High advisory for Hydra

Advisory overview

Hydra is a layer-two scalability solution for Cardano. Prior to 2.1.0, the `Claim` branch in `hydra-plutus/validators/deposit.ak` checks only that `datum.head_id` matches the attacker-controlled `redeemer.currencySymbol` and that the transaction is valid before the deposit deadline. It neither requires spending the Head state-token UTxO in the same transaction nor constrains outputs. After a deposit made through `POST /commit` is confirmed, any third party able to build a Cardano transaction can observe and consume the pending deposit UTxO before its deadline and redirect the assets to an arbitrary address. A standalone deposit spend invokes no Head validator, bypassing its safety checks; historical head-ID binding and the Aiken rewrite retained this missing invariant. The default deadline is three times depositPeriod, so the described attack applies to pending deposits before that deadline. This issue is fixed in version 2.1.0.

Related references

• https://github.com/cardano-scaling/hydra/commit/ff60e4af4ecf05545a696a6d9bf1afeffe991de0
• https://github.com/cardano-scaling/hydra/releases/tag/2.1.0
• https://github.com/cardano-scaling/hydra/security/advisories/GHSA-gfpc-m687-p7h5