# CVE-2025-43859

## Summary

- **CVE ID:** CVE-2025-43859
- **Severity:** CRITICAL
- **CVSS Score:** 9.1 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N)
- **CWE:** CWE-444
- **Published:** Apr 24, 2025
- **Last Modified:** Mar 13, 2026

## Description

h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

## Affected Products

- python-hyper — h11 (< 0.16.0)

## References

- [CNA](https://github.com/python-hyper/h11/security/advisories/GHSA-vqfr-h8mv-ghfj)
- [CNA](https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed)

## Exploitation Prediction (EPSS)

- **EPSS Score:** 0.58%
- **EPSS Percentile:** 45.8

---
_Exported from OnDuty AI Vulnerability Intelligence on 2026-09-10._