CVE-2026-63627
Received Received - Intake

Fee-Payer Policy Calldata Validation Bypass in mppx

Publication date: 2026-09-22

Last updated on: 2026-09-22

Assigner: GitHub, Inc.

Description
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.

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Meta Information

Published
2026-09-22
Last Modified
2026-09-22
Generated
2026-09-23
AI Q&A
2026-09-22
EPSS Evaluated
N/A
NVD
CVE-2026-63627
EUVD
EUVD-2026-84805

Affected Vendors & Products

Showing 3 associated CPEs
Vendor Product Version / Range
wevm mppx 0.8.2
wevm mppx to 0.8.1 (exc)
wevm mppx 0.6.27

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
CWE-20 The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.

Attack-Flow Graph

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Executive Summary

CVE-2026-63627 is a gas-draining vulnerability in the mppx npm package affecting versions prior to 0.8.2. It involves FeePayerPolicy not rejecting trailing bytes in calldata during validation. Attackers append padding bytes to increase intrinsic gas usage while staying within policy gas and fee limits, causing the server's fee-payer wallet to overpay for transactions.

Detection Guidance

Monitor for unusual gas fee spikes in transactions involving mppx versions prior to 0.8.2. Check for padded calldata in fee-payer transactions by inspecting transaction data for trailing non-zero bytes. Use viem's decodeFunctionData to validate calldata integrity and ensure no extra padding exists beyond the intended function call.

Impact Analysis

This vulnerability can lead to significant financial losses for server operators acting as fee payers. Attackers can inflate transaction costs by up to 5 times the normal amount by appending padding bytes, draining the server's wallet. It may also disrupt service operations due to unexpected high gas fees.

Compliance Impact

This vulnerability does not directly affect compliance with GDPR or HIPAA as it pertains to gas fee manipulation in blockchain transactions rather than data privacy or security controls. However, excessive gas fee drainage could impact financial record-keeping and audit compliance under standards requiring accurate transaction cost documentation.

Mitigation Strategies

Upgrade mppx to version 0.8.2 or later immediately. Implement strict validation for sponsored transactions to reject non-canonical formats, including padded calldata, access lists, or nonzero call values. Review fee-payer wallet balances for unexpected high gas costs and revoke any unauthorized transactions.

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