to focus on:
– Stricter limits and guardrails around state replay and historical queries.
– Better profiling tools to catch pathological code paths before mainnet deployment.
– Fuzzing and adversarial testing that simulates denial-of-service-like workloads via legitimate protocol operations such as attestations.
There is also a broader cultural lesson: when protocol complexity increases — as it does with data availability sampling, rollups, and more sophisticated consensus logic — the cost of implementation mistakes grows as well. Dual implementations, rigorous audits, and independent testing teams become increasingly important.
Why this incident matters beyond Ethereum
Even though the immediate crisis passed in under 24 hours, the incident will likely be studied across the broader blockchain industry. Other proof-of-stake chains that rely on a single “official” client or maintain minimal client diversity are significantly more exposed to similar failures.
Ethereum’s experience highlights a few universal principles:
– Multiple independent implementations are not a luxury; they are essential resilience infrastructure.
– Finality guarantees are only as strong as the worst-behaving client in the validator set.
– Performance bugs can manifest as consensus risks when they impact a sufficiently large fraction of validators.
A stress test passed — but a warning delivered
In the end, Ethereum’s architecture did what it was supposed to do. The network endured a serious bug in a widely used client without suffering finality loss or a total halt. Transactions continued, blocks were produced, and within a day, normalcy was restored.
Yet the narrow margin by which finality was preserved is a clear warning. As Ethereum scales to support more users, more data, and more value, the cost of such near-misses rises. Ensuring that no single client, provider, or infrastructure layer becomes too big to fail will remain a central challenge.
The Fusaka upgrade expanded Ethereum’s capacity. The Prysm incident that followed exposed the fragility that comes with complexity. Together, they form a blueprint for the network’s next phase: build bigger, but also build safer — with diversity, redundancy, and operational readiness at the core.