mpp vulnerable to Gas Draining with no limit

Details

When the server acts as the fee_payer, the mpp Elixir 0.4.0 does not validate gas_limit, max_fee_per_gas and max_priority_fee_per_gas before cosigning the client's fee-payer transaction. A malicious client embeds arbitrarily large max_fee_per_gas and max_priority_fee_per_gas values in the signed envelope. The server cosigns and broadcasts — paying those inflated gas costs from its own wallet based on the effective_gas_price calculated from the high max_fee_per_gas and max_priority_fee_per_gas set by the client.

Vulnerable function: cosign_fee_payer/3 in transaction.ex around lines 229–262.

PoC

The PoC is provided below. It is configured to reproduce the attack on Tempo Moderate testnet within a Docker environment. Download the PoC and run:

unzip mpp_elixir_PoC.zip
cd mpp_elixir
docker build -t mpp-elixir-gasdrain .
docker run --rm mpp-elixir-gasdrain

There are more details in mpp_elixir/README.md

Impact

By draining the server's wallet, an attacker causes direct financial loss to the organization. Furthermore, if the server's funds are drained completely, legitimate clients will have their requests fail, as the server can no longer fund future transactions, effectively creating a Denial of Service (DoS) attack.