Crypto glossary
Shared security
Shared security lets a smaller chain or service rely on the validators and staked capital of a larger network instead of building its own.
Shared security is the idea that one large validator set, backed by a lot of staked value, protects several chains or services at once. Instead of attracting its own validators and making its token valuable enough to deter attacks, a new chain rents protection from an established network whose stake can be slashed if it misbehaves.
The model appears in several forms: Polkadot validators check every parachain block; Cosmos offered Interchain Security so consumer chains could use Cosmos Hub validators; and restaking protocols on Ethereum let staked ETH secure actively validated services such as oracles, bridges or data availability layers. Bitcoin staking projects try to export bitcoin's value in a similar way.
Borrowed security is not unlimited. Validators may be stretched across many duties, the same stake can be counted several times, and slashing rules for external services are newer and less tested than base-layer consensus. A shared validator set also does not fix bugs in the consumer chain's own code, which is where most losses happen.
Example
A new oracle network, rather than launching its own token-secured validator set, pays restakers on Ethereum to run its nodes and accepts that their stake can be slashed for false reports.