CVE-2025-43859
h11 accepts some malformed Chunked-Encoding bodies
9.1
CRITICAL
CVSS 3.1
EPSS 0.20%
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.
How to fix CVE-2025-43859
To remediate CVE-2025-43859, upgrade the affected package to a fixed version below.
- —upgrade to 0.14.0-1.1~deb12u1 or later
- —upgrade to 0.16.0 or later
Is CVE-2025-43859 being exploited?
Low — EPSS is 0.2%, meaning exploitation activity has not been observed at scale.
Affected packages (2)
- from 0, < 0.14.0-1.1~deb12u1
- from 0, < 0.16.0
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 3.1 | CRITICAL9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |