mppx: Gas Draining with padding

Details

When the server acts as the fee_payer, mppx 0.6.27 validates calldata using viem's decodeFunctionData, which is lenient about trailing bytes. The FeePayerPolicy caps gas_limit (≤ 2 M) and max_fee_per_gas (≤ 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 gas_limit and max_fee_per_gas 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 = 2_000_000 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 gas_limit.

Note: In the experiment, a 16 KB max_header_length 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:

unzip mppx_typescript_PoC.zip
cd mppx_typescript
docker build -t mppx-padding .
docker run --rm mppx-padding

There are more details in mppx_typescript/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.

Please provide a way to share the PoC in .zip format to you. Thank you!