inactivity leak

The inactivity leak is Ethereum's emergency mechanism for recovering finality when a large fraction of validators go offline at once — for example, a major data-center outage, a country-wide internet cut, or a partition splitting the network. Casper FFG needs more than two-thirds of total stake to vote in order to finalize, so if much of the stake disappears, the chain keeps producing blocks but stops finalizing. Left alone it would stall indefinitely. The leak fixes this by gradually draining stake from the validators who are not participating.

When the chain has failed to finalize for more than four epochs, it enters leak mode. Each non-attesting validator accumulates an inactivity score, and a penalty proportional to that score is deducted every epoch — so the longer a validator stays offline, the faster its balance bleeds, a quadratic-in-time leak. Validators who are online keep their stake roughly flat (they lose nothing to the leak), so the offline cohort's share of the total shrinks over time. Eventually the active, honest validators again control more than two-thirds of the now-smaller total stake, a supermajority link forms, and finalization resumes.

The deeper purpose is to break what would otherwise be a deadlock during a network partition. If the validator set splits into two halves that cannot see each other, neither can finalize — but the leak makes each side burn the absent side's stake, so on each branch the locally-present validators eventually regain a supermajority and finalize their own history. This favors liveness recovery at the cost of allowing two partitioned forks to finalize divergently; weak subjectivity and social consensus then determine which fork the community accepts.