---
title: "CVE-2025-43859: h11 accepts some malformed Chunked-Encoding bodies"
url: https://erroneo.us/security/cve/CVE-2025-43859/
description: "CVE-2025-43859 (h11 (PyPI)): 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…"
type: vulnerability-record
section: "Security Digest"
date_published: 2025-04-24
date_modified: 2025-04-24
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2025-43859"
kev: false
cvss: 9.1
epss: 0.00584
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2025-43859: h11 accepts some malformed Chunked-Encoding bodies

**Signal:** Critical and remotely reachable. No known exploitation yet, but it is network-reachable without credentials. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.6% , percentile 47: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 47% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes h11 (PyPI) < 0.16.0, fixed in 0.16.0 . Check your lockfile, including transitive dependencies.

## What it is

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 packages

- PyPI `h11` < 0.16.0 → fixed: 0.16.0

## Affected products (CVE record)

- python-hyper h11: < 0.16.0

## Severity

- GitHub_M CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`
- GitHub CVSS 3.1: 9.1 `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable yes , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.58% probability of exploitation activity in the next 30 days; percentile 47.

## Sources

- [GitHub Advisory GHSA-vqfr-h8mv-ghfj](https://github.com/advisories/GHSA-vqfr-h8mv-ghfj)
- [CVE.org record CVE-2025-43859](https://www.cve.org/CVERecord?id=CVE-2025-43859)
- [NVD entry CVE-2025-43859](https://nvd.nist.gov/vuln/detail/CVE-2025-43859)
- [github.com](https://github.com/python-hyper/h11/security/advisories/GHSA-vqfr-h8mv-ghfj)
- [github.com](https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed)

---
Source page: https://erroneo.us/security/cve/CVE-2025-43859/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt