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.

# 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.

# 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.