The proof-of-stake analogue of the 51% attack from proof-of-work. With more than half of the active stake under one operator's control, that party can outproduce the honest network, refuse to include specific transactions, or produce a competing chain that overtakes the canonical one.
Ouroboros's security proofs assume an honest stake majority, so the protocol breaks once an attacker crosses that threshold. Cardano's mitigation is structural: ada is held by hundreds of thousands of accounts and delegated across thousands of stake pools, and protocol parameters discourage concentration: the saturation parameter (k) caps the rewards any single pool earns past its saturation point, nudging delegators toward less-saturated pools, while the pledge-influence factor (a0) makes it costly for one operator to spread stake across many low-pledge pools. Together these make acquiring majority control of the active stake economically and operationally infeasible.
Explore next
- Proof-of-Stake AttacksThe set of known attack categories against proof-of-stake blockchains and how Cardano's Ouroboros family defends against each.View term
- Proof of StakeA consensus mechanism where validators are selected to create blocks based on the amount of cryptocurrency they hold and stake (commit) to the network.View term
- OuroborosThe family of proof-of-stake consensus protocols that power Cardano, designed with formal security proofs against the known attack catalogue against PoS chains.View term
- Pool SaturationThe threshold past which extra delegation to a stake pool stops increasing rewards; a built-in incentive to spread stake across many pools rather than concentrate it in a few.View term
- k parameterThe protocol parameter that sets the target number of saturated stake pools for healthy decentralization.View term