mpp vulnerable to Gas Draining with access list

Summary

### Details When the server acts as the fee_payer, `mpp` Elixir 0.4.0 copies the client-supplied EIP-2930 access list verbatim into the cosigned fee-payer transaction. In `cosign_fee_payer`, the server re-signs the raw `base_fields` (index 5 of the 0x76 AASigned envelope) without inspecting the access list field, which is part of the signed payload. Access list gas is charged **intrinsically** — before any opcode executes — regardless of whether the listed addresses are ever touched. An attacker submits a valid `transferWithMemo` alongside fabricated address-only access list entries. The server validates calldata and amount but never inspects `access_list` length. It cosigns and broadcasts a transaction that costs the fee-payer wallet `N × 2,400` extra gas per request with no corresponding work performed on-chain. At the default of 137 entries and 100 Gwei `max_fee_per_gas`, this inflates the fee-payer cost from the normal ~51,287 gas to ~380,087 gas — a **7.4× multiplier** — while staying within Bandit's default 10,000-byte per-header-field limit (verified empirically). ### 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: ```bash unzip mpp_elixir_PoC.zip cd mpp_elixir docker build -t mpp-elixir-access-list . docker run --rm mpp-elixir-access-list ``` There are more details in `mpp_elixir/README.md` ### Impact A malicious client can force the server to pay ~**7.4x** the normal transaction fee. This dramatically increases operational costs and completely destroys the profit margin on low-cost items.