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Slashing

A validator that signs two conflicting finality votes (a GRANDPA equivocation) is slashed: part of its bond, and of every nomination and pool backing it, is taken. It is the only slashable offence.

What is and is not slashed​

BehaviourConsequence
Two GRANDPA votes for different blocks in the same roundSlashed, banned, disabled
Two blocks in the same Aura slotNot slashed
Being offlineNot slashed — the validator only misses the era points of its slots

The usual cause of an equivocation is running two nodes with the same session keys: a backup brought up without stopping the original, or a restored keystore. Never run the same keys on two machines at once.

How much​

The fraction slashed grows with how many validators equivocate in the same round: (3k / n)², where k is the number of offenders and n the size of the set. One offender in a set of 14 loses about 4.6 %; a third of the set acting together loses everything they staked, up to the cap.

An offender's slashes are capped together at 99 %.

When it applies​

  1. Report. Anyone can submit the proof of the two votes; the reporter earns 1 % of the slash.
  2. Immediately. The offender is banned — it cannot stand again until governance lifts the ban — and disabled: it stops authoring until the active set changes.
  3. 26 eras later. The slash is applied. During those 26 eras governance can cancel it (staking.cancelDeferredSlash) if the report turns out to be unjust.

Funds that are unbonding are slashed too, and withdrawals wait while a slash from the previous era is still pending.

What nominators and pools share​

  • Nominators lose the same fraction of the stake they had behind the offender in the era of the offence.
  • Pools lose it on the pool's bond. Each member pays its share from its own funds when it next withdraws, or when anyone applies the slash (nominationPools.applySlash). A member who joined after the offence shares it too; the app warns about pending slashes before you join.
  • Backing several validators that equivocate together can cost more than 99 % in total: the cap applies per offender.

Choosing who to back is the main defence: prefer operators with a clean history and a setup that cannot run the same keys twice.