ZenHive mpp up to 0.5.9 Access List cosign_fee_payer access list input validation

Summaryinfo

A vulnerability marked as problematic has been reported in ZenHive mpp up to 0.5.9. Affected by this vulnerability is the function cosign_fee_payer of the component Access List Handler. The manipulation of the argument access list leads to input validation. This vulnerability is referenced as CVE-2026-59694. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability, which was classified as problematic, has been found in ZenHive mpp up to 0.5.9. This issue affects the function cosign_fee_payer of the component Access List Handler. The manipulation of the argument access list with an unknown input leads to a input validation vulnerability. Using CWE to declare the problem leads to CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Impacted is availability. The summary by CVE is:

Improper Validation of Specified Quantity in Input in ZenHive mpp allows an unauthenticated remote client to inflate the fee-payer's gas cost per payment by a large multiplier, degrading the sponsor's operating margin. When the mpp Elixir library is configured as fee payer (fee_payer: true), MPP.Tempo.Transaction.cosign_fee_payer/3 re-signs the client-supplied base fields of the 0x76 AASigned envelope verbatim, including the EIP-2930 access list, without validating its length or contents. EIP-2930 access list entries incur intrinsic gas (~2,400 gas per address, plus 1,900 gas per storage key) charged before any opcode executes, regardless of whether the listed addresses are ever touched. A malicious client submits a valid transferWithMemo call alongside a large number of fabricated access-list entries. The server co-signs and broadcasts the transaction. The intended transfer executes normally, but the fee-payer wallet pays a large multiple of the expected gas cost with no corresponding on-chain work. At the maintainer's default of 137 access-list entries (fitting within Bandit's 10,000-byte per-header-field limit) and 100 Gwei max_fee_per_gas, per-payment gas cost rises from ~51,287 to ~380,087 gas, a 7.4x multiplier. Sustained abuse destroys the sponsor's operating margin on low-cost payments and, over time, drains the fee-payer wallet. This issue affects mpp: from 0.2.0 before 0.6.0.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-59694 since 07/06/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit.

Upgrading to version 0.6.0 eliminates this vulnerability.

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Productinfo

Vendor

• ZenHive

Name

• mpp

Version

• 0.5.0
• 0.5.1
• 0.5.2
• 0.5.3
• 0.5.4
• 0.5.5
• 0.5.6
• 0.5.7
• 0.5.8
• 0.5.9

Website

• Product: https://github.com/ZenHive/mpp/

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