CVE-2026-63627 in mppx
Summary
by MITRE • 09/22/2026
mppx is a TypeScript interface for machine payments protocol. Prior to 0.8.2, FeePayerPolicy in src/tempo/internal/fee-payer.ts used decodeFunctionData to validate fee-sponsored calldata but did not reject trailing bytes. A client could append nonzero padding that increased intrinsic calldata gas while gas_limit and max_fee_per_gas remained within policy caps, causing the server fee_payer wallet to pay substantially more than the decoded transaction required. The tested 16 KB header limit bounded the demonstrated padding to about 5,500 bytes and produced approximately five times the normal transaction fee. This issue is fixed in version 0.8.2.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 09/22/2026
The vulnerability identified within the mppx TypeScript interface for machine payments protocol represents a critical flaw in input validation logic specifically affecting the FeePayerPolicy implementation prior to version 0.8.2. The core technical deficiency lies in the handling of calldata during transaction fee sponsorship, where the system utilizes decodeFunctionData to validate sponsored transactions but fails to enforce strict length constraints on the payload data itself. This oversight allows for a discrepancy between the semantic content of the transaction and its physical byte size, creating an opportunity for economic exploitation through gas manipulation rather than traditional code execution or privilege escalation attacks.
From a technical perspective, Ethereum-based blockchains calculate transaction fees based on the total amount of gas consumed, which is heavily influenced by the length of calldata appended to the transaction request. In this specific scenario, the FeePayerPolicy correctly interprets the function signature and parameters via decodeFunctionData but neglects to verify that no extraneous bytes follow the decoded data structure. An attacker can exploit this gap by appending non-zero padding bytes to the end of a valid transaction payload. While these trailing bytes do not alter the logical execution or outcome of the smart contract call, they significantly increase the intrinsic gas cost required for processing the calldata on-chain.
The operational impact of this vulnerability is primarily financial and economic rather than functional integrity compromise. By appending approximately 5,500 bytes of padding within the tested limit headers, an attacker can inflate the transaction fee paid by the server-side fee payer wallet to roughly five times the normal cost for that specific operation. Since the gas_limit and max_fee_per_gas parameters remain within the policy caps defined in the configuration, the system accepts these inflated transactions as valid without triggering rejection mechanisms. This results in a direct financial loss for the service provider operating the mppx infrastructure, effectively allowing malicious actors to drain funds through excessive fee payments rather than exploiting logic flaws or data corruption vectors.
This vulnerability aligns with CWE-20 Improper Input Validation, specifically regarding insufficient verification of input boundaries and structure integrity. The attack vector is classified under MITRE ATT&CK technique T1496 Resource Hijacking, as the adversary leverages system resources (in this case, financial resources via gas fees) to cause economic harm or disrupt service availability through cost inflation. It also touches upon CWE-754 Improper Check for Unusual or Exceptional Conditions, where the software fails to detect and reject anomalous input patterns such as unexpected trailing data that deviates from expected protocol specifications.
Mitigation strategies require immediate updates to the mppx library version 0.8.2 which addresses this specific validation gap. For environments unable to upgrade immediately, developers must implement strict calldata length checks within custom middleware or wrapper functions before passing transactions to the FeePayerPolicy logic. This involves calculating the expected byte size of the decoded function data and ensuring that no additional bytes exist beyond that boundary in the raw transaction payload. Additionally, implementing stricter gas price monitoring and anomaly detection systems can help identify sudden spikes in fee expenditures associated with specific user agents or IP addresses, providing a secondary layer of defense against such economic exploitation attempts until the underlying code is patched.