GHSA-vc9j-9wph-qghj: Input Validation
Details When the server acts as the feepayer, mppx 0.6.27 copies the client-supplied EIP-2930 access list verbatim into the cosigned fee-payer transaction. The TypeScript SDK's fee-payer cosigning path accepts any accesslist the client includes in the 0x78 FeePayerEnvelope without inspecting its length or contents.
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 gas parameters 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 180 entries and 100 Gwei maxfeepergas, this inflates the fee-payer cost from the normal ~51,287 gas to ~483,287 gas — a 9.4× multiplier — while staying within the FeePayerPolicy cap of 500K gas, the Node.js 16 KB header limit, and the Moderato RPC's ethcall simulation budget (verified empirically at 180 entries).
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 mppxtypescriptPoC.zip cd mppxtypescript docker build -t mppx-typescript-access-list . docker run --rm mppx-typescript-access-list There are more details in mppxtypescript/README.md
Impact A malicious client can force the server to pay ~9.4x 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
npm/mppxto a version that resolves this vulnerability.Fixed in 0.8.2
Event History
Frequently Asked Questions
Who is exposed to this issue?
Servers using mppx 0.6.27 as a fee payer are exposed when they cosign client-provided 0x78 FeePayerEnvelope transactions. The affected path accepts client-supplied EIP-2930 access lists without checking their length or contents.
What does an attacker need to exploit it?
An attacker needs to submit a valid transferWithMemo request containing fabricated address-only access-list entries. They do not need the listed addresses to be used by the transaction, because access-list gas is charged intrinsically before execution.
Can this be exploited within the documented default limits?
Yes. The described 180-entry access list remains within the FeePayerPolicy 500K gas cap, the Node.js 16 KB header limit, and the Moderato RPC eth_call simulation budget. At 100 Gwei max_fee_per_gas, it can raise the fee-payer cost from about 51,287 gas to about 483,287 gas per request.
What should be checked to determine exposure?
Review fee-payer cosigning flows for FeePayerEnvelope transactions and determine whether access_list is accepted from clients and copied into the cosigned transaction without validation. Also review whether a fee-payer service permits transferWithMemo requests with access-list entries that are not required for transaction execution.