mpp vulnerable to Gas Draining with access list

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:

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.