CVE-2026-59252: Missing gas_limit validation in mpp Tempo fee-payer enables wallet drain

Improper Validation of Specified Quantity in Input in ZenHive mpp allows an unauthenticated remote client to drain the fee-payer wallet, resulting in denial of service for legitimate clients.

When the mpp Elixir library is configured as fee payer (feepayer: true), the MPP.Methods.Tempo payment method co-signs and broadcasts a client-supplied EVM transaction without first validating that the client-supplied gaslimit is sufficient to complete the intended call. A malicious client can submit a signed transferWithMemo transaction with gaslimit deliberately set just below the amount required for successful execution. The server co-signs the transaction and broadcasts it via rpcbroadcastsync. The transaction runs out of gas during EVM execution and reverts, but the fee-payer wallet is still charged for the burned gas while the client pays nothing and receives no resource. Repeated requests from one or more malicious clients drain the fee-payer wallet at near-zero cost to the attacker, ultimately preventing the server from sponsoring gas for legitimate payment requests.

The waitforconfirmation = false (optimistic) path is also affected: it invokes simulatepaymentcall via ethcall, but that simulation omits the gas parameter and therefore does not catch out-of-gas conditions.

This issue affects mpp: from 0.2.0 before 0.6.0.