A technical fault in Prysm, one of Ethereum’s key consensus clients, led to an unexpected network slowdown on December 4.
Ethereum developer Terence Tsao shared a detailed report on December 14 explaining what went wrong during the Fusaka mainnet event.
According to his post, Prysm nodes began overusing system resources when they encountered attestations from nodes that were not properly synchronized. This led to repeated calculations of previous state data, which slowed down performance.
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Instead of referencing the latest state, the affected client instances rebuilt earlier states from scratch. This process placed pressure on computing power and memory, which resulted in delays and missed validations.
The bug first appeared in a test version of the software a month before the mainnet upgrade. However, it did not trigger during testing.
The report confirmed that “the bug was introduced in Prysm PR 15965 and deployed to testnets a month before the incident without the trigger happening“. While test networks exist to catch such flaws, they do not always expose every potential failure before live deployment.
As a result of the malfunction, Ethereum experienced a 42-epoch period where almost one-fifth of slots were missed. Participation levels dropped to roughly three-quarters of normal, and validators lost about 382 Ethereum in missed attestation rewards.
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