CVE-2025-43859 CRITICAL

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

Vendor Python-Hyper
Product h11
Weakness CWE-444
Published April 24, 2025
Last update April 24, 2025
View on NVD All CVEs

CVSS base score

9.1/10
Attack vector Network
Attack complexity Low
Privileges required None
User interaction None
Confidentiality High
Integrity High

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

What the vulnerability does

01Description

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.

Key dates

02Disclosure timeline

April 24, 2025 CVE published
April 24, 2025 Record updated