---
id: CVE-2025-57804
aliases:
  - GHSA-847f-9342-265h
  - PYSEC-2026-1435
title: h2 allows HTTP Request Smuggling due to illegal characters in headers
summary: h2 allows HTTP Request Smuggling due to illegal characters in headers
severity: medium
vendor: h2
product: h2
ecosystem: pip
affected:
  - h2 < 4.3.0
patched:
  - h2 4.3.0
published: '2025-08-25'
updated: '2026-09-10'
sourceUpdated: '2026-09-10T03:50:26.965133425Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-847f-9342-265h'
references:
  - url: 'https://github.com/python-hyper/h2/security/advisories/GHSA-847f-9342-265h'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2025-57804'
  - url: >-
      https://github.com/python-hyper/h2/commit/035e9899f95e3709af098f578bfc3cd302298e3a
  - url: 'https://github.com/python-hyper/h2'
  - url: 'https://lists.debian.org/debian-lts-announce/2025/09/msg00004.html'
tags:
  - osv
  - pip
epss: 0.01725
epssPercentile: 0.76555
ingestedAt: '2026-07-08T18:25:47.291Z'
---

## Overview

### Summary

HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls.

## Affected packages

- `h2 < 4.3.0`

## Remediation

Upgrade to a patched release:

- `h2 4.3.0`