# CVE-2026-63627: mppx gas draining via calldata padding

Source: https://junglewise.ai/threats/cve-2026-63627-mppx-gas-draining-via-calldata-padding  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-22. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-63627 · Severity: medium · CVSS 4 · Published 2026-09-22

mppx is a library used to handle fee-payer transactions in blockchain applications. When the server acts as the fee payer, a malicious client can append unnecessary padding bytes to transaction calldata, inflating actual gas costs to roughly 5x normal rates while bypassing the library's fee caps. This forces the server's fee-payer wallet to cover inflated transaction fees, dramatically increasing operational costs and eliminating profitability on low-cost transactions.

Technologies: [mppx (npm)](https://junglewise.ai/threats/technologies/npm-mppx). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-63627 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-63627 being exploited in the wild.

**How severe is CVE-2026-63627?** Medium severity, with a CVSS score of 4, published 22 September 2026.

**Which products and versions are affected?** mppx (npm) SEMVER:introduced=0,fixed=0.8.2.

**Is there a fix for CVE-2026-63627?** Yes. mppx (npm): upgrade to 0.8.2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-63627 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-63627?** Check whether you run mppx (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- mppx (npm): upgrade to 0.8.2 or later

  ```sh
  npm install mppx@0.8.2
  ```


## Technical details

The vulnerability is a gas-draining attack exploiting incomplete validation in mppx's FeePayerPolicy. While the policy enforces caps on gas_limit (≤2M) and max_fee_per_gas (≤100 Gwei), it does not validate calldata length. An attacker appends non-zero padding bytes to transaction calldata, which cost 16 gas each under Tempo's legacy pricing model, inflating actual gas consumption without exceeding the declared gas_limit or fee caps. The attack requires the server to act as fee payer and co-sign the transaction; a proof-of-concept demonstrates achieving ~5x fee inflation within a 16 KB max_header_length constraint. The root cause is that intrinsic calldata gas cost is not accounted for in the FeePayerPolicy validation logic.

## Affected products

- mppx mppx 0.6.27

## Identifiers

- CVE-2026-63627
- GHSA-727h-3vm5-qwq6

## Timeline

- 2026-09-22: disclosed: GHSA advisory published

## Related threats

- [mppx gas draining via EIP-2930 access list](https://junglewise.ai/threats/cve-2026-63628-mppx-gas-draining-via-eip-2930-access-list)
- [wevm mppx credential replay in stripe/charge payment method](https://junglewise.ai/threats/cve-2026-34210-wevm-mppx-credential-replay-in-stripe-charge-payment-method)
- [wevm mppx improper validation in tempo session close handler](https://junglewise.ai/threats/cve-2026-34209-wevm-mppx-improper-validation-in-tempo-session-close-handler)