Ethereum

Fusaka

when

<p>Prysm developers released a post-mortem analysis explaining the December 4 Fusaka mainnet incident that threatened Ethereum network stability.</p>
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  <span>Summary</span>
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   <li>A Prysm bug after Fusaka caused validator participation to drop to 75%.</li>
   <li>The network missed 41 epochs and lost roughly 382 ETH in proof rewards.</li>
   <li>Ethereum avoided finality loss thanks to client diversity and rapid fixes.</li>
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<p>The consensus client suffered resource exhaustion from expensive state recomputation when processing specific attestations, causing validators to face severe operational problems.</p>
<p>The bug surfaced immediately after Fusaka activated at epoch 411392 on December 4, 2025, at 21:49 UTC.</p>
<p>The network missed 41 epochs as validator participation plummeted to 75%, resulting in approximately 382 Ethereum (<a href="https://crypto.news/price/ethereum/" rel="noopener noreferrer nofollow">ETH</a>) in lost proof rewards. Prysm developers deployed emergency runtime flags before implementing permanent fixes in versions v7.0.1 and v7.1.0.</p>
<h2 class="wp-block-heading" id="resource-exhaustion-pushed-network-toward-finality-loss">Resource exhaustion pushed network toward finality loss</h2>
<p>The technical failure centered on obsolete historical states that created denial-of-service conditions on affected nodes.</p>
<p>Prysm core developer Terence Tsao <a href="https://prysm.offchainlabs.com/docs/misc/mainnet-postmortems/" rel="noopener noreferrer nofollow">explained</a> that “historical state is compute memory heavy, a node can be dosed by large number of state replays happening in parallel.”</p>
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<p>Validators running Prysm, which represented roughly 15% to 22.71% of network validators, faced crippling performance degradation. The participation drop from normal levels above 95% to 75% pushed Ethereum dangerously close to losing finality.</p>
<p>Had the bug affected a different consensus client like Lighthouse instead of Prysm, the network could have lost finality entirely.</p>
<p>Such an event would potentially freeze Layer 2 rollup operations and block validator withdrawals until developers resolved the issue.</p>
<p>The Fusaka upgrade itself introduced PeerDAS (Peer Data Availability Sampling) technology designed to increase blob capacity eightfold for Layer 2 scaling.</p>
<p>The upgrade executed successfully with zero downtime before the Prysm bug surfaced.</p>
<h2 class="wp-block-heading" id="ten-consensus-clients-prevented-ethereum-network-collapse">Ten consensus clients prevented Ethereum network collapse</h2>
<p>Ethereum’s client diversity architecture prevented catastrophic failure. While Prysm validators struggled, ten other consensus clients including Lighthouse, Nimbus, and Teku continued validating blocks without interruption.</p>
<p>The decentralized client structure meant that roughly 75% to 85% of validators maintained normal operations throughout the crisis. This prevented finality loss and kept the network processing transactions despite Prysm’s degraded state.</p>
<p>The Ethereum Foundation quickly issued emergency guidance for Prysm operators. Validators applied the temporary fix while Prysm developers built permanent solutions.</p>
<p>By December 5, network participation recovered to nearly 99%, restoring normal operations within 24 hours of the incident.</p>
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What broke Ethereum’s Fusaka upgrade? Prysm post-mortem reveals the cause

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