The mpp Elixir library version 0.4.0 and other versions from 0.2.0 up to but not including 0.6.0 contain a vulnerability in the cosign_fee_payer/3 function. When the server acts as the fee payer, it does not validate gas-related parameters before cosigning transactions, allowing a malicious client to specify arbitrarily high gas fees. This causes the server to pay inflated gas costs from its own wallet, leading to direct financial loss and potential denial of service if funds are depleted.