EEF-CVE-2026-82750

## Summary

Improper Validation of Specified Quantity in Input in ZenHive mpp allows an unauthenticated remote client to inflate the fee-payer's gas cost per sponsored payment by a large multiplier and to have the sponsor pay for EIP-7702 account delegations of the client's choosing.

When the server sponsors Tempo payments, MPP.Methods.Tempo.FeePayerPolicy.measure/3 in lib/mpp/methods/tempo/fee\_payer\_policy.ex bounds the gas fields, the fee budget, the validity window and the access list of the client-signed 0x76 envelope, but never reads its aa\_authorization\_list field. Every signed delegation in that list is charged as intrinsic gas before the payment call runs, so a client attaching delegations from throwaway authority keys makes the sponsor pay for them within the default gas\_limit ceiling. At the reporter's default of seven entries the sponsored cost rises from about 46,575 gas to about 1,884,087 gas. Because each entry is applied as a persistent set-code delegation, a client can also upgrade its own accounts to delegated code at the sponsor's expense.

This issue affects mpp: from 0.2.0 before 0.16.1.

## Workaround

Disable Tempo fee sponsorship by leaving "fee\_payer" at its default of false and setting no fee\_payer\_url, so the client pays its own gas. Where sponsorship must stay enabled, lower the max\_gas override in fee\_payer\_policy to just above what a legitimate sponsored payment needs (a TIP-20 transfer is roughly 50,000 gas), so an envelope padded with delegations either fails the policy or runs out of gas at a bounded cost. This bounds the inflation without closing it.

## Configuration

Only deployments that enable Tempo server-side fee sponsorship are affected, through "fee\_payer" => true in method\_config with either local co-signing (fee\_payer\_private\_key) or hosted sponsorship (fee\_payer\_url). Sponsorship is disabled by default.