GHSA-qpxh-ff8m-c62v: Input Validation
Details When the server acts as the feepayer, mpp Elixir 0.4.0 copies the client-supplied EIP-2930 access list verbatim into the cosigned fee-payer transaction. In cosignfeepayer, the server re-signs the raw basefields (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 accesslist 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 maxfeepergas, 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 mppelixirPoC.zip cd mppelixir docker build -t mpp-elixir-access-list . docker run --rm mpp-elixir-access-list There are more details in mppelixir/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.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
erlang/mppto a version that resolves this vulnerability.Fixed in 0.6.0
Event History
Frequently Asked Questions
Who is exposed to this issue?
Servers using mpp Elixir 0.4.0 as a fee payer are exposed when they cosign client-submitted transactions. The direct financial impact falls on the fee-payer wallet, which pays the extra intrinsic gas.
What does an attacker need to exploit it?
An attacker needs to submit a valid transferWithMemo request that passes the server's calldata and amount validation. They can attach fabricated address-only EIP-2930 access-list entries, which are copied into the cosigned transaction without length validation.
Are default settings affected?
Yes. The described attack uses 137 access-list entries, which remains within Bandit's default 10,000-byte per-header-field limit. At 100 Gwei max_fee_per_gas, the example increases transaction gas from about 51,287 to about 380,087.
How can operators tell whether they may be affected?
Review whether the deployment uses mpp Elixir 0.4.0 and operates as a fee payer. Affected request handling cosigns the raw base_fields of a 0x76 AASigned envelope without inspecting or limiting the access_list field.