CVE-2026-62162 in Hydra
Summary
by MITRE • 10/30/2026
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.
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Analysis
by VulDB Data Team • 10/30/2026
The vulnerability identified within Hydra, a layer-two scalability solution for Cardano, represents a critical flaw in the smart contract logic governing deposit claims prior to version 2.1.0. The core of the issue resides in the Claim branch located at hydra-plutus/validators/deposit.ak. This specific code path is responsible for processing withdrawals or claims related to deposits made into Hydra heads. However, the validation logic implemented here was insufficiently restrictive regarding transaction structure and state management. Specifically, the validator only verified that the datum.head_id matched the attacker-controlled redeemer.currencySymbol and confirmed that the transaction occurred before a specified deposit deadline. It failed to enforce two crucial security invariants: it did not require the spending of the Head state-token UTxO within the same transaction, nor did it impose constraints on the outputs generated by the claim transaction. This omission created a significant gap between the intended protocol behavior and its actual execution, allowing for unauthorized asset redirection.
The operational impact of this flaw is severe, as it enables any third party to intercept pending deposits before their expiration deadline. Once a deposit is confirmed via the POST /commit endpoint, the resulting UTxO becomes visible on the Cardano blockchain. An attacker can monitor these pending transactions and construct a separate transaction that consumes the deposit UTxO without invoking the Head validator logic. By bypassing the Head state-token spending requirement, the attacker effectively sidesteps the safety checks designed to ensure proper head lifecycle management. Consequently, the assets associated with the deposit are redirected to an arbitrary address controlled by the attacker rather than being properly processed within the Hydra protocol framework. This attack vector exploits the lack of binding between the claim transaction and the specific Head state, allowing for theft of funds that were intended to be locked until valid withdrawal conditions were met through proper head operations.
This vulnerability stems from a historical design oversight where the Aiken rewrite retained the missing invariant regarding the mandatory spending of the Head state-token during deposit claims. The default deadline for such attacks is set at three times the depositPeriod, providing a window of opportunity for attackers to observe and exploit pending deposits before they expire or are claimed through legitimate channels. The absence of output constraints further exacerbates the risk, as it allows the attacker to direct funds without adhering to protocol-defined distribution rules. This issue highlights the importance of comprehensive invariant checking in smart contracts, particularly when dealing with state transitions that involve external asset movements and time-sensitive operations.
Mitigation for this vulnerability requires upgrading to Hydra version 2.1.0 or later, where the logic has been corrected to enforce the necessary constraints. The fix ensures that any transaction claiming a deposit must include the spending of the corresponding Head state-token UTxO in the same transaction block. This change restores the critical link between the claim operation and the head's active state, preventing unauthorized third parties from intercepting deposits. Additionally, implementing strict output validation rules would further harden the contract against similar structural attacks. From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation, as the validator failed to adequately check transaction inputs for required components like the Head state-token. It also relates to CWE-841 Improvement of Transaction Consistency, given that the protocol's consistency guarantees were bypassed by allowing out-of-band spending of deposit UTxOs. In terms of MITRE ATT&CK techniques, this scenario reflects aspects of T1567 Exfiltration Over Web Protocol if viewed through the lens of data theft via transaction manipulation, though it is primarily a smart contract logic flaw rather than an external exfiltration method.