mpp vulnerable to Gas Draining with low gas limit

## Vulnerability When the server acts as the fee payer, `mpp` Elixir 0.4.0 (ZenHive/mpp) does not validate whether the `gas_limit` set by the client is enough before broadcasting. A malicious client can drain the server's gas without paying. A `transferWithMemo` call on Tempo Moderato testnet requires **~51,299 gas** to complete successfully. By setting `gas_limit = 51,298`: 1. The Tx gets cosigned and broadcast by the server. 2. The Tx runs out of gas during EVM execution. All state reverts. 3. The server's fee-payer wallet is charged for gas used. 4. The client pays nothing and receives no resource. ```bash # Run the PoC unzip mpp_elixir_low_gas_PoC.zip cd mpp_elixir_low_gas_PoC docker build -t mpp-elixir-low-gas . docker run --rm mpp-elixir-low-gas ``` **Zero-Cost DoS Attack:** Unlike gas draining with `access list` or `padding`, where the malicious client needs to complete a payment to drain the gas, this vulnerability allows malicious users to continuously drain the server's gas at virtually no financial cost (requiring only computing power). Therefore, an attacker can spawn *N* malicious clients to completely drain the funds from the server's wallet to perform a Denial of Service (DoS) attack. Once the server's wallet is empty, it has no more funds to pay the gas fees for upcoming requests from legitimate clients. ```bash # Run the DoS PoC unzip mpp_elixir_low_gas_dos_PoC.zip cd mpp_elixir_low_gas_dos_PoC docker build -t mpp-elixir-dos . docker run --rm mpp-elixir-dos ``` **Vulnerable code path:** `broadcast_and_verify/7` in `mpp/methods/tempo.ex` (ZenHive/mpp 0.4.0). When `wait_for_confirmation = true` (the default), it calls `rpc_broadcast_sync` directly without any gas-adequacy check or simulation. The alternative `wait_for_confirmation = false` path does call `simulate_payment_call` via `eth_call`, but that simulation omits the `gas` parameter and therefore does not catch out-of-gas conditions. ## Impact A malicious client can drain the server's wallet without any financial cost.