Details When the server acts as the feepayer, mppx 0.6.27 validates calldata using viem's decodeFunctionData, which is lenient about trailing bytes. The FeePayerPolicy caps gaslimit (≤ 2 M) and maxfeepergas (≤ 100 Gwei) but does not check calldata length.
Tempo uses legacy calldata gas pricing: 16 gas per non-zero byte. Appending N bytes of 0x01 padding inflates actual gas used by N × 16 while keeping gaslimit and maxfeepergas within policy caps. The server cosigns and broadcasts the padded transaction, draining its fee-payer wallet.
Vulnerable code path: FeePayerPolicy in fee-payer.ts (mppx 0.6.27) enforces maxGas = 2000000 and maxFeePerGas = 100 Gwei but has no calldata.length bound. The policy check passes because the inflated gas comes from intrinsic calldata cost, not the declared gaslimit.
Note: In the experiment, a 16 KB maxheaderlength is enforced, which caps the maximum effective padding at roughly 5,500 bytes. The default padding in this PoC (5,500 bytes) is within that limit.
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-padding . docker run --rm mppx-padding There are more details in mppxtypescript/README.md
Impact A malicious client can force the server to pay ~5x the normal transaction fee. This dramatically increases operational costs and completely destroys the profit margin on low-cost items.
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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.